Attack Model and Threat for Certificate Transparency
draft-ietf-trans-threat-analysis-04
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Expired".
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|---|---|---|---|
| Author | Stephen Kent | ||
| Last updated | 2016-03-26 (Latest revision 2016-02-01) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Melinda Shore | ||
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| Responsible AD | (None) | ||
| Send notices to | "Melinda Shore" <melinda.shore@gmail.com> |
draft-ietf-trans-threat-analysis-04
Public Notary Transparency S. Kent
Internet Draft BBN Technologies
Expires: July 2016 January 30, 2016
Intended Status: Informational
Attack Model and Threat for Certificate Transparency
draft-ietf-trans-threat-analysis-04
Abstract
This document describes an attack model and discusses threats for
the Web PKI context in which security mechanisms to detect mis-
issuance of web site certificates are being developed. The model
provides an analysis of detection and remediation mechanisms for
both syntactic and semantic mis-issuance. The model introduces an
outline of attacks to organize the discussion. The model also
describes the roles played by the elements of the Certificate
Transparency (CT) system, to establish a context for the model.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
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This Internet-Draft will expire on July 30, 2016.
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Table of Contents
1. Introduction....................................................... 3
2. Threats............................................................ 8
3. Semantic mis-issuance.............................................. 9
3.1. Non-malicious Web PKI CA context ............................... 9
3.1.1. Certificate logged ........................................ 10
3.1.1.1. Benign log............................................. 10
3.1.1.1.1. Self-monitoring Subject ............................ 10
3.1.1.1.2. Benign third party Monitor ......................... 10
3.1.1.2. Misbehaving log........................................ 10
3.1.1.2.1. Self-monitoring Subject ............................ 11
3.1.1.2.2. Benign third party Monitor ......................... 11
3.1.1.3. Misbehaving third party Monitor........................ 12
3.1.2. Certificate not logged .................................... 12
3.1.2.1. Self-monitoring Subject................................ 12
3.1.2.2. CT-enabled browser..................................... 12
3.2. Malicious Web PKI CA context .................................. 13
3.2.1. Certificate logged ........................................ 13
3.2.1.1. Benign log............................................. 13
3.2.1.1.1. Self-monitoring Subject ............................ 13
3.2.1.1.2. Benign third party Monitor ......................... 13
3.2.1.2. Misbehaving log........................................ 14
3.2.1.2.1. Monitors - third party and self .................... 14
3.2.1.3. Misbehaving third party Monitor........................ 14
3.2.2. Certificate not logged .................................... 14
3.2.2.1. CT-aware browser....................................... 15
4. Syntactic mis-issuance............................................ 15
4.1. Non-malicious Web PKI CA context .............................. 15
4.1.1. Certificate logged ........................................ 15
4.1.1.1. Benign log............................................. 15
4.1.1.2. Misbehaving log or third party Monitor................. 16
4.1.1.3. Self-monitoring Subject and Benign third party Monitor. 17
4.1.1.4. CT-enabled browser..................................... 17
4.1.2. Certificate not logged .................................... 17
4.2. Malicious Web PKI CA context .................................. 17
4.2.1. Certificate logged ........................................ 18
4.2.1.1. Benign log............................................. 18
4.2.1.2. Misbehaving log or third party Monitor................. 18
4.2.1.3. Self-monitoring Subject and Benign third party Monitor. 18
4.2.1.4. CT-enabled browser..................................... 18
4.2.2. Certificate is not logged ................................. 19
5. Issues Applicable to Sections 3 and 4............................. 19
5.1. How does a Subject know which Monitor(s) to use? .............. 19
5.2. How does a Monitor discover new logs? ......................... 19
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5.3. CA response to report of a bogus or erroneous certificate ..... 20
5.4. Browser behavior .............................................. 20
5.5. Remediation for a malicious CA ................................ 20
5.6. Auditing - detecting misbehaving logs ......................... 21
6. Security Considerations........................................... 22
7. IANA Considerations............................................... 22
8. Acknowledgments................................................... 23
9. References........................................................ 24
9.1. Normative References .......................................... 24
9.2. Informative References ........................................ 24
Author's Addresses................................................... 24
Copyright Statement.................................................. 25
1. Introduction
Certificate transparency (CT) is a set of mechanisms designed to
detect, deter, and facilitate remediation of certificate mis-
issuance. The term certificate mis-issuance is defined here to
encompass violations of either semantic or syntactic constraints.
The fundamental semantic constraint for a certificate is that it was
issued to an entity that is authorized to represent the Subject (or
Subject Alternative) named in the certificate. (It is also assumed
that the entity requested the certificate from the CA that issued
it.) Throughout the remainder of this document we refer to a
semantically mis-issued certificate as "bogus.")
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A certificate is characterized as syntactically mis-issued (aka
erroneous) if it violates syntax constraints associated with the
class of certificate that it purports to represent. Syntax
constraints for certificates are established by certificate
profiles, and typically are application-specific. For example,
certificates used in the Web PKI environment might be characterized
as domain validation (DV) or extended validation (EV) certificates.
Certificates used with applications such as IPsec or S/MIME have
different syntactic constraints from those in the Web PKI context.
There are two classes of beneficiaries of CT: certificate Subjects
and relying parties (RPs). In the initial focus context of CT, the
Web PKI, Subjects are web sites and RPs are browsers employing HTTPS
to access these web sites.
A certificate Subject benefits from CT because CT helps detect
certificates that have been mis-issued in the name of that Subject.
A Subject learns of a bogus certificate (issued in its name), via
the Monitor function of CT. The Monitor function may be provided by
the Subject itself, i.e., self-monitoring, or by a third party
trusted by the Subject. When a Subject is informed of certificate
mis-issuance by a Monitor, the Subject is expected to request/demand
revocation of the bogus certificate. Revocation of a bogus
certificate is the primary means of remedying mis-issuance.
Certificate Revocations Lists (CRLs) [RFC5280] are the primary means
of certificate revocation established by IETF (and ISO) standards.
Unfortunately, most browsers do not make use of CRLs to check the
revocation status of certificates presented by a TLS Server
(Subject). Some browsers make use of Online Certificate Status
Protocol(OCSP) data [RFC4366] as a standards-based alternative to
CRLs. Also, most browser vendors employ proprietary means of
revoking certificates, e.g., via a "blacklist" or a bad-CA-list.
Such capabilities enable a browser vendor to cause browsers to
reject any certificates on the blacklist or for which the issuing CA
is on the bad-CA-list. This approach also remedies mis-issuance.
Throughout the remainder of this document references to certificate
revocation as a remedy encompass this and analogous forms of browser
behavior, if available. Note: there are no IETF standards defining a
browser blacklist capability.)
Note that a Subject can benefit from the Monitor function of CT even
if the Subject's certificate has not been logged. Monitoring of logs
for certificates issued in the Subject's name suffices to detect
mis-issuance targeting the Subject, if the bogus/erroneous
certificate is logged.
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A relying party (e.g., browser) benefits from CT if it rejects a
bogus certificate, i.e., treats it as invalid. An RP is protected
from accepting a bogus certificate if that certificate is revoked,
and if the RP checks the revocation status of the certificate. (An
RP is also protected if a browser vendor "blacklists" a certificate
or "bad-CA-lists" a CA as noted above.) An RP also may benefit from
CT if the RP validates an SCT associated with a certificate, and
rejects the certificate if the Signed certificate Timestamp (SCT)
[TRANS] is invalid. If an RP verified that a certificate that claims
to have been logged has a valid log entry, the RP would have a
higher degree of confidence that the certificate is genuine.
However, checking logs in this fashion imposes a burden on RPs and
on logs. Moreover, the existence of a log entry does not ensure that
the certificate is not mis-issued. Unless the certificate Subject is
monitoring the log(s) in question, a bogus certificate will not be
detected by CT mechanisms. Finally, if an RP were to check logs for
individual certificates, that would disclose to logs the identity of
web sites being visited by the RP, a privacy violation. Thus this
attack model assumes that RPs will not check log entries.
Note that all RPs may benefit from CT even if they do nothing with
SCTs. If Monitors inform Subjects of mis-issuance, and if a CA
revokes a certificate in response to a request from the
certificate's legitimate Subject, then an RP benefits without having
to implement any CT-specific mechanisms.
Also note that one proposal for distributing Audit information (to
detect misbehaving logs) calls for a browser to send SCTs it
receives to the corresponding website when visited by the browser.
If a website acquires an inclusion proof from a log for each
(unique) SCT it receives in this fashion, this would cause a bogus
SCT to be discovered, and, presumably, trigger a revocation request.
Logging [TRANS] is the central element of CT. Logging enables a
Monitor to detect a bogus certificate based on reference information
provided by the certificate Subject. Logging of certificates is
intended to deter mis-issuance, by creating a publicly-accessible
record that associates a CA with any certificates that it mis-
issues. Logging does not remedy mis-issuance; but it does facilitate
remediation by providing the information needed to enable detection
and consequently revocation of bogus certificates in some
circumstances.
Auditing is a function employed by CT to detect misbehavior by logs
and to deter mis-issuance that is abetted by misbehaving logs.
Auditing detects several types of log misbehavior, including
failures to adhere to the advertised Maximum Merge Delay (MMD) and
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Signed Tree Head (STH) frequency count [TRANS] violating the append-
only property, and providing inconsistent views of the log to
different log clients. The first three of these are relatively easy
for an individual auditor to detect, but the last form of
misbehavior requires communication among multiple log clients.
Monitors ought not trust logs that are detected misbehaving. Thus
the Audit function does not detect mis-issuance per se. There is no
agreed-upon Audit function design for CT at the time of this
writing. As a result, the model merely notes where Auditing is
needed to detect certain classes of attacks.
Figure 1 (below) illustrates the data exchanges among the major
elements of the CT system, based on the log specification [TRANS]
and on the assumed behavior of other CT system elements as described
above. This Figure does not include the Audit function, because
there is not yet agreement on how that function will work in a
distributed, privacy-preserving fashion.
+----+ +---------+ +---------+
| CA |---[ 1]-->| Log |<---[8]---| Monitor |
| | | | | |
| |<--[ 2]---| |----[9]-->| |
| | | | | |
| |---[ 3]-->| |<--[10]---| |
| | | | | |--------+
| |<--[ 4]---| |---[11]-->| | |
| | | | +---------+ |
| | | | |
| | | | +---------+ |
| | | |<--[8]----| Self- | |
| | | | | Monitor | |
| | | |---[9]--->|(Subject)| |
| | | | | | |
| | | |<--[10]---| | [12]
| | | | | | |
| | | |---[11]-->| | |
| | +---------+ +---------+ |
| | |
| | +---------+ +---------+ |
| |---[ 5]-->| Website |---[7]--->| Browser | |
| | |(Subject)| +---------+ |
| |<--[ 6]-->| |<----------------------------+
+----+ +---------+
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[ 1] Retrieve accepted root certs
[ 2] accepted root certs
[ 3] Add chain to log/add PreCertChain to log
[ 4] SCT (embedded in pre-cert, if applicable)
[ 5] send cert + SCTs (or cert with embedded SCTs)
[ 6] Revocation request/response (in response to detected
mis-issuance)
[ 7] cert + SCTs (or cert with embedded SCTs)
[ 8] Retrieve entries from Log
[ 9] returned entries from log
[10] Retrieve latest STH
[11] returned STH
[12] bogus/erroneous cert notification
Figure 1. Data Exchanges Between Major Elements of the CT System
Certificate mis-issuance may arise in one of several ways. The ways
by which CT enables a Subject (or others) to detect and redress mis-
issuance depends on the context and the entities involved in the
mis-issuance. This attack model applies to use of CT in the Web PKI
context. If CT is extended to apply to other contexts, each context
will require its own attack model, although most elements of the
model described here are likely to be applicable.
Because certificates are issued by CAs, the top level
differentiation in this analysis is whether the CA that mis-issued a
certificate did so maliciously or not. Next, for each scenario, the
model considers whether or not the certificate was logged. Scenarios
are further differentiated based on whether the logs and monitors
are benign or malicious and whether a certificate's Subject is self-
monitoring or is using a third party Monitoring service. Finally,
the analysis considers whether a browser is performing checking
relevant to CT. The scenarios are organized as illustrated by the
following outline:
Web PKI CA - malicious vs non-malicious
Certificate - logged vs not logged
Log - benign vs malicious
Third party Monitor - benign vs malicious
Certificate's Subject - self-monitoring (or not)
Browser - CT-supporting (or not)
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The next section of the document briefly discusses threats.
Subsequent sections examine each of the cases described above. As
noted earlier, the focus here is on the Web PKI context, although
most of the analysis is applicable to other PKI contexts.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Threats
A threat is defined, traditionally, as a motivated, capable
adversary. An adversary who is not motivated to attack a system is
not a threat. An adversary who is motivated but not "capable" also
is not a threat. Threats change over time; new classes of
adversaries may arise, new motivations may come into play, and the
capabilities of adversaries may change. Nonetheless, it is useful to
document perceived threats against a system to provide a context for
understanding attacks. Even if the assumptions about adversaries
prove to be incorrect, documenting the assumptions is valuable.
As noted above, the goals of CT are to deter, detect, and facilitate
remediation of attacks on the web PKI. Such attacks can enable an
attacker to spoof the identity of TLS-enabled web sites. Spoofing
enables an adversary to acquire information that a TLS-enabled
client would not communicate if the client were aware of the
spoofing. Such information may include personal identification and
authentication information and electronic payment authorization
information. Because of the nature of the information that may be
divulged, the principal adversaries in the CT context are perceived
to be (cyber) criminals and nation states. Both adversaries are
motivated to acquire personal identification and authentication
information. Criminals are also motivated to acquire electronic
payment authorization information.
To make use of forged web site certificates, an adversary must be
able to direct a TLS client to a spoofed web site, so that it can
present the forged certificate during a TLS handshake. An adversary
may achieve this in various ways, e.g., by causing a user to click
on a link to the website, or by manipulation of the DNS response
sent to a TLS client. The former type of attack is well within the
perceived capabilities of both classes of adversary. The latter
attack may be possible for criminals and is certainly a capability
available to a nation state within its borders. Nation states also
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may be able to compromise DNS servers outside their own
jurisdiction.
The elements of CT may themselves be targets of attacks, as
described below. A criminal organization might compromise a CA and
cause it to issue bogus certificates, or it may exert influence over
a CA (or CA staff) to do so, e.g., through extortion or physical
threat. (Even though the CA is not intentionally malicious in this
case, the action is equivalent to a malicious CA, hence the use of
the term "bogus" here.) A nation state may operate or influence a CA
that is part of the large set of "root CAs" in browsers. A CA,
acting in this fashion, is termed a "malicious" CA. A nation state
also might compromise a CA in another country, to effect issuance of
bogus certificates. In this case the (non-malicious) CA, upon
detecting the compromise (perhaps because of CT) is expected to work
with Subjects to remedy the mis-issuance.
A log also might be compromised by a suitably sophisticated criminal
organization or by a nation state. Compromising a log would enable a
compromised or rogue CA to acquire SCTs, but log entries would be
suppressed, either for all log clients or for targeted clients
(e.g., to selected Monitors or Auditors). It seems unlikely that a
compromised, non-malicious, log would persist in presenting multiple
views of its data, but a malicious log would.
Finally, note that a browser trust store may include a CA that is
intended to issue certificates to enable monitoring of encrypted
browser sessions. The inclusion of a trust anchor for such a CA is
intended to facilitate monitoring encrypted content, via an
authorized man-in-the-middle (MITM) attack. CT is not designed to
detect and counter this type of locally-authorized interception.
3. Semantic mis-issuance
3.1. Non-malicious Web PKI CA context
In this section, we address the case where the CA has no intent to
issue a bogus certificate.
A CA may have mis-issued a certificate as a result of an error or,
in the case of a bogus certificate, because it was the victim of a
social engineering attack or an attack such as the one that affected
DigiNotar
[https://www.vasco.com/company/about_vasco/press_room/news_archive/2
011/news_diginotar_reports_any security_incident.aspx]. In the case
of an error, the CA should have a record of the erroneous
certificate and be prepared to revoke this certificate once it has
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discovered and confirmed the error. In the event of an attack, a CA
may have no record of a bogus certificate.
3.1.1. Certificate logged
3.1.1.1. Benign log
The log (or logs) is benign and thus is presumed to provide
consistent, accurate responses to requests from all clients.
If a bogus (pre-)certificate has been submitted to one or more logs
prior to issuance to acquire an embedded SCT, or post-issuance to
acquire a standalone SCT, detection of this mis-issuance is the
responsibility of a Monitor.
3.1.1.1.1. Self-monitoring Subject
If a Subject is tracking the log(s) to which a certificate was
submitted, and is performing self-monitoring, then it will be able
to detect a bogus (pre-)certificate and request revocation. In this
case, the CA will make use of the log entry (supplied by the
Subject) to determine the serial number of the bogus certificate,
and investigate/revoke it. (See Sections 5.1, 5.2 and 5.3.)
3.1.1.1.2. Benign third party Monitor
If a benign third party monitor is checking the logs to which a
certificate was submitted and is protecting the targeted Subject, it
will detect a bogus certificate and will alert the Subject. The
Subject, in turn, will ask the CA to revoke the bogus certificate.
In this case, the CA will make use of the log entry (supplied by the
Subject) to determine the serial number of the bogus certificate,
and revoke it (after investigation). (See Sections 5.1, 5.2 and
5.3.)
3.1.1.2. Misbehaving log
In this case, the bogus (pre-)certificate has been submitted to one
or more logs, each of which generate an SCT for the submission. A
misbehaving log probably will suppress a bogus certificate log
entry, or it may create an entry for the certificate but report it
selectively. (A misbehaving log also could create and report entries
for bogus certificates that have not been issued by the indicated CA
(hereafter called "fake"). Unless a Monitor validates the associated
certificate chains up to roots that it trusts, these fake bogus
certificates could cause the Monitors to report non-existent
semantic problems to the Subject who would in turn report them to
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the purported issuing CA. This might cause the CA to do needless
investigative work or perhaps incorrectly revoke and re-issue the
Subject's real certificate. Note that for every certificate
submitted to a log, the log MUST verify a complete certificate chain
up to one of the roots it accepts. So creating a log entry for a
fake bogus certificate marks the log as misbehaving.
3.1.1.2.1. Self-monitoring Subject
If a misbehaving log suppresses a bogus certificate log entry, a
Subject performing self-monitoring will not detect the bogus
certificate. CT relies on an Audit mechanism to detect log
misbehavior, as a deterrent. It is anticipated that logs that are
identified as persistently misbehaving will cease to be trusted by
Monitors, non-malicious CAs, and by browser vendors. This assumption
forms the basis for the perceived deterrent. It is not clear if
mechanisms to detect this sort of log misbehavior will be viable.
3.1.1.2.2. Benign third party Monitor
Because a misbehaving log will suppress a bogus certificate log
entry (or report such entries inconsistently) a benign third party
Monitor that is protecting the targeted Subject also will not detect
a bogus certificate. In this scenario, CT relies on a distributed
Auditing mechanism [gossip] to detect log misbehavior, as a
deterrent. (See Section 5.6 below.) However, a Monitor (third-party
or self) must participate in the Audit mechanism in order to become
aware of log misbehavior.
If the misbehaving log has logged the bogus certificate when issuing
the associated SCT, it will try to hide this from the Subject (if
self-monitoring) or from the Monitor protecting the Subject. It does
so by presenting them with a view of its log entries and STH that
does not contain the bogus certificate. To other entities, the log
presents log entries and an STH that include the bogus certificate.
This discrepancy can be detected if there is an exchange of
information about the log entries and STH between the entities
receiving the view that excludes the bogus certificate and entities
that receive a view that includes it, i.e., a distributed Audit
mechanism.
If a malicious log does not create an entry for a bogus certificate
(for which an SCT has been issued), then any Monitor/Auditor that
sees the bogus certificate will detect this when it checks with the
log for log entries and STH (see Section 3.1.2.)
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3.1.1.3. Misbehaving third party Monitor
A third party Monitor that misbehaves will not notify the targeted
Subject of a bogus certificate. This is true irrespective of whether
the Monitor checks the logs or whether the logs are benign or
malicious/conspiring.
Note that independent of any mis-issuance on the part of the CA, a
misbehaving Monitor could issue false warnings to a Subject that it
protects. These could cause the Subject to report non-existent
semantic problems to the issuing CA and cause the CA to do needless
investigative work or perhaps incorrectly revoke and re-issue the
Subject's certificate.
3.1.2. Certificate not logged
If the CA does not submit a pre-certificate to a log, whether a log
is benign or misbehaving does not matter. The same is true if a
Subject is issued a certificate without an SCT and does not log the
certificate itself, to acquire an SCT. Also, since there is no log
entry in this scenario, there is no difference in outcome between a
benign and a misbehaving third party Monitor. In both cases, no
Monitor (self or third-party) will detect a bogus certificate based
on Monitor functions and there will be no consequent reporting of
the problem to the Subject or by the Subject to the CA based on
examination of log entries.
3.1.2.1. Self-monitoring Subject
A Subject performing self-monitoring will be able to detect the lack
of an embedded SCT in the certificate it received from the CA, or
the lack of an SCT supplied to the Subject via an out-of-band
channel. A Subject ought to notify the CA if the Subject expected
that its certificate was to be logged. (A Subject would expect its
certificate to be logged if there is an agreement between the
Subject and the CA to do so, or because the CA advertises that it
logs all of the certificates that it issues.) If the certificate was
supposed to be logged, but was not, the CA can use the certificate
supplied by the Subject to investigate and remedy the problem. In
the context of a benign CA, a failure to log the certificate might
be the result of an operations error, or evidence of an attack on
the CA.
3.1.2.2. CT-enabled browser
If a browser rejects certificates without SCTs (see Section 5.4),
CAs may be "encouraged" to log the certificates they issue. This, in
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turn, would make it easier for Monitors to detect bogus
certificates. However, the CT architecture does not describe how
such behavior by browsers can be deployed incrementally throughout
the Internet. As a result, this attack model does not assume that
browsers will reject a certificate that is not accompanied by an
SCT. In the CT architecture certificates have to be logged to enable
Monitors to detect mis-issuance, and to trigger subsequent
revocation [CTarch]. Thus the effectiveness of CT is diminished in
this context.
3.2. Malicious Web PKI CA context
In this section, we address the scenario in which the mis-issuance
is intentional, not due to error. The CA is not the victim but the
attacker.
3.2.1. Certificate logged
3.2.1.1. Benign log
A bogus (pre-)certificate may be submitted to one or more benign
logs prior to issuance, to acquire an embedded SCT, or post-issuance
to acquire a standalone SCT. The log (or logs) replies correctly to
requests from clients.
3.2.1.1.1. Self-monitoring Subject
If a Subject is checking the logs to which a certificate was
submitted and is performing self-monitoring, it will be able to
detect the bogus certificate and will request revocation. The CA may
refuse to revoke, or may substantially delay revoking, the bogus
certificate. For example, the CA could make excuses about inadequate
proof that the certificate is bogus, or argue that it cannot quickly
revoke the certificate because of legal concerns, etc. In this case,
the CT mechanisms will have detected mis-issuance, but the
information logged by CT does not help remedy the problem. (See
Sections 4 and 6.)
3.2.1.1.2. Benign third party Monitor
If a benign third party monitor is checking the logs to which a
certificate was submitted and is protecting the targeted Subject, it
will detect the bogus certificate and will alert the Subject. The
Subject will then ask the CA to revoke the bogus certificate. As in
3.2.1.1.1, the CA may or may not revoke the certificate.
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3.2.1.2. Misbehaving log
A bogus (pre-)certificate may have been submitted to one or more
logs that are misbehaving, e.g., conspiring with an attacker. These
logs may or may not issue SCTs, but will hide the log entries from
some or all Monitors.
3.2.1.2.1. Monitors - third party and self
If log entries are hidden from a Monitor (third party or self), the
Monitor will not be able to detect issuance of a bogus certificate.
The Audit function of CT is intended to detect logs that conspire to
delay or suppress log entries (potentially selectively), based on
consistency checking of logs. (See 3.1.1.2.2.) If a Monitor learns
of misbehaving log operation, it alerts the Subjects that it is
protecting, so that they no longer acquire SCTs from that log. The
Monitor also avoids relying upon such a log in the future. However,
unless a distributed Audit mechanism proves effective in detecting
such misbehavior, CT cannot be relied upon to detect this form of
mis-issuance. (See Section 5.6 below.)
3.2.1.3. Misbehaving third party Monitor
If the third party Monitor that is "protecting" the targeted Subject
is misbehaving, then it will not notify the targeted Subject of any
mis-issuance or of any malfeasant log behavior that it detects
irrespective of whether the logs it checks are benign or
malicious/conspiring. The CT architecture does not include any
measures to detect misbehavior by third-party monitors.
3.2.2. Certificate not logged
Because the CA is presumed malicious, it may choose to not submit a
(pre-)certificate to a log. This means there is no SCT for the
certificate.
When a CA does not submit a certificate to a log, whether a log is
benign or misbehaving does not matter. Also, since there is no log
entry, there is no difference in behavior between a benign and a
misbehaving third-party Monitor. Neither will report a problem to
the Subject.
A bogus certificate would not be delivered to the legitimate
Subject. So the Subject, acting as a self-Monitor, cannot detect the
issuance of a bogus certificate in this case.
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3.2.2.1. CT-aware browser
If careful browsers reject certificates without SCTs, CAs may be
"encouraged" to log certificates (see section 5.4.) However, the CT
architecture does not describe how such behavior by browsers can be
deployed incrementally throughout the Internet. As a result, this
attack model does not assume that browsers will reject a certificate
that is not accompanied by an SCT. Since certificates have to be
logged to enable detection of mis-issuance by Monitors, and to
trigger subsequent revocation, the effectiveness of CT is diminished
in this context.
4. Syntactic mis-issuance
4.1. Non-malicious Web PKI CA context
This section analyzes the scenario in which the CA has no intent to
issue a syntactically incorrect certificate. As noted in Section 1,
we refer to a syntactically incorrect certificate as erroneous.
4.1.1. Certificate logged
4.1.1.1. Benign log
If a (pre-)certificate is submitted to a benign log, syntactic mis-
issuance can (optionally) be detected, and noted. This will happen
only if the log performs syntactic checks in general, and if the log
is capable of performing the checks applicable to the submitted
(pre-)certificate. (A (pre-)certificate SHOULD be logged even if it
fails syntactic validation; logging takes precedence over detection
of syntactic mis-issuance.) If syntactic validation fails, this can
be noted in an SCT extension returned to the submitter.
If the (pre-)certificate is submitted by the non-malicious issuing
CA, and if the CA has a record of the certificate content, then the
CA SHOULD remedy the syntactic problem and re-submit the
(pre-)certificate to a log or logs. If this is a pre-certificate
submitted prior to issuance, syntactic checking by a log helps avoid
issuance of an erroneous certificate. If the CA does not have a
record of the certificate contents, then presumably it was a bogus
certificate and the CA SHOULD revoke it.
If a certificate is submitted by its Subject, and is deemed
erroneous, then the Subject SHOULD contact the issuing CA and
request a new certificate. If the Subject is a legitimate subscriber
of the CA, then the CA will either have a record of the certificate
content or can obtain a copy of the certificate from the Subject.
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The CA will remedy the syntactic problem and either re-submit a
corrected (pre-)certificate to a log and send it to the Subject or
the Subject will re-submit it to a log. Here too syntactic checking
by a log enables a Subject to be informed that its certificate is
erroneous and thus may hasten issuance of a replacement certificate.
If a certificate is submitted by a third party, that party might
contact the Subject or the issuing CA, but because the party is not
the Subject of the certificate it is not clear how the CA will
respond.
This analysis suggests that syntactic mis-issuance of a certificate
can be avoided by a CA if it makes use of logs that are capable of
performing these checks for the types of certificates that are
submitted, and if the CA acts on the feedback it receives. If a CA
uses a log that does not perform such checks, or if the CA requests
checking relative to criteria not supported by the log, then
syntactic mis-issuance will not be detected or avoided by this
mechanism. Similarly, syntactic mis-issuance can be remedied if a
Subject submits a certificate to a log that performs syntactic
checks, and if the Subject asks the issuing CA to fix problems
detected by the log. (The issuer is presumed to be willing to re-
issue the certificate, correcting any problems, because the issuing
CA is not malicious.)
4.1.1.2. Misbehaving log or third party Monitor
A log or Monitor that is conspiring with the attacker or is
independently malicious, will either not perform syntactic checks,
even though it claims to do so, or simply not report errors. The log
entry and the SCT for an erroneous certificate will assert that the
certificate syntax was verified.
As with detection of semantic mis-issuance, a distributed Audit
mechanism could, in principle, detect misbehavior by logs or
Monitors with respect to syntactic checking. For example, if for a
given certificate, some logs (or Monitors) are reporting syntactic
errors and some that claim to do syntactic checking, are not
reporting these errors, this is indicative of misbehavior by these
logs and/or Monitors.
Note that a malicious log (or Monitor) could report syntactic errors
for a syntactically valid certificate. This could result in
reporting of non-existent syntactic problems to the issuing CA,
which might cause the CA to do needless investigative work or
perhaps incorrectly revoke and re-issue the Subject's certificate.
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4.1.1.3. Self-monitoring Subject and Benign third party Monitor
If a Subject or benign third party Monitor performs syntactic
checks, it will detect the erroneous certificate and the issuing CA
will be notified (by the Subject). If the Subject is a legitimate
subscriber of the CA, then the CA will either have a record of the
certificate content or can obtain a copy of the certificate from the
Subject. The CA SHOULD revoke the erroneous certificate (after
investigation) and remedy the syntactic problem. The CA SHOULD
either re-submit the corrected (pre-)certificate to one or more logs
and then send the result to the Subject, or send the corrected
certificate to the Subject, who will re-submit it to one or more
logs.
4.1.1.4. CT-enabled browser
If a browser rejects an erroneous certificate and notifies the
Subject and/or the issuing CA, then syntactic mis-issuance will be
detected (see Section 5.) Unfortunately, experience suggests that
many browsers do not perform thorough syntactic checks on
certificates, and so it seems unlikely that browsers will be a
reliable way to detect erroneous certificates. Moreover, a protocol
used by a browser to notify a Subject and/or CA of an erroneous
certificate represents a DoS potential, and thus may not be
appropriate. Additionally, if a browser directly contacts a CA when
an erroneous certificate is detected, this is a potential privacy
violation, i.e., the CA learns that the browser user is visiting the
web site in question. These observations argue for syntactic
checking to be performed by other elements of the CT system, e.g.,
logs and/or Monitors.
4.1.2. Certificate not logged
If a CA does not submit a certificate to a log, there can be no
syntactic checking by the log. Detection of syntactic errors will
depend on a Subject performing the requisite checks when it receives
its certificate from a CA. A Monitor that performs syntactic checks
on behalf of a Subject also could detect such problems, but the CT
architecture does not require Monitors to perform such checks.
4.2. Malicious Web PKI CA context
This section analyzes the scenario in which the CA's issuance of a
syntactically incorrect certificate is intentional, not due to
error. The CA is not the victim but the attacker.
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4.2.1. Certificate logged
4.2.1.1. Benign log
Because the CA is presumed to be malicious, the CA may cause the log
to not perform checks, in one of several ways. (See [DOMVAL] and
[EXTVAL] for more details on validation checks and CCIDs).
1. The CA may assert that the certificate is being issued w/o
regard to any guidelines (the "no guidelines" reserved CCID).
2. The CA may assert a CCID that has not been registered, and
thus no log will be able to perform a check.
3. The CA may check to see which CCIDs a log declares it can
check, and chose a registered CCID that is not checked by the log
in question.
4. The CA may submit a (pre-) certificate to a log that is known
to not perform any syntactic checks, and thus avoid syntactic
checking.
4.2.1.2. Misbehaving log or third party Monitor
A misbehaving log or third party Monitor will either not perform
syntactic checks or not report any problems that it discovers. (See
4.1.1.2 for further problems). Also, as noted above, the CT
architecture includes no explicit provisions for detecting a
misbehaving third-party Monitor.
4.2.1.3. Self-monitoring Subject and Benign third party Monitor
Irrespective of whether syntactic checks are performed by a log, a
malicious CA will acquire an embedded SCT, or post-issuance will
acquire a standalone SCT. If Subjects or Monitors perform syntactic
checks that detect the syntactic mis-issuance and report the problem
to the CA, a malicious CA may do nothing or may delay the action(s)
needed to remedy the problem.
4.2.1.4. CT-enabled browser
As noted above (4.1.1.4), many browsers fail to perform thorough
syntax checks on certificates. Such browsers would benefit from
having such checks performed by a log and reported in the SCT.
(Remember, in this scenario, the log is benign.) However, if a
browser does not discriminate against certificates that do not
contain SCTs (or that are not accompanied by an SCT in the TLS
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handshake), only minimal benefits might accrue to the browser from
syntax checks perform by logs or Monitors.
If a browser accepts certificates that do not appear to have been
syntactically checked by a log (as indicated by the SCT), a
malicious CA need not worry about failing a log-based check.
Similarly, if there is no requirement for a browser to reject a
certificate that was logged by an operator that does not perform
syntactic checks, the fourth attack noted in 4.2.1.1 will succeed as
well. If a browser were configured to know which versions of
certificate types are applicable to its use of a certificate, the
second and third attack strategies noted above could be thwarted.
4.2.2. Certificate is not logged
Since certificates are not logged in this scenario, a Monitor
(third-party or self) cannot detect the issuance of an erroneous
certificate. Thus there is no difference between a benign or a
malicious/conspiring log or a benign or conspiring/malicious
Monitor. (A Subject MAY detect a syntax error by examining the
certificate returned to it by the Issuer.) However, even if errors
are detected and reported to the CA, a malicious/conspiring CA may
do nothing to fix the problem or may delay action.
5. Issues Applicable to Sections 3 and 4
5.1. How does a Subject know which Monitor(s) to use?
If a CA submits a bogus certificate to one or more logs, but these
logs are not tracked by a Monitor that is protecting the targeted
Subject, CT will not remedy this type of mis-issuance attack. If
third-party Monitors advertise which logs they track, Subjects may
be able to use this information to select an appropriate Monitor (or
set thereof). Also, it is not clear whether every third-party
Monitor MUST offer to track every Subject that requests protection.
If a Subject acts as its own Monitor, this problem is solved for
that Subject.
5.2. How does a Monitor discover new logs?
It is not clear how a (self-)Monitor becomes aware of all (relevant)
logs, including newly created logs. The means by which Monitors
become aware of new logs MUST accommodate self-monitoring by a
potentially very large number of web site operators. If there are
many logs, it may not be feasible for a (self-) Monitor to track all
of them, or to determine what set of logs suffice to ensure an
adequate level of coverage.
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5.3. CA response to report of a bogus or erroneous certificate
A CA being presented with evidence of a bogus or erroneous
certificate, in the form of a log entry and/or SCT, will need to
examine its records to determine if it has knowledge of the
certificate in question. It also will likely require the targeted
Subject to provide assurances that it is the authorized entity
representing the Subject name (subjectAltname) in question. Thus a
Subject should not expect immediate revocation of a contested
certificate. The time frame in which a CA will respond to a
revocation request usually is described in the CPS for the CA. Other
certificate fields and extensions may be of interest for forensic
purposes, but are not required to effect revocation nor to verify
that the certificate to be revoked is bogus or erroneous, based on
applicable criteria. The SCT and log entry, because each contains a
timestamp from a third party, is probably valuable for forensic
purposes (assuming a non-conspiring log operator).
5.4. Browser behavior
If a browser is to reject a certificate that lacks an embedded SCT,
or is not accompanied by an SCT transported via the TLS handshake,
this behavior needs to be defined in a way that is compatible with
incremental deployment. Issuing a warning to a (human) user is
probably insufficient, based on experience with warnings displayed
for expired certificates, lack of certificate revocation status
information, and similar errors that violate RFC 5280 path
validation rules [RFC5280]. Unless a mechanism is defined that
accommodates incremental deployment of this capability, attackers
probably will avoid submitting bogus certificates to (benign) logs
as a means of evading detection.
5.5. Remediation for a malicious CA
A targeted Subject might ask the parent of a malicious CA to revoke
the certificate of the non-cooperative CA. However, a request of
this sort may be rejected, e.g., because of the potential for
significant collateral damage. A browser might be configured to
reject all certificates issued by the malicious CA, e.g., using a
bad-CA-list distributed by a browser vendor. However, if the
malicious CA has a sufficient number of legitimate clients, treating
all of their certificates as bogus or erroneous still represents
serious collateral damage. If this specification were to require
that a browser can be configured to reject a specific, bogus or
erroneous certificate identified by a Monitor, then the bogus or
erroneous certificate could be rejected in that fashion. This
remediation strategy calls for communication between Monitors and
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browsers, or between Monitors and browser vendors. Such
communication has not been specified, i.e., there are no standard
ways to configure a browser to reject individual bogus or erroneous
certificates based on information provided by an external entity
such as a Monitor. Moreover, the same or another malicious CA could
issue new bogus or erroneous certificates for the targeted Subject,
which would have to be detected and rejected in this (as yet
unspecified) fashion. Thus, for now, CT does not seem to provide a
way to remedy this form of attack, even though it provides a basis
for detecting such attacks.
5.6. Auditing - detecting misbehaving logs
The combination of a malicious CA and one or more conspiring logs
motivates the definition of an audit function, to detect conspiring
logs. If a Monitor protecting a Subject does not see bogus
certificates, it cannot alert the Subject. If one or more SCTs are
present in a certificate, or passed via the TLS handshake, a browser
has no way to know that the logged certificate is not visible to
Monitors. Only if Monitors and browsers reject certificates that
contain SCTs from conspiring logs (based on information from an
auditor) will CT be able to detect and deter use of such logs. Thus
the means by which a Monitor performing an audit function detects
such logs, and informs browsers must be specified for CT to be
effective in the context of misbehaving logs.
Absent a well-defined mechanism that enables Monitors to verify that
data from logs are reported in a consistent fashion, CT cannot claim
to provide protection against logs that are malicious or may
conspire with, or are victims of, attackers effecting certificate
mis-issuance. The mechanism needs to protect the privacy of users
with respect to which web sites they visit. It needs to scale to
accommodate a potentially large number of self-monitoring Subjects
and a vast number of browsers, if browsers are part of the
mechanism. Even when an Audit mechanism is defined, it will be
necessary to describe how the CT system will deal with a misbehaving
or compromised log. For example, will there be a mechanism to alert
all browsers to reject SCTs issued by such a log? Absent a
description of a remediation strategy to deal with misbehaving or
compromised logs, CT cannot ensure detection of mis-issuance in a
wide range of scenarios.
Monitors play a critical role in detecting semantic certificate mis-
issuance, for Subjects that have requested monitoring of their
certificates. A monitor (including a Subject performing self-
monitoring) examines logs for certificates associated with one or
more Subjects that are being "protected". A third-party Monitor must
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obtain a list of valid certificates for the Subject being monitored,
in a secure manner, to use as a reference. It also must be able to
identify and track a potentially large number of logs on behalf of
its Subjects. This may be a daunting task for Subjects that elect to
perform self-monitoring.
Note: A Monitor must not rely on a CA or RA database for its
reference information or use certificate discovery protocols; this
information must be acquired by the Monitor based on reference
certificates provided by a Subject. If a Monitor were to rely on a
CA or RA database (for the CA that issued a targeted certificate),
the Monitor would not detect mis-issuance due to malfeasance on the
part of that CA or the RA, or due to compromise of the CA or the
RA. If a CA or RA database is used, it would support detection of
mis-issuance by an unauthorized CA. A Monitor must not rely on
certificate discovery mechanisms to build the list of valid
certificates since such mechanisms might result in bogus or
erroneous certificates being added to the list.
As noted above, Monitors represent another target for adversaries
who wish to effect certificate mis-issuance. If a Monitor is
compromised by, or conspires with, an attacker, it will fail to
alert a Subject to a bogus or erroneous certificate targeting that
Subject, as noted above. It is suggested that a Subject request
certificate monitoring from multiple sources to guard against such
failures. Operation of a Monitor by a Subject, on its own behalf,
avoids dependence on third party Monitors. However, the burden of
Monitor operation may be viewed as too great for many web sites, and
thus this mode of operation ought not be assumed to be universal
when evaluating protection against Monitor compromise.
6. Security Considerations
An attack and threat model is, by definition, a security-centric
document. Unlike a protocol description, a threat model does not
create security problems nor does it purport to address security
problems. This model postulates a set of threats (i.e., motivated,
capable adversaries) and examines classes of attacks that these
threats are capable of effecting, based on the motivations ascribed
to the threats. It then analyses the ways in which the CT
architecture addresses these attacks.
7. IANA Considerations
None.
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8. Acknowledgments
The author would like to thank David Mandelberg and Karen Seo for
their assistance in reviewing and preparing this document, and other
members of the TRANS working group for reviewing it.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC 2119, March 1997.
[CTarch] Kent, S., Mandelberg, D., Seo, K., "Certificate
Transparency (CT) System Architecture", draft-kent-trans-
architecture-00, (October 2015), work in progress.
9.2. Informative References
[TRANS] Laurie, B., Langley, A., Kasper, E., Messeri, E.,
Stradling, R., "Certificate Transparency," draft-ietf-
trans-rfc6962-bis-07 (March 9, 2015), work in progress.
[DOMVAL] Kent, S., "Syntactic and Semantic Checks for Domain
Validation Certificates," draft-kent-trans-domain-
validation-cert-checks-01, (December 2014), work in
progress.
[EXTVAL] Kent, S., "Syntactic and Semantic Checks for Extended
Validation Certificates," draft-kent-trans-extended-
validation-cert-checks-01 (December 2014), work in
progress.
[gossip] Nordberg, L, Gillmore, D, "Gossiping in CT," draft-linus-
trans-gossip-ct-01, (March 9, 2015), work in progress
[RFC5280] Cooper, D., Santession, S., Farrell, S., Boeyen, S.,
Housley, R., Polk, W., "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile," RFC 5280, May 2008.
Author's Addresses
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge MA 02138
USA
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Phone: +1 (617) 873-3988
Email: skent@bbn.com
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