Network Working Group D. Thaler, Ed.
Internet-Draft Microsoft
Intended status: Informational July 16, 2012
Expires: January 17, 2013
Issues in Identifier Comparison for Security Purposes
draft-iab-identifier-comparison-03.txt
Abstract
Identifiers such as hostnames, URIs, and email addresses are often
used in security contexts to identify security principals and
resources. In such contexts, an identifier supplied via some
protocol is often compared against some policy to make security
decisions such as whether the principal may access the resource, what
level of authentication or encryption is required, etc. If the
parties involved in a security decision use different algorithms to
compare identifiers, then failure scenarios ranging from denial of
service to elevation of privilege can result.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 17, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Canonicalization . . . . . . . . . . . . . . . . . . . . . 4
2. Security Uses . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Types of Identifiers . . . . . . . . . . . . . . . . . . . 6
2.2. False Positives and Negatives . . . . . . . . . . . . . . 7
2.3. Hypothetical Example . . . . . . . . . . . . . . . . . . . 8
3. Common Identifiers . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Hostnames . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. IPv4 Literals . . . . . . . . . . . . . . . . . . . . 9
3.1.2. IPv6 Literals . . . . . . . . . . . . . . . . . . . . 11
3.1.3. Internationalization . . . . . . . . . . . . . . . . . 12
3.1.4. Resolution for comparison . . . . . . . . . . . . . . 12
3.2. Ports and Service Names . . . . . . . . . . . . . . . . . 13
3.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3.1. Scheme component . . . . . . . . . . . . . . . . . . . 15
3.3.2. Authority component . . . . . . . . . . . . . . . . . 15
3.3.3. Path component . . . . . . . . . . . . . . . . . . . . 16
3.3.4. Query component . . . . . . . . . . . . . . . . . . . 16
3.3.5. Fragment component . . . . . . . . . . . . . . . . . . 16
3.3.6. Resolution for comparison . . . . . . . . . . . . . . 17
3.4. Email Address-like Identifiers . . . . . . . . . . . . . . 17
4. General Internationalization Issues . . . . . . . . . . . . . 17
5. General Scope Issues . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
9. Informative References . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
In computing and the Internet, various types of "identifiers" are
used to identify humans, devices, content, etc. Before discussing
security issues, we first give some background on some typical
processes involving identifiers.
As depicted in Figure 1, there are multiple processes relevant to our
discussion.
1. An identifier must first be generated. If the identifier is
intended to be unique, the generation process includes some
mechanism, such as allocation by a central authority, to help
ensure uniqueness. However the notion of "unique" involves
determining whether a putative identifier matches any other
already-allocated identifier. As we will see, for many types of
identifiers, this is not simply an exact binary match.
As a result of generating the identifier, it is often stored in
two locations: with the requester or "holder" of the identifier,
and with some repository of identifiers (e.g., DNS). For
example, if the identifier was allocated by a central authority,
the repository might be that authority. If the identifier
identifies a device or content on a device, the repository might
be that device.
2. The identifier must be distributed, either by the holder of the
identifier or by a repository of identifiers, to others who could
use the identifier. This distribution might be electronic, but
sometimes it is via other channels such as voice, business card,
billboard, or other form of advertisement. The identifier itself
might be distributed directly, or it might be used to generate a
portion of another type of identifier that is then distributed.
For example, a URI or email address might include a server name,
and hence distributing the URI or email address also inherently
distributes the server name.
3. The identifier must be used by some party. Generally the user
supplies the identifier which is (directly or indirectly) sent to
the repository of identifiers. For example, using an email
address to send email to the holder of an identifier may result
in the email arriving at the holder's email server which has
access to the mail stores.
The repository of identifiers must then attempt to match the
user-supplied identifier with an identifier in its repository.
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+------------+
| Holder of | 1. Generation
| identifier +<---------+
+----+-------+ |
| | Match
| v/
| +-------+-------+
+----------+ Repository of |
| | identifiers |
| +-------+-------+
2. Distribution | ^\
| | Match
v |
+---------+-------+ |
| User of | |
| identifier +----------+
+-----------------+ 3. Use
Typical Identifier Processes
Figure 1
One key aspect is that the identifier values passed in generation,
distribution, and use, may all be different forms. For example,
generation might be exchanged in printed form, distribution done via
voice, and use done electronically. As such, the match process can
be complicated.
Furthermore, in many uses, the relationship between holder,
repositories, and users may be more involved. For example, when a
hierarchy of web caches exist, each cache is itself a repository of a
sort, and the match process is usually intended to be the same as on
the origin server.
1.1. Canonicalization
Perhaps the most common algorithm for comparison involves first
converting each identifier to a canonical form (a process known as
"canonicalization" or "normalization"), and then testing . the
resulting canonical representations for bitwise equality. In so
doing, it is thus critical that all entities involved agree on the
same canonical form and use the same canonicalization algorithm so
that the overall comparison process is also the same.
Note that in some contexts, such as in internationalization, the
terms "canonicalization" and "normalization" have a precise meaning.
In this document, however, we use these terms synonymously in their
more generic form, to mean conversion to some standard form.
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While the most common method of comparison includes canonicalization,
comparison can also be done by defining an equivalence algorithm,
where no single form is canonical. However in most cases, a
canonical form is useful for other purposes, such as output, and so
in such cases defining a canonical form suffices to define a
comparison method.
2. Security Uses
Identifiers such as hostnames, URIs, and email addresses are used in
security contexts to identify principals and resources as well as
other security parameters such as types and values of claims. Those
identifiers are then used to make security decisions based on an
identifier supplied via some protocol. For example:
o Authentication: a protocol might match a security principal
identifier to look up expected keying material, and then match
keying material.
o Authorization: a protocol might match a resource name to look up
an access control list (ACL), and then look up the security
principal identifier (or a surrogate for it) in that ACL.
o Accounting: a system might create an accounting record for a
security principal identifier or resource name, and then might
later need to match a supplied identifier to allow (for example)
law enforcement to follow up based on the records, or add new
filtering rules based on the records in order to stop an attack.
If the parties involved in a security decision use different matching
algorithms for the same identifiers, then failure scenarios ranging
from denial of service to elevation of privilege can result, as we
will see.
This is especially complicated in cases involving multiple parties
and multiple protocols. For example, there are many scenarios where
some form of "security token service" is used to grant to a requester
permission to access a resource, where the resource is held by a
third party that relies on the security token service (see Figure 2).
The protocol used to request permission (e.g., Kerberos or OAuth) may
be different from the protocol used to access the resource (e.g.,
HTTP). Opportunities for security problems arise when two protocols
define different comparison algorithms for the same type of
identifier, or when a protocol is ambiguously specified and two
endpoints (e.g., a security token service and a resource holder)
implement different algorithms within the same protocol.
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+----------+
| security |
| token |
| service |
+----------+
^
| 1. supply credentials and
| get token for resource
| +--------+
+----------+ 2. supply token and access resource |resource|
|requester |=------------------------------------->| holder |
+----------+ +--------+
Simple Security Exchange
Figure 2
In many cases the situation is more complex. With certificates, the
name in a certificate gets compared against names in ACLs or other
things. In the case of web site security, the name in the
certificate gets compared to a portion of the URI that a user may
have typed into a browser. The fact that many different people are
doing the typing, on many different types of systems, complicates the
problem.
Add to this the certificate enrollment step, and the certificate
issuance step, and two more parties have an opportunity to adjust the
encoding or worse, the software that supports them might make changes
that the parties are unaware are happening.
2.1. Types of Identifiers
In this document we will refer to the following types of identifiers:
o Absolute: identifiers that can be compared byte-by-byte for
equality. Two identifiers that have different bytes are defined
to be different. For example, binary IP addresses are in this
class.
o Definite: identifiers that have a well-defined comparison
algorithm on which all parties agree. For example, URI scheme
names are required to be ASCII and are defined to match in a case-
insensitive way; the comparison is thus definite since all parties
agree on how to do a case-insensitive match among ASCII strings.
o Indefinite: identifiers that have no single comparison algorithm
on which all parties agree. For example, human names are in this
class. Everyone might want the comparison to be tailored for
their locale, for some definition of locale. In some cases, there
may be limited subsets of parties that might be able to agree
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(e.g., ASCII users might all agree on a common comparison
algorithm whereas users of other Latin scripts, such as Turkish,
may not), but identifiers often tend to leak out of such limited
environments.
2.2. False Positives and Negatives
It is first worth discussing in more detail the effects of errors in
the comparison algorithm. A "false positive" results when two
identifiers compare as if they were equal, but in reality refer to
two different objects (e.g., security principals or resources). When
privilege is granted on a match, a false positive thus results in an
elevation of privilege, for example allowing execution of an
operation that should not have been permitted otherwise. When
privilege is denied on a match (e.g., matching an entry in a block/
deny list or a revocation list), a permissible operation is denied.
At best, this can cause worse performance (e.g., a cache miss, or
forcing redundant authentication), and at worst can result in a
denial of service.
A "false negative" results when two identifiers that in reality refer
to the same thing compare as if they were different, and the effects
are the reverse of those for false positives. That is, when
privilege is granted on a match, the result is at best worse
performance and at worst a denial of service; when privilege is
denied on a match, elevation of privilege results.
Figure 3 summarizes these effects.
| "Grant on match" | "Deny on match"
---------------+------------------------+-----------------------
False positive | Elevation of privilege | Denial of service
---------------+------------------------+-----------------------
False negative | Denial of service | Elevation of privilege
---------------+------------------------+-----------------------
Effect of False Positives/Negatives
Figure 3
Elevation of privilege is almost always seen as far worse than denial
of service. Hence, for URIs for example, Section 6.1 of [RFC3986]
states: "comparison methods are designed to minimize false negatives
while strictly avoiding false positives".
Thus URIs were defined with a "grant privilege on match" paradigm in
mind, where it is critical to prevent elevation of privilege while
minimizing denial of service. Using URIs in a "deny privilege on
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match" system can thus be problematic.
2.3. Hypothetical Example
In this example, both security principals and resources are
identified using URIs. Foo Corp has paid example.com for access to
the Stuff service. Foo Corp allows its employees to create accounts
on the Stuff service. Alice gets the account
"http://example.com/Stuff/FooCorp/alice" and Bob gets
"http://example.com/Stuff/FooCorp/bob". It turns out, however, that
Foo Corp's URI canonicalizer includes URI fragment components in
comparisons whereas example.com's does not, and Foo Corp does not
disallow the # character in the account name. So Chuck, who is a
malicious employee of Foo Corp, asks to create an account at
example.com with the name alice#stuff. Foo Corp's URI logic checks
its records for accounts it has created with stuff and sees that
there is no account with the name alice#stuff. Hence, in its
records, it associates the account alice#stuff with Chuck and will
only issue tokens good for use with
"http://example.com/Stuff/FooCorp/alice#stuff" to Chuck.
Chuck, the attacker, goes to a security token service at Foo Corp and
asks for a security token good for
"http://example.com/Stuff/FooCorp/alice#stuff". Foo Corp issues the
token since Chuck is the legitimate owner (in Foo Corp's view) of the
alice#stuff account. Chuck then submits the security token in a
request to "http://example.com/Stuff/FooCorp/alice".
But example.com uses a URI canonicalizer that, for the purposes of
checking equality, ignores fragments. So when example.com looks in
the security token to see if the requester has permission from Foo
Corp to access the given account it successfully matches the URI in
the security token, "http://example.com/Stuff/FooCorp/alice#stuff",
with the requested resource name
"http://example.com/Stuff/FooCorp/alice".
Leveraging the inconsistencies in the canonicalizers used by Foo Corp
and example.com, Chuck is able to successfully launch an elevation of
privilege attack and access Alice's resource.
Furthermore, consider an attacker using a similar corporation such as
"foocorp" (or any variation containing a non-ASCII character that
some humans might expect to represent the same corporation). If the
resource holder treats them as different, but the security token
service treats them as the same, then again elevation of privilege
can occur.
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3. Common Identifiers
In this section, we walk through a number of common types of
identifiers and discuss various issues related to comparison that may
affect security whenever they are used to identify security
principals or resources. These examples illustrate common patterns
that may arise with other types of identifiers.
3.1. Hostnames
Hostnames (composed of dot-separated labels) are commonly used either
directly as identifiers, or as components in identifiers such as in
URIs and email addresses. Another example is in [RFC5280], sections
7.2 and 7.3 (and updated in section 3 of
[I-D.ietf-pkix-rfc5280-clarifications]), which specify use in
certificates.
In this section we discuss a number of issues in comparing strings
that appear to be some form of hostname.
Section 3 of [RFC6055] discusses the differences between a "hostname"
vs. a "DNS name", where the former is a subset of the latter by using
a restricted set of characters. If one canonicalizer uses the "DNS
name" definition whereas another uses a "hostname" definition, a name
might be valid in the former but invalid in the latter. As long as
invalid identifiers are denied privilege, this difference will not
result in elevation of privilege.
[IAB1123] briefly discusses issues with the ambiguity around whether
a label will be "alphabetic", including among other issues, how
"alphabetic" should be interpreted in an internationalized
environment, and whether a hostname can be interpreted as an IP
address. We explore this last issue in more detail below.
3.1.1. IPv4 Literals
[RFC0952] defined an entry in the "Internet host table" as follows:
A "name" (Net, Host, Gateway, or Domain name) is a text string up
to 24 characters drawn from the alphabet (A-Z), digits (0-9),
minus sign (-), and period (.). Note that periods are only
allowed when they serve to delimit components of "domain style
names". [...] No blank or space characters are permitted as part
of a name. No distinction is made between upper and lower case.
The first character must be an alpha character. The last
character must not be a minus sign or period. [...] Single
character names or nicknames are not allowed.
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[RFC1123] section 2.1 then updates the definition with:
The syntax of a legal Internet host name was specified in RFC-952
[DNS:4]. One aspect of host name syntax is hereby changed: the
restriction on the first character is relaxed to allow either a
letter or a digit. Host software MUST support this more liberal
syntax.
and
Whenever a user inputs the identity of an Internet host, it SHOULD
be possible to enter either (1) a host domain name or (2) an IP
address in dotted-decimal ("#.#.#.#") form. The host SHOULD check
the string syntactically for a dotted-decimal number before
looking it up in the Domain Name System.
and
This last requirement is not intended to specify the complete
syntactic form for entering a dotted-decimal host number; that is
considered to be a user-interface issue.
In specifying the inet_addr() API, the POSIX standard [IEEE-1003.1]
defines "IPv4 dotted decimal notation" as allowing not only strings
of the form "10.0.1.2", but also allows octal and hexadecimal, and
addresses with less than four parts. For example, "10.0.258",
"0xA000001", and "012.0x102" all represent the same IPv4 address in
standard "IPv4 dotted decimal" notation. We will refer to this as
the "loose" syntax of an IPv4 address literal.
In section 6.1 of [RFC3493] getaddrinfo() is defined to support the
same (loose) syntax as inet_addr():
If the specified address family is AF_INET or AF_UNSPEC, address
strings using Internet standard dot notation as specified in
inet_addr() are valid.
In contrast, section 6.3 of the same RFC states, specifying
inet_pton():
If the af argument of inet_pton() is AF_INET, the src string shall
be in the standard IPv4 dotted-decimal form: ddd.ddd.ddd.ddd where
"ddd" is a one to three digit decimal number between 0 and 255.
The inet_pton() function does not accept other formats (such as
the octal numbers, hexadecimal numbers, and fewer than four
numbers that inet_addr() accepts).
As shown above, inet_pton() uses what we will refer to as the
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"strict" form of an IPv4 address literal. Some platforms also use
the strict form with getaddrinfo() when the AI_NUMERICHOST flag is
passed to it.
Both the strict and loose forms are standard forms, and hence a
protocol specification is still ambiguous if it simply defines a
string to be in the "standard IPv4 dotted decimal form". And, as a
result of these differences, names such as "10.11.12" are ambiguous
as to whether they are an IP address or a hostname, and even
"10.11.12.13" can be ambiguous because of the "SHOULD" in RFC 1123
above making it optional whether to treat it as an address or a name.
Protocols and data formats that can use addresses in string form for
security purposes need to resolve these ambiguities. For example,
for the host component of URIs, section 3.2.2 of [RFC3986] resolves
the first ambiguity by only allowing the strict form, and the second
ambiguity by specifying that it is considered an IPv4 address
literal. New protocols and data formats should similarly consider
using the strict form rather than the loose form in order to better
match user expectations.
Thus, whereas (binary) IPv4 addresses are Absolute identifiers, IPv4
address literals are at best Definite identifiers, and often turn out
to be Indefinite identifiers.
Furthermore, when strings can contain non-ASCII characters, they can
contain other characters that may look like dots or digits to a human
viewing and/or entering the identifier, especially to one who might
expect digits to appear in his or her native script.
3.1.2. IPv6 Literals
IPv6 addresses similarly have a wide variety of alternate but
semantically identical string representations, as defined in section
2.2 of [RFC4291] and section 2 of [I-D.ietf-6man-uri-zoneid]. As
discussed in section 3.2.5 of [RFC5952], this fact causes problems in
security contexts if comparison (such as in X.509 certificates), is
done between strings rather than between the binary representations
of addresses.
[RFC5952] recently specified a recommended canonical string format as
an attempt to solve this problem, but it may not be ubiquitously
supported at present. And, when strings can contain non-ASCII
characters, the same issues (and more, since hexadecimal and colons
are allowed) arise as with IPv4 literals.
Whereas (binary) IPv6 addresses are Absolute identifiers, IPv6
address literals are Definite identifiers, since string-to-address
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conversion for IPv6 address literals is unambiguous.
3.1.3. Internationalization
The IETF policy on character sets and languages [RFC2277] requires
support for UTF-8 in protocols, and as a result many protocols now do
support non-ASCII characters. When a hostname is sent in a UTF-8
field, there are a number of ways it may be encoded. For example,
hostname labels might be encoded directly in UTF-8, or might first be
Punycode-encoded [RFC3492] or even percent-encoded from UTF-8.
For example, in URIs, [RFC3986] section 3.2.2 specifically allows for
the use of percent-encoded UTF-8 characters in the hostname, as well
as the use of IDNA encoding [RFC3490] using the Punycode algorithm.
Percent-encoding is unambiguous for hostnames since the percent
character cannot appear in the strict definition of a "hostname",
though it can appear in a DNS name.
Punycode-encoded labels (or "A-labels") on the other hand can be
ambiguous if hosts are actually allowed to be named with a name
starting with "xn--", and false positives can result. While this may
be extremely unlikely for normal scenarios, it nevertheless provides
a possible vector for an attacker.
A hostname comparator thus needs to decide whether a Punycode-encoded
label should or should not be considered a valid hostname label, and
if so, then whether it should match a label encoded in some other
form such as a percent-encoded Unicode label (U-label).
For example, Section 3 of "Transport Layer Security (TLS) Extensions"
[RFC6066], states:
"HostName" contains the fully qualified DNS hostname of the
server, as understood by the client. The hostname is represented
as a byte string using ASCII encoding without a trailing dot.
This allows the support of internationalized domain names through
the use of A-labels defined in [RFC5890]. DNS hostnames are case-
insensitive. The algorithm to compare hostnames is described in
[RFC5890], Section 2.3.2.4.
For some additional discussion of security issues that arise with
internationalization, see [TR36].
3.1.4. Resolution for comparison
Some systems (specifically Java URLs [JAVAURL]) use the rule that if
two hostnames resolve to the same IP address(es) then the hostnames
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are considered equal. That is, the canonicalization algorithm
involves name resolution with an IP address being the canonical form.
For example, if resolution was done via DNS, and DNS contained:
example.com. IN A 10.0.0.6
example.net. CNAME example.com.
example.org. IN A 10.0.0.6
then the algorithm might treat all three names as equal, even though
the third name might refer to a different entity.
With the introduction of dynamic IP addresses, private IP addresses,
multiple IP addresses per name, multiple address families (e.g., IPv4
vs. IPv6), devices that roam to new locations, commonly deployed DNS
tricks that result in the answer depending on factors such as the
requester's location and the load on the server whose address is
returned, etc., this method of comparison cannot be relied upon.
There is no guarantee that two names for the same host will resolve
the name to the same IP addresses, nor that the addresses resolved
refer to the same entity such as when the names resolve to private IP
addresses, nor even that the system has connectivity (and the
willingness to wait for the delay) to resolve names at the time the
answer is needed.
In addition, a comparison mechanism that relies on the ability to
resolve identifiers such as hostnames to other identifies such as IP
addresses leaks information about security decisions to outsiders if
these queries are publicly observable.
3.2. Ports and Service Names
Port numbers and service names are discussed in depth in [RFC6335].
Historically, there were port numbers, service names used in SRV
records, and mnemonic identifiers for assigned port numbers (known as
port "keywords" at [IANA-PORT]). The latter two are now unified, and
various protocols use one or more of these types in strings. For
example, the common syntax used by many URI schemes allows port
numbers but not service names. Some implementations of the
getaddrinfo() API support strings that can be either port numbers or
port keywords (but not service names).
For protocols that use service names that must be resolved, the
issues are the same as those for resolution of addresses in
Section 3.1.4. In addition, Section 5.1 of [RFC6335] clarifies that
service names/port keywords must contain at least one letter. This
prevents confusion with port numbers in strings where both are
allowed.
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3.3. URIs
This section looks at issues related to using URIs for security
purposes. For example, [RFC5280], section 7.4, specifies comparison
of URIs in certificates. Examples of URIs in security token-based
access control systems include WS-*, SAML-P and OAuth WRAP. In such
systems, a variety of participants in the security infrastructure are
identified by URIs. For example, requesters of security tokens are
sometimes identified with URIs. The issuers of security tokens and
the relying parties who are intended to consume security tokens are
frequently identified by URIs. Claims in security tokens often have
their types defined using URIs and the values of the claims can also
be URIs.
Also, when a URI is embedded in plain text (e.g., an email message),
there is an additional concern because there is no termination
criterion for a URI. For example, consider
http://unicode.org/cldr/utility/list-unicodeset.jsp?a=a&g=gc.
Some applications that detect URIs will stop before the first '.' in
the path, while others go to last '.', and yet others may stop at the
';'. As another point of comparison, Section 2.37 of [EE] (a
standard for history citations) specifies the use of a space after a
URI and before the punctuation.
URIs are defined with multiple components, each of which has its own
rules. We cover each in turn below. However, it is also important
to note that there exist multiple comparison algorithms. [RFC3986]
section 6.2 states:
A variety of methods are used in practice to test URI equivalence.
These methods fall into a range, distinguished by the amount of
processing required and the degree to which the probability of
false negatives is reduced. As noted above, false negatives
cannot be eliminated. In practice, their probability can be
reduced, but this reduction requires more processing and is not
cost-effective for all applications.
If this range of comparison practices is considered as a ladder,
the following discussion will climb the ladder, starting with
practices that are cheap but have a relatively higher chance of
producing false negatives, and proceeding to those that have
higher computational cost and lower risk of false negatives.
The ladder approach has both pros and cons. On the pro side, it
allows some uses to optimize for security, and other uses to optimize
for cost, thus allowing URIs to be applicable to a wide range of
uses. A disadvantage is that when different approaches are taken by
different components in the same system using the same identifiers,
the inconsistencies can result in security issues.
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3.3.1. Scheme component
[RFC3986] defines URI schemes as being case-insensitive ASCII and in
section 6.2.2.1 specifies that scheme names should be normalized to
lower-case characters.
New schemes can be defined over time. In general two URIs with an
unrecognized scheme cannot be safely compared, however. This is
because the canonicalization and comparison rules for the other
components may vary by scheme. For example, a new URI scheme might
have a default port of X, and without that knowledge, a comparison
algorithm cannot know whether "example.com" and "example.com:X"
should be considered to match in the authority component. Hence for
security purposes, it is safest for unrecognized schemes to be
treated as invalid identifiers. However, if the URIs are only used
with a "grant access on match" paradigm then unrecognized schemes can
be supported by doing a generic case-sensitive comparison, at the
expense of some false negatives.
3.3.2. Authority component
The authority component is scheme-specific, but many schemes follow a
common syntax that allows for userinfo, host, and port.
3.3.2.1. Host
Section 3.1 discussed issues with hostnames in general. In addition,
[RFC3986] section 3.2.2 allows future changes using the IPvFuture
production. As with IPv4 and IPv6 literals, IPvFuture formats may
have issues with multiple semantically identical string
representations, and may also be semantically identical to an IPv4 or
IPv6 address. As such, false negatives may be common if IPvFuture is
used.
3.3.2.2. Port
See discussion in Section 3.2.
3.3.2.3. Userinfo
[RFC3986] defines the userinfo production that allows arbitrary data
about the user of the URI to be placed before '@' signs in URIs. For
example: "http://alice:bob:chuck@example.com/bar" has the value
"alice:bob:chuck" as its userinfo. When comparing URIs in a security
context, one must decide whether to treat the userinfo as being
significant or not. Some URI comparison services for example treat
"http://alice:ick@example.com" and "http://example.com" as being
equal.
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When the userinfo is treated as being significant, it has additional
considerations (e.g., whether it is case-sensitive or not) which we
cover in Section 3.4.
3.3.3. Path component
[RFC3986] supports the use of path segment values such as "./" or
"../" for relative URIs. Strictly speaking, including such path
segment values in a fully qualified URI is syntactically illegal but
[RFC3986] section 4.1 nevertheless defines an algorithm to remove
them.
Unless a scheme states otherwise, the path component is defined to be
case-sensitive. However, if the resource is stored and accessed
using a filesystem using case-insensitive paths, there will be many
paths that refer to the same resource. As such, false negatives can
be common in this case.
3.3.4. Query component
There is the question as to whether "http://example.com/foo",
"http://example.com/foo?", and "http://example.com/foo?bar" are each
considered equal or different.
Similarly, it is unspecified whether the order of values matters.
For example, should "http://example.com/blah?ick=bick&foo=bar" be
considered equal to "http://example.com/blah?foo=bar&ick=bick"? And
if a domain name is permitted to appear in a query component (e.g.,
in a reference to another URI), the same issues in Section 3.1 apply.
3.3.5. Fragment component
Some URI formats include fragment identifiers. These are typically
handles to locations within a resource and are used for local
reference. A classic example is the use of fragments in HTTP URIs
where a URI of the form "http://example.com/blah.html#ick" means
retrieve the resource "http://example.com/blah.html" and, once it has
arrived locally, find the HTML anchor named ick and display that.
So, for example, when a user clicks on the link
"http://example.com/blah.html#baz" a browser will check its cache by
doing a URI comparison for "http://example.com/blah.html" and, if the
resource is present in the cache, a match is declared.
Hence comparisons for security purposes typically ignore the fragment
component and treat all fragments as equal to the full resource.
However, if one were actually trying to compare the piece of a
resource that was identified by the fragment identifier, ignoring it
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would result in potential false positives. For example, there is at
least one well known site today (Twitter) that requires the fragment
component in order to uniquely identify a user profile.
3.3.6. Resolution for comparison
As with Section 3.1.4 for hostnames, it may be tempting to define a
URI comparison algorithm based on whether they resolve to the same
content. Similar problems exist, however, including content that
dynamically changes over time or based on factors such as the
requester's location, potential lack of external connectivity at the
time/place comparison is done, potentially undesirable delay
introduced, etc.
In addition, as noted in Section 3.1.4, resolution leaks information
about security decisions to outsiders if the queries are publicly
observable.
3.4. Email Address-like Identifiers
Section 3.4.1 of [RFC5322] defines the syntax of an email address-
like identifier, and Section 3.2 of [RFC6532] updates it to support
internationalization. [RFC5280], section 7.5, further discusses the
use of internationalized email addresses in certificates.
[RFC6532] use in certificates points to [RFC6530], where Section 13
of that document contains a discussion of many issues resulting from
internationalization.
Email address-like identifiers have a local part and a domain part.
The issues with the domain part are essentially the same as with
hostnames, covered earlier.
The local part is left for each domain to define. People quite
commonly use email addresses as usernames with web sites such as
banks or shopping sites, but the site doesn't know whether
foo@example.com is the same person as FOO@example.com. Thus email-
like identifiers are typically Indefinite identifiers.
To avoid false positives, some security mechanisms (such as
[RFC5280]) compare the local part using an exact match. Hence, like
URIs, email address-like identifiers are designed for use in grant-
on-match security schemes, not in deny-on-match schemes.
4. General Internationalization Issues
In addition to the issues with hostnames discussed in Section 3.1.3,
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there are a number of internationalization issues that apply to many
types of Definite and Indefinite identifiers.
First, there is no DNS mechanism for identifying whether non-
identical strings would be seen by a human as being equivalent.
There are problematic examples even with ASCII (Basic Latin) strings
including regional spelling variations such as "color" and "colour"
and many non-English cases including partially-numeric strings in
Arabic script contexts, Chinese strings in Simplified and Traditional
forms, and so on. Attempts to produce such alternate forms
algorithmically could produce false positives and hence have an
adverse affect on security.
Second, some strings are visually confusable with others, and hence
if a security decision is made by a user based on visual inspection,
many opportunities for false positives exist. As such, using visual
inspection for security is unreliable. In addition to the security
issues, visual confusability also adversely affects the usability of
identifiers distributed via visual mediums. Similar issues can arise
with audible confusability when using audio (e.g., for radio
distribution, accessibility to the blind, etc.) in place of a visual
medium.
Determining whether a string is a valid identifier should typically
be done after, or as part of, canonicalization. Otherwise an
attacker might use the canonicalization algorithm to inject (e.g.,
via percent encoding, NFKC, or non-shortest-form UTF-8) delimiters
such as '@' in an email address-like identifier, or a '.' in a
hostname.
Any case-insensitive comparisons need to define how comparison is
done, since such comparisons may vary by locale of the endpoint. As
such, using case-insensitive comparisons in general often result in
identifiers being either Indefinite or, if the legal character set is
restricted (e.g., to ASCII), then Definite.
See also [WEBER] for a more visual discussion of many of these
issues.
Finally, the set of permitted characters and the canonical form of
the characters (and hence the canonicalization algorithm) sometimes
varies by protocol today, even when the intent is to use the same
identifier, such as when one protocol passes identifiers to the
other. See [I-D.ietf-precis-problem-statement] for further
discussion.
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5. General Scope Issues
Another issue arises when an identifier (e.g., "localhost",
"10.11.12.13", etc.) is not globally unique. [RFC3986] Section 1.1
states:
URIs have a global scope and are interpreted consistently
regardless of context, though the result of that interpretation
may be in relation to the end-user's context. For example,
"http://localhost/" has the same interpretation for every user of
that reference, even though the network interface corresponding to
"localhost" may be different for each end-user: interpretation is
independent of access.
Whenever a non-globally-unique identifier is passed to another entity
outside of the scope of uniqueness, it will refer to a different
resource, and can result in a false positive. This problem is often
addressed by using the identifier together with some other unique
identifier of the context. For example "alice" may uniquely identify
a user within a system, but must be used with "example.com" (as in
"alice@example.com") to uniquely identify the context outside of that
system.
It is also worth noting that non-globally-scoped IPv6 addresses can
be written with, or otherwise associated with, a "zone ID" to
identify the context (see [RFC4007] for more information). However,
zone IDs are only unique within a host, so they typically narrow,
rather than expand, the scope of uniqueness of the resulting
identifier.
6. Security Considerations
This entire document is about security considerations.
To minimize elevation of privilege issues, any system that requires
the ability to use both deny and allow operations within the same
identifier space, should avoid the use of Indefinite identifiers in
security comparisons.
To minimize future security risks, any new identifiers being designed
should specify an Absolute or Definite comparison algorithm, and if
extensibility is allowed (e.g., as new schemes in URIs allow) then
the comparison algorithm should remain invariant so that unrecognized
extensions can be compared. That is, security risks can be reduced
by specifying the comparison algorithm, making sure to resolve any
ambiguities pointed out in this document (e.g., "standard dotted
decimal").
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Some issues (such as unrecognized extensions) can be mitigated by
treating such identifiers as invalid. Validity checking of
identifiers is further discussed in [RFC3696].
Perhaps the hardest issues arise when multiple protocols are used
together, such as in the figure in Section 2, where the two protocols
are defined or implemented using different comparison algorithms.
When constructing an architecture that uses multiple such protocols,
designers should pay attention to any differences in comparison
algorithms among the protocols, in order to fully understand the
security risks. An area for future work is how to deal with such
security risks in current systems.
7. Acknowledgements
Yaron Goland contributed to the discussion on URIs. Patrick
Faltstrom contributed to the background on identifiers. John Klensin
contributed text in a number of different sections. Additional
helpful feedback and suggestions came from Bernard Aboba, Leslie
Daigle, Mark Davis, Russ Housley, Magnus Nystrom, and Chris Weber.
8. IANA Considerations
This document requires no actions by the IANA.
9. Informative References
[EE] Mills, E., "Evidence Explained: Citing History Sources
from Artifacts to Cyberspace", 2007.
[I-D.ietf-6man-uri-zoneid]
Carpenter, B. and R. Hinden, "Representing IPv6 Zone
Identifiers in Address Literals and Uniform Resource
Identifiers", draft-ietf-6man-uri-zoneid-02 (work in
progress), July 2012.
[I-D.ietf-pkix-rfc5280-clarifications]
Yee, P., "Updates to the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", draft-ietf-pkix-rfc5280-clarifications-05
(work in progress), June 2012.
[I-D.ietf-precis-problem-statement]
Blanchet, M. and A. Sullivan, "Stringprep Revision and
PRECIS Problem Statement",
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draft-ietf-precis-problem-statement-06 (work in progress),
July 2012.
[IAB1123] IAB, "The interpretation of rules in the ICANN gTLD
Applicant Guidebook", February 2012, <http://www.iab.org/
documents/correspondence-reports-documents/2012-2/
iab-statement-the-interpretation-of-rules-in-the-icann-
gtld-applicant-guidebook>.
[IANA-PORT]
IANA, "PORT NUMBERS", June 2011,
<http://www.iana.org/assignments/port-numbers>.
[IEEE-1003.1]
IEEE and The Open Group, "The Open Group Base
Specifications, Issue 6 IEEE Std 1003.1, 2004 Edition",
IEEE Std 1003.1, 2004.
[JAVAURL] Oracle, "Class URL, Java(TM) Platform, Standard Ed. 7",
2011, <http://docs.oracle.com/javase/7/docs/api/java/net/
URL.html>.
[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, October 1985.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2277] Alvestrand, H., "IETF Policy on Character Sets and
Languages", BCP 18, RFC 2277, January 1998.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003.
[RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
Stevens, "Basic Socket Interface Extensions for IPv6",
RFC 3493, February 2003.
[RFC3696] Klensin, J., "Application Techniques for Checking and
Transformation of Names", RFC 3696, February 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
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RFC 3986, January 2005.
[RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
March 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 2010.
[RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055,
February 2011.
[RFC6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, August 2011.
[RFC6530] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 6530, February 2012.
[RFC6532] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012.
[TR36] Unicode Consortium, "Unicode Security Considerations",
Unicode Technical Report 36, August 2004.
[WEBER] Weber, C., "Attacking Software Globalization", March 2010,
<http://www.lookout.net/files/
Chris_Weber_Character%20Transformations%20v1.7_IUC33.pdf>.
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Author's Address
Dave Thaler (editor)
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1 425 703 8835
Email: dthaler@microsoft.com
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