Threat Model for BGP Path Security
draft-ietf-sidr-bgpsec-threats-02

Secure Inter-Domain Routing                                      S. Kent
Internet-Draft                                                    A. Chi
Intended status: Standards Track                                     BBN
Expires: August 25, 2012                               February 22, 2012

                   Threat Model for BGP Path Security
                   draft-ietf-sidr-bgpsec-threats-02

Abstract

   This document describes a threat model for BGP path security
   (BGPSEC).  It assumes the context established by the SIDR WG charter,
   as of April 19, 2011.  The charter established two goals for the SIDR
   work:

   o  Enabling an AS to verify the authorization of an origin AS to
      originate a specified set of prefixes

   o  Enabling an AS to verify that the AS-PATH represented in a route
      matches the path travelled by the NLRI for the route

   The charter further mandates that SIDR build upon the Resource Public
   Key Infrastructure (RPKI), the first product of the WG.  Consistent
   with the charter, this threat model includes an analysis of the RPKI,
   and focuses on the ability of an AS to verify the authenticity of the
   AS path info received in a BGP update.

   The model assumes that BGP path security is achieved through the
   application of digital signatures to AS_Path Info.  The document
   characterizes classes of potential adversaries that are considered to
   be threats, and examines classes of attacks that might be launched
   against BGPSEC.  It concludes with brief discussion of residual
   vulnerabilities.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 25, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Threat Characterization  . . . . . . . . . . . . . . . . . . .  9
   4.  Attack Characterization  . . . . . . . . . . . . . . . . . . . 11
     4.1.  Active wiretapping of links between routers  . . . . . . . 11
     4.2.  Attacks on a BGP router  . . . . . . . . . . . . . . . . . 11
     4.3.  Attacks on network operator management computers
           (non-CA computers) . . . . . . . . . . . . . . . . . . . . 13
     4.4.  Attacks on a repository publication point  . . . . . . . . 14
     4.5.  Attacks on an RPKI CA  . . . . . . . . . . . . . . . . . . 16
   5.  Residual Vulnerabilities . . . . . . . . . . . . . . . . . . . 19
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 22
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 24
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 24
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25

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1.  Introduction

   This document describes the security context in which BGPSEC is
   intended to operate.  It discusses classes of potential adversaries
   that are considered to be threats, and classes of attacks that might
   be launched against BGPSEC.  Because BGPSEC depends on the Resource
   Public Key Infrastructure (RPKI) [RFC6480], threats and attacks
   against the RPKI are included.  This model also takes into
   consideration classes of attacks that are enabled by the use of
   BGPSEC (based on the current BGPSEC design.)

   The motivation for developing BGPSEC, i.e., residual security
   concerns for BGP, is well described in several documents, including
   "BGP Security Vulnerabilities Analysis" [RFC4272] and "Design and
   Analysis of the Secure Border Gateway Protocol (S-BGP)" [Kent2000].
   All of these papers note that BGP does not include mechanisms that
   allow an Autonomous System (AS) to verify the legitimacy and
   authenticity of BGP route advertisements.  (BGP now mandates support
   for mechanisms to secure peer-peer communication, i.e., for the links
   that connect BGP routers.  There are several secure protocol options
   to addresses this security concern, e.g., IPsec [RFC4301] and TCP-AO
   [RFC5925].  This document briefly notes the need to address this
   aspect of BGP security, but focuses on application layer BGP security
   issues that are addressed by BGPSEC.)

   RFC 4272 [RFC4272] succinctly notes:

      BGP speakers themselves can inject bogus routing information,
      either by masquerading as any other legitimate BGP speaker, or by
      distributing unauthorized routing information as themselves.
      Historically, misconfigured and faulty routers have been
      responsible for widespread disruptions in the Internet.  The
      legitimate BGP peers have the context and information to produce
      believable, yet bogus, routing information, and therefore have the
      opportunity to cause great damage.  The cryptographic protections
      of [TCPMD5] and operational protections cannot exclude the bogus
      information arising from a legitimate peer.  The risk of
      disruptions caused by legitimate BGP speakers is real and cannot
      be ignored.

   BGPSEC is intended to address the concerns cited above, to provide
   significantly improved path security, building upon the secure route
   origination foundation offered by use of the RPKI.  Specifically, the
   RPKI enables relying parties (RPs) to determine if the origin AS for
   a path was authorized to advertise the prefix contained in a BGP
   update message.  This security feature is enabled by the use of two
   types of digitally signed data: a PKI [RFC6487] that associates one
   or more prefixes with the public key(s) of an address space holder,

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   and Route Origination Authorizations (ROAs) [RFC6482] that allows a
   prefix holder to specify the AS(es) that are authorized to originate
   routes for a prefix.

   The security model adopted for BGPSEC does not assume an "oracle"
   that can see all of the BGP inputs and outputs associated with every
   AS or every BGP router.  Instead, the model is based on a local
   notion of what constitutes legitimate, authorized behavior by the BGP
   routers associated with an AS.  This is an AS-centric model of secure
   operation, consistent with the AS-centric model that BGP employs for
   routing.  This model forms the basis for the discussion that follows.

   This document begins with a brief set of definitions relevant to the
   subsequent sections.  It then discusses classes of adversaries that
   are perceived as viable threats against routing in the public
   Internet.  It continues to explore a range of attacks that might be
   effected by these adversaries, against both path security and the
   infrastructure upon which BGPSEC relies.  It concludes with a brief
   review of residual vulnerabilities.

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2.  Terminology

   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].

   The following security and routing terminology definitions are
   employed in this document.

   Adversary - An adversary is an entity (e.g., a person or an
   organization) perceived as malicious, relative to the security policy
   of a system.  The decision to characterize an entity as an adversary
   is made by those responsible for the security of a system.  Often one
   describes classes of adversaries with similar capabilities or
   motivations, rather than specific individuals or organizations.

   Attack - An attack is an action that attempts to violate the security
   policy of a system, e.g., by exploiting a vulnerability.  There is
   often a many to one mapping of attacks to vulnerabilities, because
   many different attacks may be used to exploit a vulnerability.

   Autonomous System (AS) - An AS is a set of one or more IP networks
   operated by a single administrative entity.

   AS Number (ASN) - An ASN is a 2 or 4 byte number issued by a registry
   to identify an AS in BGP.

   Certification Authority (CA) - An entity that issues digital
   certificates (e.g., X.509 certificates) and vouches for the binding
   between the data items in a certificate.

   Countermeasure - A countermeasure is a procedure or technique that
   thwarts an attack, preventing it from being successful.  Often
   countermeasures are specific to attacks or classes of attacks.

   Border Gateway Protocol (BGP) - A path vector protocol used to convey
   "reachability" information among autonomous systems, in support of
   inter-domain routing.

   False (Route) Origination - If a network operator originates a route
   for a prefix that the network operator does not hold (and that it has
   not been authorized to originate by the prefix holder, this is termed
   false route origination.

   Internet Service Provider (ISP) - An organization managing (and,
   typically, selling,) Internet services to other organizations or
   individuals.

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   Internet Number Resources (INRs) - IPv4 or IPv6 address space and
   ASNs

   Internet Registry - An organization that manages the allocation or
   distribution of INRs.  This encompasses the Internet Assigned Number
   Authority (IANA), Regional Internet Registries (RIRs), National
   Internet Registries (NIRs), and Local Internet Registries (LIRs,
   network operators).

   Man in the Middle (MITM) - A MITM is an entity that is able to
   examine and modify traffic between two (or more) parties on a
   communication path.

   NOC (Network Operations Center) - A network operator employs a set
   equipment and a staff to manage a network, typically on a 24/7 basis.
   The equipment and staff are often referred to as the NOC for the
   network.

   Prefix - A prefix is an IP address and a mask used to specify a set
   of addresses that are grouped together for purposes of routing.

   Public Key Infrastructure (PKI) - A PKI is a collection of hardware,
   software, people, policies, and procedures used to create, manage,
   distribute, store, and revoke digital certificates.

   Relying Parties (RPs) - An RP is an entity that makes use of signed
   products from a PKI, i.e., relies on signed data that is verified
   using certificates, and CRLs from a PKI.

   RPKI Repository System - The RPKI repository system consists of a
   distributed set of loosely synchronized databases.

   Resource PKI (RPKI) - A PKI operated by the entities that manage
   INRs, and that issues X509 certificates (and CRLs) that attest to the
   holdings of INRs.

   RPKI Signed Object - An RPKI signed object is a Cryptographic Message
   Syntax (CMS)-encapsulated data object complying with the format and
   semantics defined in [RFC6488].

   Route - In the Internet, a route is a prefix and an associated
   sequence of ASNs that indicates a path via which traffic destined for
   the prefix can be directed.  (The route includes the origin AS.)

   Route leak - A route leak is said to occur when AS-A advertises
   routes that it has received from an AS-B to AS-A's neighbors, but
   AS-A is not viewed as a transit provider for the prefixes in the
   route.

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   Threat - A threat is a motivated, capable adversary.  An adversary
   that is not motivated to launch an attack is not a threat.  An
   adversary that is motivated but not capable of launching an attack
   also is not a threat.

   Vulnerability - A vulnerability is a flaw or weakness in a system's
   design, implementation, or operation and management that could be
   exploited to violate the security policy of a system.

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3.  Threat Characterization

   The following classes of threats are addressed in this document.

   Network Operators - A network operator may be a threat.  A network
   operator may be motivated to cause BGP routers it controls to emit
   update messages with inaccurate routing info, e.g. to cause traffic
   to flow via paths that are economically advantageous for the
   operator.  Such updates might cause traffic to flow via paths that
   would otherwise be rejected as less advantageous by other network
   operators.  Because a network operator controls the BGP routers in
   its network, it is in a position to modify their operation in
   arbitrary ways.  Routers managed by a network operator are vehicles
   for mounting MITM attacks on both control and data plane traffic.  If
   a network operator participates in the RPKI, it will have at least CA
   resource certificate and may be able to generate an arbitrary number
   of subordinate CA certificates and ROAs.  It will be authorized to
   populate (and may even host) its own repository publication point.
   If it implements BGPSEC, it will have the ability to issue
   certificates for its routers, and to sign updates in a fashion that
   will be recognized by BGPSEC-enabled neighbors.

   Hackers - Hackers are considered a threat.  A hacker might assume
   control of network management computers and routers controlled by
   network operators, including network operators that implement BGPSEC.
   In such cases, hackers would be able to act as a rogue network
   operators (see above).  It is assumed that hackers generally do not
   have the capability to effect MITM attacks on most links between
   networks (links used to transmit BGP and subscriber traffic).  A
   hacker might be recruited, without his/her knowledge, by criminals or
   by nations, to act on their behalf.  Hackers may be motivated by a
   desire for "bragging rights" or for profit.

   Criminals - Criminals may be a threat.  Criminals might persuade (via
   threats or extortion) a network operator to act as a rogue network
   operator (see above), and thus be able to effect a wide range of
   attacks.  Criminals might persuade the staff of a telecommunications
   provider to enable MITM attacks on links between routers.
   Motivations for criminals may include the ability to extort money
   from network operators or network operator clients, e.g., by
   adversely affecting routing for these network operators or their
   clients.  Criminals also may wish to manipulate routing to conceal
   the sources of spam, DoS attacks, or other criminal activities.

   Registries - Any registry in the RPKI could be a threat.  Staff at
   the registry are capable of manipulating repository content or
   mismanaging the RPKI certificates that they issue.  These actions
   could adversely affect a network operator or a client of a network

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   operator.  The staff could be motivated to do this based on political
   pressure from the nation in which the registry operates (see below)
   or due to criminal influence (see above).

   Nations - A nation may be a threat.  A nation may control one or more
   network operators that operate in the nation, and thus can cause them
   to act as rogue network operators.  A nation may have a technical
   active wiretapping capability (e.g., within its territory) that
   enables it to effect MITM attacks on inter-network traffic.  (This
   capability may be facilitated by control or influence over a
   telecommunications provider operating within the nation.)  It may
   have an ability to attack and take control of routers or management
   network computers of network operators in other countries.  A nation
   may control a registry (e.g., an RIR) that operates within its
   territory, and might force that registry to act in a rogue capacity.
   National threat motivations include the desire to control the flow of
   traffic to/from the nation or to divert traffic destined for other
   nations (for passive or active wiretapping, including DoS).

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4.  Attack Characterization

   This section describes classes of attacks that may be effected
   against Internet routing (relative to the context described in
   Section 1).  Attacks are classified based on the target of the
   attack, as an element of the routing system, or the routing security
   infrastructure on which BGPSEC relies.  In general, attacks of
   interest are ones that attempt to violate the integrity or
   authenticity of BGP traffic, or which violate the authorizations
   associated with entities participating in the RPKI.  Attacks that
   violate the implied confidentiality of routing traffic are not
   considered significant (see Section 4.1 below).

4.1.  Active wiretapping of links between routers

   An adversary may attack the links that connect BGP routers.  Passive
   attacks are not considered, because it is assumed that most of the
   info carried by BGP will otherwise be accessible to adversaries.
   Several classes of adversaries are assumed to be capable of MITM
   effecting attacks against the control plane traffic.  MITM attacks
   may be directed against BGP, BGPSEC, or against TCP or IP.  Such
   attacks include replay of selected BGP messages, selective
   modification of BGP messages, and DoS attacks against BGP routers.

4.2.  Attacks on a BGP router

   An adversary may attack a BGP router, whether it implements BGPSEC or
   not.  Any adversary that controls routers legitimately, or that can
   assume control of a router, is assumed to be able to effect the types
   of attacks described below.  Note that any router behavior that can
   be ascribed to a local routing policy decision is not considered to
   be an attack.  This is because such behavior could be explained as a
   result of local policy settings, and thus is beyond the scope of what
   BGPSEC can detect as unauthorized behavior.  Thus, for example, a
   router may fail to propagate some or all route withdrawals or effect
   "route leaks".  (These behaviors are not precluded by the
   specification for BGP, and might be the result of a local policy that
   is not publicly disclosed.  As a result, they are not considered
   attacks.  See Section 5 for additional discussion.)

   Attacks on a router are active wiretapping attacks (in the most
   general sense) that manipulate (forge, tamper with, or suppress) data
   contained in BGP updates.  The list below illustrates attacks of this
   type.

      AS Insertion: A router might insert one or more ASNs, other than
      its own ASN, into an update message.  This violates the BGP spec
      and thus is considered an attack.

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      False (Route) Origination: A router might originate a route for a
      prefix, when the AS that the router represents is not authorized
      to originate routes for that prefix.  This is an attack.

      Secure Path Downgrade: A router might remove signatures from a
      BGPSEC update that it receives, when forwarding this update to a
      BGPSEC-enabled neighbor.  This behavior violates the BGPSEC spec
      and thus is considered an attack.

      Invalid Signature Insertion: A router might emit a signed update
      with a "bad" signature, i.e., a signature that cannot be validated
      by other BGPSEC routers.  This might be an intentional act, or it
      might occur due to use of a revoked or expired certificate, a
      computational error, or a syntactic error.  Such behavior violates
      the BGPSEC spec and thus is considered an attack.

      Stale Path Announcement: An announcement may be propagated with an
      origination signature segment that has expired.  This behavior
      violates the BGPSEC spec and is considered a possible replay
      attack.

      Premature Path Announcement Expiration: A router might emit a
      signed update with an origin expiry time that is very short.
      Unless the BGPSEC protocol specification mandates a minimum expiry
      time, this is not an attack.  However, if such a time is mandates,
      this behavior becomes an attack.  BGP speakers along a path
      generally cannot determine if an expiry time is "suspiciously
      short" since they cannot know how long a route may have been held
      by an earlier AS, prior to being released.  Thus only an immediate
      neighbor of a route originator could be expected to detect this
      type of attack.

      MITM Attack: A cryptographic key used for point-to-point security
      (e.g., TCP-AO, TLS, or IPsec) between two BGP routers might be
      compromised (e.g., by extraction from a router).  This would
      enable an adversary to effect MITM attacks on the link(s) where
      the key is used.  Use of specific security mechanisms to protect
      inter-router links between ASes is outside the scope of BGPSEC.

      Compromised Router Private Key: The private key associated with an
      RPKI EE certificate issued to a router might be compromised by an
      attack against the router.  An adversary with access to this key
      would be able to generate updates that appear to have passed
      through the AS that this router represents.  Such updates might be
      in injected on a link between the compromised router and its
      neighbors, if that link is accessible to the adversary.  If the
      adversary controls another network, it could use this key to forge
      signatures that appear to come from the AS or router(s) in

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      question, with some contraints.  So, for example, an adversary
      that controls another AS could use a compromised router key to
      issue signed routes that include the targeted AS/router, with
      limits.  (Neighbors of the adversary's AS ought not accept a route
      that purports to emanate directly from the targeted AS.  So, an
      adversary can take a legitimate route that passes through the
      compromised AS, add itself as the next hop, and then forward the
      resulting route to neighbors.)

      Replay Attack: A BGPSEC-protected update may be signed and
      announced, and later withdrawn.  An adversary controlling
      intermediate routers could fail to propagate the withdrawal, and
      instead re-announce (i.e., replay) a previous announcement (that
      has not yet expired).  BGP is already vulnerable to behavior of
      this sort; re-announcement cannot be characterized as an attack,
      under the assumptions upon which this mode is based (i.e., no
      oracle).

4.3.  Attacks on network operator management computers (non-CA
      computers)

   An adversary may choose to attack computers used by a network
   operator to manage its network, especially its routers.  Such attacks
   might be effected by an adversary that has compromised the security
   of these computers.  This might be effected via remote attacks,
   extortion of selected network operations staff, etc.  If an adversary
   compromises NOC computers, it can execute any management function
   that authorized network operations staff would have performed.  Thus
   the adversary could modify local routing policy to change
   preferences, to black-hole certain routes, etc.  This type of
   behavior cannot be externally detected as an attack.  Externally,
   this appears as a form of rogue network operator behavior.

   If a network operator participates in the RPKI, an adversary could
   manipulate the RP tools that extract data from the RPKI, causing the
   output of these tools to be corrupted in various ways.  For example,
   an attack of this sort could cause the network operator to view valid
   routes as not validated, which could alter its routing behavior.

   If an adversary invoked the tool used to manage the repository
   publication point for this network operator, it could delete any
   objects stored there (certificates, CRLs, manifests, ROAs, or
   subordinate CA certificates).  This could affect the routing status
   of entities that have allocations/assignments from this network
   operator (e.g., by deleting their CA certificates).

   An adversary could invoke the tool used to request certificate
   revocation, causing router certificates, ROAs, or subordinate CA

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   certificates to be revoked.  An attack of this sort could affect not
   only this network operator, but also any network operators that
   receive allocations/assignments from it, e.g., because their CA
   certificates were revoked.

   If a network operator is BGPSEC-enabled, an attack of this sort could
   cause the affected network operator to be viewed as not BGPSEC-
   enabled, possibly making routes it emits be less preferred by other
   network operators.

   If an adversary invoked a tool used to request ROAs, it could
   effectively re-allocate some of the prefixes allocated/assigned to
   the network operator (e.g., by modifying the origin AS in ROAs).
   This might cause other BGPSEC-enabled networks to view the affected
   network as no longer originating routes for these prefixes.  Multi-
   homed subscribers of this network operator who received an allocation
   from the network operator might find their traffic was now routed via
   other connections.

   If the network operator is BGPSEC-enabled, and the adversary invoked
   a tool used to request certificates, it could replace valid
   certificates for routers with ones that might be rejected by BGPSEC-
   enabled neighbors.

4.4.  Attacks on a repository publication point

   A critical element of the RPKI is the repository system.  An
   adversary might attack a repository, or a publication point within a
   repository, to adversely affect routing.

   This section considers only those attacks that can be launched by any
   adversary who controls a computer hosting one or more repository
   publication points, without access to the cryptographic keys needed
   to generate valid RPKI signed products.  Such attacks might be
   effected by an inside or an external threat.  Because all repository
   objects are digitally signed, attacks of this sort translate into DoS
   attacks against the RPKI RPs.  There are a few distinct forms of such
   attacks, as described below.

   Note first that the RPKI calls for RPs to cache the data they acquire
   and verify from the repository system.  Attacks that delete signed
   products, that insert products with "bad" signatures, that tamper
   with object signatures, or that replace newer objects with older
   (valid) ones, can be detected by RPs (with a few exceptions).  RPs
   are expected to make use of local caches.  If repository publication
   points are unavailable or the retrieved data is corrupted, an RP can
   revert to using the cached data.  This behavior helps insulate RPs
   from the immediate effects of DoS attacks on publication points.

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   Each RPKI data object has an associated date at which it expires, or
   is considered stale.  (Certificates expire, CRLs become stale.)  When
   an RP uses cached data it is a local decision how to deal with stale
   or expired data.  It is common in PKIs to make use of stale
   certificate revocation status data, when fresher data is not
   available.  Use of expired certificates is less common, although not
   unknown.  Each RP will decide, locally, whether to continue to make
   use of or ignore cached RPKI objects that are stale or expired.

   If an adversary inserts an object into a publication point, and the
   object has a "bad" signature, the object will not be accepted and
   used by RPs.

   If an adversary modifies any signed product at a publication point,
   the signature on the product will fail, causing RPs to not accept it.
   This is equivalent to deleting the object, in many respects.

   If an adversary deletes one or more CA certificates, ROAs or the CRL
   for a publication point, the manifest for that publication point will
   allow an RP to detect this attack.  (The RP would be very unhappy if
   there is no CRL for the CA instance anyway.)  An RP can continue to
   use the last valid instance of the deleted object as a local policy
   option), thus minimizing the impact of such an attack.

   If an adversary deletes a manifest (and does not replace it with an
   older instance), that is detectable by RPs.  Such behavior should
   result in the CA (or publication point maintainer) being notified of
   the problem.  An RP can continue to use the last valid instance of
   the deleted manifest (a local policy option), thus minimizing the
   impact of such an attack.

   If an adversary deletes newly added CA certificates or ROAs, and
   replaces the current manifest with the previous manifest, the
   manifest (and the CRL that it matches) will be "stale" (see
   [RFC6486]).  This alerts an RP that there may be a problem, and,
   hopefully, the entity responsible for the publication point will be
   asked to remedy the problem (e.g., republish the missing CA
   certificates and/or ROAs).  An RP cannot know the content of the new
   certificates or ROAs that are not present, but it can continue to use
   what it has cached.  An attack of this sort will, at least
   temporarily, cause RPs to be un aware of the newly published objects.
   INRs associated with these objects will be treated as
   unauthenticated.

   If a CA revokes a CA certificate or a ROA (via deleting the
   corresponding EE certificate), and the adversary tries to reinstate
   that CA certificate or ROA, the adversary would have to rollback the
   CRL and the manifest to undo this action by the CA.  As above, this

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   would make the CRL and manifest stale, and this is detectable by RPs.
   An RP cannot know which CA certificates or ROAs were deleted.
   Depending on local policy, the RP might use the cached instances of
   the affected objects, and thus be tricked into making decisions based
   on these revoked objects.  Here too the hope is that the CA will be
   notified of the problem (by RPs) and will remedy the error.

   In the attack scenarios above, when a CRL or manifest is described as
   stale, this means that the next issue date for the CRL or manifest
   has passed.  Until the next issue date, an RP will not be detect the
   attack.  Thus it behooves CAs to select CRL/manifest lifetimes (the
   two are linked) that represent an acceptable tradeoff between risk
   and operational burdens.

   Attacks effected by adversaries that are legitimate managers of
   publication points can have much greater effects, and are discussed
   below under attacks on or by CAs.

4.5.  Attacks on an RPKI CA

   Every entity to which INRs have been allocated/assigned is a CA in
   the RPKI.  Each CA is nominally responsible for managing the
   repository publication point for the set of signed products that it
   generates.  (An INR holder may choose to outsource the operation of
   the RPKI CA function, and the associated publication point.  In such
   cases, the organization operating on behalf of the INR holder becomes
   the CA, from an operational and security perspective.  The following
   discussion does not distinguish such outsourced CA operations.)

   Note that attacks attributable to a CA may be the result of malice by
   the CA (i.e., the CA is the adversary) or they may result from a
   compromise of the CA.

   All of adversaries listed in Section 2 are presumed to be capable of
   launching attacks against the computers used to perform CA functions.
   Some adversaries might effect an attack on a CA by violating
   personnel or physical security controls as well.  The distinction
   between CA as adversary vs. CA as an attack victim is important.
   Only in the latter case should one expect the CA to remedy problems
   caused by a attack once the attack has been detected.  (If a CA does
   not take such action, the effects are the same as if the CA is an
   adversary.)

   Note that most of the attacks described below do not require
   disclosure of a CA's private key to an adversary.  If the adversary
   can gain control of the computer used to issue certificates, it can
   effect these attacks, even though the private key for the CA remains
   "secure" (i.e., not disclosed to unauthorized parties).  However, if

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   the CA is not the adversary, and if the CA's private key is not
   compromised, then recovery from these attacks is much easier.  This
   motivates use of hardware security modules to protect CA keys, at
   least for higher tiers in the RPKI.

   An attack by a CA can result in revocation or replacement of any of
   the certificates that the CA has issued.  Revocation of a certificate
   should cause RPs to delete the (formerly) valid certificate (and
   associated signed object, in the case of a revoked EE certificate)
   that they have cached.  This would cause repository objects (e.g., CA
   certificates and ROAs) that are verified under that certificate to be
   considered invalid, transitively.  As a result, RPs would not
   consider as valid any ROAs or BGPSEC-signed updates based on these
   certificates, which would make routes dependent on them to be less
   preferred.  Because a CA that revokes a certificate is authorized to
   do so, this sort of attack cannot be detected, intrinsically, by most
   RPs.  However, the entities affected by the revocation or replacement
   of CA certificates can be expected to detect the attack and contact
   the CA to effect remediation.  If the CA was not the adversary, it
   should be able to issue new certificates and restore the publication
   point.

   An adversary that controls the CA for a publication point can publish
   signed products that create more subtle types of DoS attacks against
   RPs.  For example, such an attacker could create subordinate CA
   certificates with Subject Information Access (SIA) pointers that lead
   RPs on a "wild goose chase" looking for additional publication points
   and signed products.  An attacker could publish certificates with
   very brief validity intervals, or CRLs and manifests that become
   "stale" very quickly.  This sort of attack would cause RPs to access
   repositories more frequently, and that might interfere with
   legitimate accesses by other RPs.

   An attacker with this capability could create very large numbers of
   ROAs to be processed (with prefixes that are consistent with the
   allocation for the CA), and correspondingly large manifests.  An
   attacker could create very deep subtrees with many ROAs per
   publication point, etc.  All of these types of DoS attacks against
   RPs are feasible within the syntactic and semantic constraints
   established for RPKI certificates, CRLs, and signed objects.

   An attack that results in revocation and replacement (e.g., key
   rollover or certificate renewal) of a CA certificate would cause RPs
   to replace the old, valid certificate with the new one.  This new
   certificate might contain a public key that does not correspond to
   the private key held by the certificate subject.  That would cause
   objects signed by that subject to be rejected as invalid, and prevent
   the affected subject from being able to sign new objects.  As above,

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   RPs would not consider as valid any ROAs issued under the affected CA
   certificate, and updates based on router certificates issued by the
   affected CA would be rejected.  This would make routes dependent on
   these signed products to be less preferred.  However, the constraints
   imposed by the use of RFC 3779 [RFC3779] extensions do prevent a
   compromised CA from issuing (valid) certificates with INRs outside
   the scope of the CA, thus limiting the impact of the attack.

   An adversary that controls a CA could issue CA certificates with
   overlapping INRs to different entities, when no transfer of INRs is
   intended.  This could cause confusion for RPs as conflicting ROAs
   could be issued by the distinct (subordinate) CAs.

   An adversary could replace a CA certificate, use the corresponding
   private key to issue new signed products, and then publish them at a
   publication point controlled by the attacker.  This would effectively
   transfer the affected INRs to the adversary, or to a third party of
   his choosing.  The result would be to cause RPs to view the entity
   that controls the private key in question as the legitimate INR
   holder.  Again the constraints imposed by the use of RFC 3779
   extensions prevent a compromised CA from issuing (valid) certificates
   with INRs outside the scope of the CA, thus limiting the impact of
   the attack.

   Finally, an entity that manages a repository publication point can
   inadvertently act as an attacker (as first noted by Pogo).  For
   example, a CA might fail to replace its own certificate in a timely
   fashion (well before it expires).  If might fail to issue its CRL and
   manifest prior to expiration, creating stale instances of these
   products that cause concern for RPs.  A CA with many subordinate CAs
   (e.g., an RIR or NIR) might fail to distribute the expiration times
   for the CA certificates that it issues.  A network with many ROAs
   might do the same for the EE certificates associated with the ROAs it
   generates.  A CA could rollover its key, but fail to reissue
   subordinate CA certificates under its new key.  Poor planning with
   regard to rekey intervals for managed CAs could impose undue burdens
   for RPs, despite a lack of malicious intent.  All of these example of
   mismanagement could adversely affect RPs, despite the absence of
   malicious intent.

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5.  Residual Vulnerabilities

   The RPKI, upon which BGPSEC relies, has several residual
   vulnerabilities that were discussed in the preceding text
   (Section 4.4 and Section 4.5).  These vulnerabilities are of two
   principle forms:

   o  the RPKI repository system may be attacked in ways that make its
      contents unavailable, not current, or inconsistent.  The principle
      defense against most forms of DoS attacks is the use of a local
      cache by each RP.  The local cache ensures availability of
      previously-acquired RPKI data, in the event that a repository is
      inaccessible or if repository contents are deleted (maliciously).
      Nonetheless, the system cannot ensure that every RP will always
      have access to up-to-date RPKI data.  An RP, when it detects a
      problem with acquired repository data has two options:

      1.  The RP may choose to make use of its local cache, employing
          local configuration settings that tolerate expired or stale
          objects.  (Such behavior is, nominally, always within the
          purview of an RP in PKI.)  Using cached, expired or stale data
          subjects the RP to attacks that take advantage of the RP's
          ignorance of changes to this data.

      2.  The RP may chose to purge expired objects.  Purging expired
          objects removes the security info associated with the real
          world INRs to which the objects refer.  This is equivalent to
          the affected INRs not having been afforded protection via the
          RPKI.  Since use of the RPKI (and BGPSEC) is voluntary, there
          may always be set of INRs that are not protected by these
          mechanisms.  Thus purging moves the affected INRs to the set
          of non-participating INR holders.  This more conservative
          response enables an attacker to move INRs from the protected
          to the unprotected set.

   o  any CA in the RPKI may misbehave within the bounds of the INRs
      allocated to it, e.g., it may issue certificates with duplicate
      resource allocations or revoke certificates inappropriately.  This
      vulnerability is intrinsic in any PKI, but its impact is limited
      in the RPKI because of the use or RFC 3779 extensions.  It is
      anticipated that RPs will deal with such misbehavior through
      administrative means, once it is detected.

   BGPSEC has a separate set of residual vulnerabilities:

   o  "Route leaks" are viewed as a routing security problem by many
      network operators, even though there is no IETF-codified
      definition of a route leak.  BGP itself does not include semantics

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      that preclude what many perceive as route leaks.  Moreover, route
      leaks are outside the scope of BGPSEC, at this time, based on the
      SIDR charter.  Thus route leaks are not addressed in this threat
      model.

   o  BGPSEC signatures do not protect all attributes associated with an
      AS_path.  Some of these attributes are employed as inputs to
      routing decisions.  Thus attacks that modify (or strip) these
      other attributes are not detected by BGPSEC.  The SIDR charter
      calls for protecting only the info needed to verify that a
      received route traversed the ASes on question, and that the NLRI
      in the route is what was advertised.  Thus, protection of other
      attributes is outside the scope of the charter, at the time this
      document was prepared.

   o  BGPSEC cannot ensure that an AS will withdraw a route when the AS
      no longer has a route for a prefix, as noted in Section 4.2.
      BGPSEC may incorporate features to limit the lifetime of an
      advertisement.  Such lifetime limits provide an upper bound on the
      time that the failure to withdraw a route will remain effective.

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6.  Security Considerations

   A threat model is, by definition, a security-centric document.
   Unlike a protocol description, a threat model does not create
   security problems nor 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 describes the impact of these types of attacks on
   BGPSEC, including on the RPKI on which BGPSEC relies.  It describes
   how the design of the RPKI (and the current BGPSEC design) address
   classes of attacks, where applicable.  It also notes residual
   vulnerabilities.

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7.  IANA Considerations

   [Note to IANA, to be removed prior to publication: there are no IANA
   considerations stated in this version of the document.]

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8.  Acknowledgements

   The author wishes to thank...

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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.

9.2.  Informative References

   [Kent2000]
              Kent, S., Lynn, C., and K. Seo, "Design and Analysis of
              the Secure Border Gateway Protocol (S-BGP)", IEEE DISCEX
              Conference, June 2000.

   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779, June 2004.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, January 2006.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, February 2012.

   [RFC6482]  Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
              Origin Authorizations (ROAs)", RFC 6482, February 2012.

   [RFC6486]  Austein, R., Huston, G., Kent, S., and M. Lepinski,
              "Manifests for the Resource Public Key Infrastructure
              (RPKI)", RFC 6486, February 2012.

   [RFC6487]  Huston, G., Michaelson, G., and R. Loomans, "A Profile for
              X.509 PKIX Resource Certificates", RFC 6487,
              February 2012.

   [RFC6488]  Lepinski, M., Chi, A., and S. Kent, "Signed Object
              Template for the Resource Public Key Infrastructure
              (RPKI)", RFC 6488, February 2012.

   [TCPMD5]   Heffernan, A., "Protection of BGP Sessions via the TCP MD5
              Signature Option", RFC 2385, August 1998.

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Authors' Addresses

   Stephen Kent
   BBN Technologies
   10 Moulton St.
   Cambridge, MA  02138
   US

   Email: kent@bbn.com

   Andrew Chi
   BBN Technologies
   10 Moulton St.
   Cambridge, MA  02138
   US

   Email: achi@bbn.com

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