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Versions: (draft-kent-bgpsec-threats) 00 01 02 03 04 05 06 07 08 09 RFC 7132

Secure Inter-Domain Routing                                      S. Kent
Internet-Draft                                                       BBN
Intended status: Informational                                    A. Chi
Expires: April 11, 2014                                           UNC-CH
                                                        October 08, 2013


                   Threat Model for BGP Path Security
                   draft-ietf-sidr-bgpsec-threats-07

Abstract

   This document describes a threat model for the context in which
   (E)BGP path security mechanisms will be developed.  The 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.  We use the term PATHSEC to refer to any BGP path security
   technology that makes use of the RPKI.  PATHSEC will secure BGP
   [RFC4271], consistent with the inter-AS security focus of the RPKI
   [RFC6480].

   The document characterizes classes of potential adversaries that are
   considered to be threats, and examines classes of attacks that might
   be launched against PATHSEC.  It does not revisit attacks against
   unprotected BGP, as that topic has already been addressed in
   [RFC4271].  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
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 11, 2014.

Copyright Notice





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   Copyright (c) 2013 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
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Threat Characterization . . . . . . . . . . . . . . . . . . .   6
   4.  Attack Characterization . . . . . . . . . . . . . . . . . . .   7
     4.1.  Active wiretapping of sessions between routers  . . . . .   8
     4.2.  Attacks on a BGP router . . . . . . . . . . . . . . . . .   8
     4.3.  Attacks on network operator management computers (non-CA
           computers)  . . . . . . . . . . . . . . . . . . . . . . .  10
     4.4.  Attacks on a repository publication point . . . . . . . .  11
     4.5.  Attacks on an RPKI CA . . . . . . . . . . . . . . . . . .  13
   5.  Residual Vulnerabilities  . . . . . . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   9.  Informative References  . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This document describes the security context in which PATHSEC is
   intended to operate.  (The term "PATHSEC" is employed in this
   document to refer to any design used to achieve the path security
   goal described in the SIDR WG charter.  The charter focuses on
   mechanisms that will enable an AS to determine if the AS_PATH
   represented in a route represents the path via which the Network
   Layer Reachability Information traveled.  Other SIDR documents use
   the term "BGPSEC" to refer to a specific design.)  It discusses
   classes of potential adversaries that are considered to be threats,
   and classes of attacks that might be launched against PATHSEC.
   Because PATHSEC will rely 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 PATHSEC (based on the current PATHSEC design.)



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   The motivation for developing PATHSEC, 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 documents 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 must be addressed by PATHSEC.)

   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."

   PATHSEC is intended to address the concerns cited above, to provide
   significantly improved path security, building upon the route
   origination validation capability offered by use of the RPKI
   [RFC6810].  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, 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 PATHSEC 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.



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   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 PATHSEC relies.  It concludes with a brief
   review of residual vulnerabilities, i.e., vulnerabilities that are
   not addressed by use of the RPKI and that appear likely to be outside
   the scope of PATHSEC mechanisms.

2.  Terminology

   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.







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   False (Route) Origination - If a network operator originates a route
   for a prefix that the 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.

   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.

   Network Operator - An entity that manages an AS and thus emits (E)BGP
   updates, e.g., an ISP.

   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 Certificate Revocation Lists (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 X.509 certificates (and CRLs) that attest to
   the holdings of INRs.



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   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.

   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.

3.  Threat Characterization

   As noted in Section 2 above, a threat is defined as a motivated,
   capable, adversary.  The following classes of threats represent
   classes of adversaries viewed as relevant to this environment.

   Network Operators - A network operator may be a threat.  An 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 an
   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 an operator participates in the
   RPKI, it will have at least one 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 PATHSEC, and if
   PATHSEC makes use of certificates associated with routers or ASes, it
   will have the ability to issue such certificates for itself.  If
   PATHSEC digitally signs updates, it will be able to do so in a
   fashion that will be accepted by PATHSEC-enabled neighbors.





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   Hackers - Hackers are considered a threat.  A hacker might assume
   control of network management computers and routers controlled by
   operators, including operators that implement PATHSEC.  In such
   cases, hackers would be able to act as 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 or to express support for a cause
   ("hacktivists" [Sam04]).

   Criminals - Criminals may be a threat.  Criminals might persuade (via
   threats or extortion) a network operator to act as a rogue 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
   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).

4.  Attack Characterization




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   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 PATHSEC 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, e.g., passive
   wiretapping attacks, are not considered a requirement for BGP
   security (see [RFC4272]).

4.1.  Active wiretapping of sessions between routers

   An adversary may attack the BGP (TCP) session that connects a pair of
   BGP speakers.  An active attack against a BGP (TCP) session can be
   effected by directing traffic to a BGP speaker from some remote
   point, or by being positioned as a MITM on the link that carries BGP
   session traffic.  Remote attacks can be effected by any adversary.  A
   MITM attack requires access to the link.  Modern transport networks
   may be as complex as the packet networks that utilize them for inter-
   AS links.  Thus these transport networks may present significant
   attack surfaces.  Nonetheless, only some classes of adversaries are
   assumed to be capable of MITM attacks against a BGP session.  MITM
   attacks may be directed against BGP, PATHSEC-protected BGP, or
   against TCP or IP.  Such attacks include replay of selected BGP
   messages, selective modification of BGP messages, and DoS attacks
   against BGP routers.  [RFC4272] describes several countermeasures for
   such attacks, and thus this document does not further address such
   attacks.

4.2.  Attacks on a BGP router

   An adversary may attack a BGP router, whether it implements PATHSEC
   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 PATHSEC 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.)





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   Attacks on a router are equivalent to 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.

      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, but it is
      addressed by the use of the RPKI [RFC6480].

      Secure Path Downgrade: A router might remove AS_PATH data from a
      PATHSEC-protected update that it receives, when forwarding this
      update to a PATHSEC-enabled neighbor.  This behavior violates the
      PATHSEC security goals and thus is considered an attack.

      Invalid AS_PATH Data Insertion: A router might emit a PATHSEC-
      protected update with "bad" data (such as a signature), i.e.,
      PATHSEC data that cannot be validated by other PATHSEC routers.
      Such behavior is assumed to violate the PATHSEC goals and thus is
      considered an attack.

      Stale Path Announcement: If PATHSEC-secured announcements can
      expire, such an announcement may be propagated with PATHSEC data
      that is "expired".  This behavior would violate the PATHSEC goals
      and is considered a type of replay attack.

      Premature Path Announcement Expiration: If a PATHSEC-secured
      announcement has an associated expiration time, a router might
      emit a PATHSEC-secured announcement with an expiry time that is
      very short.  Unless the PATHSEC protocol specification mandates a
      minimum expiry time, this is not an attack.  However, if such a
      time is mandated, 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.

      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 PATHSEC.





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      Compromised Router Private Key: If PATHSEC mechanisms employ
      public key cryptography, e.g., to digitally sign data in an
      update, then a private key associated with a router or an AS 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 question, with some constraints.  So, for example,
      an adversary that controls another AS could use a compromised
      router/AS key to issue PATHSEC-signed data that include the
      targeted AS/router.  (Neighbors of the adversary's AS ought not
      accept a route that purports to emanate directly from the targeted
      AS.  So, an adversary could take a legitimate, protected route
      that passes through the compromised AS, add itself as the next
      hop, and then forward the resulting route to neighbors.)

      Withdrawal Suppression Attack: A PATHSEC-protected update may be
      signed and announced, and later withdrawn.  An adversary
      controlling intermediate routers could fail to propagate the
      withdrawal.  BGP is already vulnerable to behavior of this sort,
      so withdrawal suppression is not 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 who has compromised the security of
   these computers.  This might be effected via remote attacks,
   extortion of network operations staff, etc.  If an adversary
   compromises NOC computers, he 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 operator behavior.  (Such behavior
   might be perceived as accidental or malicious by other operators.)

   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 operator to view valid routes
   as not validated, which could alter its routing behavior.




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   If an adversary invoked the tool used to manage the repository
   publication point for this 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
   certificates to be revoked.  An attack of this sort could affect not
   only this operator, but also any operators that receive allocations/
   assignments from it, e.g., because their CA certificates were
   revoked.

   If an operator is PATHSEC-enabled, an attack of this sort could cause
   the affected operator to be viewed as not PATHSEC-enabled, possibly
   making routes it emits be less preferred by other 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 PATHSEC-enabled networks to view the affected
   network as no longer originating routes for these prefixes.  Multi-
   homed subscribers of this operator who received an allocation from
   the operator might find their traffic was now routed via other
   connections.

   If the network operator is PATHSEC-enabled, and make use of
   certificates associated with routers/ASes, an adversary could invoke
   a tool used to request such certificates.  The adversary could then
   replace valid certificates for routers/ASes with ones that might be
   rejected by PATHSEC-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 insider 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.




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   Note first that the RPKI calls for RPs to cache the data they acquire
   and verify from the repository system [RFC6480][RFC6481].  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.

   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.  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.  The RP
   should use the information from a Ghostbuster record [RFC6493] to
   contact the entity responsible for the publication point, requesting
   that entity to remedy the problem (e.g., republish the missing CA



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   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 unaware 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
   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 goal 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 detect the
   attack.  Thus it behooves CAs to select CRL/manifest lifetimes (the
   two are linked) that represent an acceptable trade-off 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



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   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
   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 PATHSEC-protected 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



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   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,
   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 (an example of Walt Kelly's most
   famous "Pogo" quote [Kelly70]).  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



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   a lack of malicious intent.  All of these example of mismanagement
   could adversely affect RPs, despite the absence of malicious intent.

5.  Residual Vulnerabilities

   The RPKI, upon which PATHSEC 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 PATHSEC) 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 of RFC 3779 extensions.  It is
      anticipated that RPs will deal with such misbehavior through
      administrative means, once it is detected.

   PATHSEC has a separate set of residual vulnerabilities:




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   o  It has been stated that "route leaks" are viewed as a routing
      security problem by many operators.  However, BGP itself does not
      include semantics that preclude what many perceive as route leaks,
      and there is no definition of the term in any RFC.  This makes it
      inappropriate to address route leaks in this document.
      Additionally, route leaks are outside the scope of PATHSEC, based
      on the SIDR charter.  That charter focuses on the integrity and
      authenticity of the data contained in the AS_path.  If, at a later
      time, the SIDR charter is amended to include route leaks, and an
      appropriate definition exists, this document should be revised.

   o  PATHSEC is not required to protect all attributes associated with
      an AS_PATH, even though some of these attributes may be employed
      as inputs to routing decisions.  Thus attacks that modify (or
      strip) these other attributes are not prevented/detected by
      PATHSEC.  The SIDR charter calls for protecting only the info
      needed to verify that a received route traversed the ASes in
      question, and that the NLRI in the route is what was advertised.
      (The AS_PATH data also may have traversed ASes within a
      confederation that are not represented.  However, these ASes are
      not externally visible, and thus do not influence route selection,
      so their omission in this context is not a security concern.)
      Thus, protection of other attributes is outside the scope of the
      charter, at the time this document was prepared.  If, at a later
      time, the SIDR charter is amended to include protection of
      additional BGP attributes, this document should be revised.

   o  PATHSEC 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.
      PATHSEC 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.

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
   PATHSEC, including on the RPKI on which PATHSEC relies.  It describes
   how the design of the RPKI (and the PATHSEC design goals) 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.]

8.  Acknowledgements

   TBD

9.  Informative References

   [Kelly70]  Kelly, W., "'We Have Met the Enemy, and He is Us': Pogo
              Earth Day Poster", April 1970.

   [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.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [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.

   [RFC6481]  Huston, G., Loomans, R., and G. Michaelson, "A Profile for
              Resource Certificate Repository Structure", RFC 6481,
              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.




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   [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.

   [RFC6493]  Bush, R., "The Resource Public Key Infrastructure (RPKI)
              Ghostbusters Record", RFC 6493, February 2012.

   [RFC6810]  Bush, R. and R. Austein, "The Resource Public Key
              Infrastructure (RPKI) to Router Protocol", RFC 6810,
              January 2013.

   [Sam04]    Samuel, A., "Hacktivism and the Future of Political
              Participation", Ph.D. dissertation, Harvard University,
              August 2004.

Authors' Addresses

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

   Email: kent@bbn.com


   Andrew Chi
   University of North Carolina - Chapel Hill
   c/o Department of Computer Science
   CB 3175, Sitterson Hall
   Chapel Hill, NC  27599
   US

   Email: achi@cs.unc.edu













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