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BGP UPDATE with malformed Path Attribute tears down BGP session

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Article ID: JSA10458 SECURITY_ADVISORIES Last Updated: 09 May 2013Version: 4.0
Legacy Advisory Id:
PSN-2010-10-969
Product Affected:
This issue affects all Junos routers configured for BGP.
Problem:

Per RFC 4271, Junos will tear down a BGP session upon receiving a malformed UPDATE. The issue is that Junos may recognize optional, transitive attributes that are not yet recognized by other vendors' routers. In this case, non-Junos routers which don't recognize the attribute just pass the UPDATE through with the Partial bit ON. Junos, however, would recognize the path attribute and perform validation. If the packet has errors, a Junos router will send a NOTIFICATION (Error code = 3, UPDATE message error, possible subcodes: 1, 5, or 11) and tear down the BGP session.
Solution:

All Junos software releases built on or after 2009-06-29 have been enhanced to be more tolerant of malformed optional, transitive attributes. Releases containing the coding change specifically include: 9.1S2, 9.3R3, 9.6R1 and all subsequent releases (i.e. all releases built after 9.6R1).

Note that Junos 9.0 and earlier remains unchanged, and continues to operate per RFC 4271 (see below). Any corrupt optional, transitive attributes understood by the running version of Junos will cause a session flap.

Summary of BGP behavioral changes upon reception of MALFORMED OPTIONAL-TRANSITIVE attributes

Pre 9.1
Flap - Operating as per RFC. Risk to networks are contained as per Section 6 of RFC 4271. Changes to this behavior are described in the IETF draft "Error Handling for Optional Transitive BGP Attributes" (draft-ietf-idr-optional-transitive-03).

9.1, 9.2, 9.3, 9.4, and 9.5
Partial bit RESET: Flap the session

Partial bit SET:
  • AS4Path, Aggregator4, Aggregator, PMSI, ATTR_SET:
    • Strip the attribute and process the rest of the attribute as is
  • Communities, Extended Communities:
    • Flap the session
9.6 Onwards
Partial bit RESET: Flap the session

Partial bit SET:
  • AS4Path, Aggregator4, Aggregator:
    • Strip the attribute and process the rest of the attribute as is
  • PMSI, ATTR_SET, Communities, Extended Communities:
    • Treat the update as a WITHDRAW
This issue is being tracked as PR 423487. While this PR is not viewable by customers, it can be used as a reference when discussing the issue with JTAC.

KB16765: "In which releases are vulnerabilities fixed?" describes which release vulnerabilities are fixed as per our End of Engineering and End of Life support policies.

Workaround:

Not applicable. This bulletin describes a feature enhancement.
Implementation:

How to obtain Service Releases:

Security vulnerabilities are fixed in the next available Maintenance Release of each supported Junos version. In some cases, a Maintenance Release is not planned to be available in an appropriate time-frame. For these cases, Service Releases are made available in order to be more timely. Security Advisory notices will indicate which Maintenance and/or Service Releases contain fixes for the issues described. Upon request to JTAC, customers will be provided download instructions for a Service Release. Although Juniper does not provide formal Release Note documentation for a Service Release, a list of "PRs fixed" can be provided on request.
CVSS Score:
7.1 (AV:N/AC:M/Au:N/C:N/I:N/A:C)
Severity Level:
High
Severity Assessment:
Information for how Juniper Networks uses CVSS can be found at KB 16446: "Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories."

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