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as-path-prepend and specific length AS_PATH we can cause a JUNOS router to send corrupted UPDATE packets to eBGP neighbors



Article ID: JSA10418 SECURITY_ADVISORIES Last Updated: 09 May 2013Version: 5.0
Legacy Advisory Id:
Product Affected:
All JUNOS Devices
Data corruption in a BGP update packet with a long AS_PATH using the as-path-prepend option, could cause the a neighboring eBGP peer to drop the session with an update message error. The number of AS_PATH prepends are excessive (greater than 254), but can happen through normal operational configuration. Given the nature of AS_PATH, the as-path-prepend can be crafted to impact routers several AS hops away from the origin.

Limiting the number of AS numbers in the AS_PATH will not mitigate this issue on the receiving routers. Malformed Transitive Attributes are handled before the BGP logic for filtering the number of AS paths.

Credit: This issue was reported to Juniper Networks by Mark Bailey, Euan Galloway and Danny Vernals. Juniper Networks SIRT thanks them for their efforts.
Customers are recommended to upgrade JUNOS through planned and methodical upgrade processes.

All JUNOS software releases built on or after March 12, 2009 has fixed this BGP as-path-prepend malformed attribute. This specifically includes 8.1S2, 8.5S1, 9.0S2, 9.1-20090321-SR, 9.2-20090320-SR, 9.2R4, 9.3R3, 9.4S1, 9.4R3, 9.5R1, and all subsequent releases.

PR Reference for this issue is PR 430077

A JUNOS Router can be prevented from sending excessive AS_PATHs using the following policy on all outbound BGP prefixes:

policy-options {
    policy-statement block-very-long-paths {
        from as-path too-many-hops;
        then reject;

    as-path too-many-hops ".{200,}";

This outbound policy (or something similar) would limit the spread of the risk if a JUNOS router were to receive a BGP prefix with more than 254 AS paths.
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:
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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