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 JSA11218 - 2021-10 Security Bulletin: Junos OS and Junos OS Evolved: RPD
         core upon receipt of specific BGP update (CVE-2021-31353)
                              15 October 2021


        AusCERT Security Bulletin Summary

Product:           Junos OS
                   Junos OS Evolved
Publisher:         Juniper Networks
Operating System:  Juniper
Impact/Access:     Denial of Service -- Remote/Unauthenticated
Resolution:        Patch/Upgrade
CVE Names:         CVE-2021-31353  

Original Bulletin: 

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2021-10 Security Bulletin: Junos OS and Junos OS Evolved: RPD core upon receipt of specific BGP update (CVE-2021-31353)

Article ID  : JSA11218
Last Updated: 13 Oct 2021
Version     : 1.0

Product Affected:
This issue affects specific versions of Junos OS and Junos OS Evolved (see

An Improper Handling of Exceptional Conditions vulnerability in Juniper
Networks Junos OS and Junos OS Evolved allows an attacker to inject a specific
BGP update, causing the routing protocol daemon (RPD) to crash and restart,
leading to a Denial of Service (DoS).

Continued receipt and processing of the BGP update will create a sustained
Denial of Service (DoS) condition.

This issue affects very specific versions of Juniper Networks Junos OS:

  o 19.3R3-S2;
  o 19.4R3-S3;
  o 20.2 versions 20.2R2-S3 and later, prior to 20.2R3-S2;
  o 20.3 versions 20.3R2 and later, prior to 20.3R3;
  o 20.4 versions 20.4R2 and later, prior to 20.4R3;
  o 21.1 versions prior to 21.1R2.

Juniper Networks Junos OS 20.1 is not affected by this issue.

This issue also affects Juniper Networks Junos OS Evolved:

  o All versions prior to 20.4R2-S3-EVO, 20.4R3-EVO;
  o 21.1-EVO versions prior to 21.1R2-EVO;
  o 21.2-EVO versions prior to 21.2R2-EVO.

This issue can occur when multipath is enabled:

routing-instance <vrf> routing-options multipath

and one of the following two TTL propagation options (but not both) are

protocols mpls no-propagate-ttl
routing-instance <vrf> no-vrf-propagate-ttl

Juniper SIRT is not aware of any malicious exploitation of this vulnerability.

This issue was seen during production usage.

This issue has been assigned CVE-2021-31353 .


The following software releases have been updated to resolve this specific

Junos OS 18.4R2-S9, 19.1R3-S7, 19.3R3-S3, 19.4R1-S4, 19.4R3-S4, 20.1R3,
20.2R3-S2, 20.3R3, 20.4R3, 21.1R2, 21.2R1, 21.2R2, 21.3R1, and all subsequent

Junos OS Evolved 20.4R2-S3-EVO, 20.4R3-EVO, 21.1R2-EVO, 21.2R2-EVO, 21.3R1-EVO,
and all subsequent releases.

Note: Only those releases listed in the PROBLEM section above are affected.
This fix has also been proactively committed into other releases that are not
vulnerable to this issue.

This issue is being tracked as 1595165 .


This issue can be mitigated in two ways:

 1. ensure that TTL propagation is either enabled or disabled in both places
    protocols mpls no-propagate-ttl
    routing-instance <vrf> no-vrf-propagate-ttl
 2. Disable multipath:
    routing-instance <vrf> routing-options multipath

Software releases or updates are available for download at https://
Modification History:

2021-10-13: Initial Publication.

CVSS Score:
7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
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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