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2021-10 Security Bulletin: Junos OS and Junos OS Evolved: Receipt of a specific LDP message will cause a Denial of Service (CVE-2021-31363)



Article ID: JSA11225 SECURITY_ADVISORIES Last Updated: 13 Oct 2021Version: 3.0
Product Affected:
This issue affects Junos OS versions 19.2R2 and higher, and all versions of Junos OS Evolved.

In an MPLS P2MP environment a Loop with Unreachable Exit Condition vulnerability in the routing protocol daemon (RPD) of Juniper Networks Junos OS and Junos OS Evolved allows an unauthenticated adjacent attacker to cause high load on RPD which in turn may lead to routing protocol flaps.

If a system with sensor-based-stats enabled receives a specific LDP FEC this can lead to the above condition.

Continued receipted of such an LDP FEC will create a sustained Denial of Service (DoS) condition.

This issue affects

Juniper Networks Junos OS

  • 19.2 version 19.2R2 and later versions prior to 19.2R3-S3;
  • 19.3 versions prior to 19.3R2-S6, 19.3R3-S2;
  • 19.4 versions prior to 19.4R1-S4, 19.4R2-S4, 19.4R3-S2;
  • 20.1 versions prior to 20.1R2-S1, 20.1R3;
  • 20.2 versions prior to 20.2R2-S1, 20.2R3;
  • 20.3 versions prior to 20.3R1-S2, 20.3R2.

This issue does not affect Juniper Networks Junos OS versions prior to 19.2R2.

Juniper Networks Junos OS Evolved

  • All versions prior to 20.1R2-S3-EVO;
  • 20.3 versions prior to 20.3R1-S2-EVO.

A vulnerable Junos OS configuration would need to have:

[protocols ldp p2mp]


[protocols ldp traffic-statistics sensor-based-stats]

For Junos OS Evolved only:

[protocols ldp p2mp]

is need as sensor-based-stats is the default setting which can't be changed.

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-2021-31363.


The following software releases have been updated to resolve this specific issue in Junos OS: 19.2R3-S3, 19.3R2-S6, 19.3R3-S2, 19.4R1-S4, 19.4R2-S4, 19.4R3-S2, 20.1R2-S1, 20.1R3, 20.2R2-S1, 20.2R3, 20.3R1-S2, 20.3R2, 20.4R1, and all subsequent releases.

The following software releases have been updated to resolve this specific issue in Junos OS Evolved: 20.1R2-S3-EVO, 20.3R1-S2-EVO, 20.4R1-EVO, and all subsequent releases.

This issue is being tracked as 1552041.


For Junos OS turn off sensor-based-stats by deleting:

[protocols ldp traffic-statistics sensor-based-stats]

For Junos OS Evolved mLDP will have to be turned off as sensor-based-stats is the default:

[protocols ldp p2mp]
Software releases or updates are available for download at
Modification History:
2021-10-13: Initial Publication.
CVSS Score:
6.5 (CVSS:3.1/AV:A/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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