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:

and

For Junos OS Evolved only:

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 .

Software releases or updates are available for download at https://support.juniper.net/support/downloads/

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

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

2021-10-13: Initial Publication.

• KB16613: Overview of the Juniper Networks SIRT Quarterly Security Bulletin Publication Process [juniper.net]
• KB16765: In which releases are vulnerabilities fixed? [juniper.net]
• KB16446: Common Vulnerability Scoring System (CVSS) and Juniper's Security Advisories [juniper.net]
• Report a Vulnerability - How to Contact the Juniper Networks Security Incident Response Team
• CVE-2021-31363: Junos OS and Junos OS Evolved: Receipt of a specific LDP message will cause a Denial of Service