# CVE-2026-98101: ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source()

> A race condition in Linux kernel IPv6 multicast handling allows concurrent readers to observe torn or inconsistent source filter data, potentially leading to information disclosure or system instability.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-104636, BELL-CVE-2026-98101, DEBIAN-CVE-2026-98101, GHSA-wv36-qwc6-r8c9, UBUNTU-CVE-2026-98101

## What is CVE-2026-98101?

CVE-2026-98101 is a vulnerability in Linux, published on 25 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

## Which versions are affected by CVE-2026-98101?


## How do I fix CVE-2026-98101?

Apply the vendor's patch.

## Is CVE-2026-98101 being exploited?

There is no known exploitation of CVE-2026-98101 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98101 and fix it if it is affected.

What it is: CVE-2026-98101 is a vulnerability in Linux, published on 25 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-98101 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-98101/ (Markdown: https://insights.sbomify.com/CVE-2026-98101/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98101
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98101
- OSV: https://osv.dev/vulnerability/CVE-2026-98101

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98101/
Summaries are generated and may contain mistakes; the official description above is the record.