# CVE-2026-98099: ipv6: mcast: use rcu_assign_pointer() for __rcu list updates

> A race condition in Linux kernel IPv6 multicast handling due to improper RCU pointer updates can lead to system instability or denial of service.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **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:** 21 Oct 2026
- **Also known as:** AZL-104745, BELL-CVE-2026-98099, DEBIAN-CVE-2026-98099, GHSA-5fxh-xwwm-73v4, UBUNTU-CVE-2026-98099

## What is CVE-2026-98099?

CVE-2026-98099 is a medium-severity vulnerability in Linux kernel, 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 rcu_assign_pointer() for __rcu list updates

Several places in net/ipv6/mcast.c update RCU-protected lists
(np->ipv6_mc_list, idev->mc_list, idev->mc_tomb) using direct pointer
assignments instead of rcu_assign_pointer():

1. In __ipv6_dev_mc_dec(), unlinking a group from idev->mc_list did:
       *map = ma->next;
   without rcu_assign_pointer() while concurrent readers traverse
   idev->mc_list locklessly under rcu_read_lock().
2. In ipv6_sock_mc_drop() and __ipv6_sock_mc_close(), unlinking a group
   from np->ipv6_mc_list directly assigned *lnk = mc_lst->next and
   np->ipv6_mc_list = mc_lst->next without rcu_assign_pointer(), racing
   with lockless readers in inet6_mc_check().
3. In __ipv6_sock_mc_join(), mc_lst->next was initialized to
   np->ipv6_mc_list via raw assignment before publishing mc_lst.
4. In mld_del_delrec() and __ipv6_dev_mc_inc(), __rcu source pointers
   passed into rcu_assign_pointer() lacked explicit dereference helpers.

Fix these by consistently using rcu_assign_pointer() along with
mc_dereference() / sock_dereference().

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2026-98099?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-98099?

Apply the vendor's patch.

## Is CVE-2026-98099 being exploited?

There is no known exploitation of CVE-2026-98099 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98099 and fix it if it is affected.

What it is: CVE-2026-98099 is a medium-severity vulnerability in Linux kernel, 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-2026-98099 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-98099/ (Markdown: https://insights.sbomify.com/CVE-2026-98099/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98099
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98099
- OSV: https://osv.dev/vulnerability/CVE-2026-98099

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98099/
Summaries are generated and may contain mistakes; the official description above is the record.