# CVE-2026-98103: igmp: convert struct ip_sf_list to RCU

> A race condition in the Linux kernel IGMP implementation can cause a deadlock, leading to denial of service.

- **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:** 21 Oct 2026
- **Also known as:** AZL-104577, BELL-CVE-2026-98103, DEBIAN-CVE-2026-98103, GHSA-2hrc-mwqg-6fr2, UBUNTU-CVE-2026-98103

## What is CVE-2026-98103?

CVE-2026-98103 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:

igmp: convert struct ip_sf_list to RCU

Commit 23d2b94043ca ("igmp: Add ip_mc_list lock in ip_check_mc_rcu")
added spin_lock_bh(&im->lock) to ip_check_mc_rcu() to prevent a
use-after-free while iterating im->sources during concurrent deletions.

However, ip_check_mc_rcu() is called from RCU read-side critical
sections in packet receive and route lookup fast paths (e.g.
__mkroute_output(), ip_route_input_rcu(), and __udp4_lib_rcv()).

When igmpv3_send_cr() or igmpv3_send_report() holds &pmc->lock and
calls add_grec() -> igmpv3_newpack() -> ip_route_output_ports(),
an XFRM policy matching a multicast destination triggers
xfrm_tmpl_resolve_one() -> xfrm4_get_saddr() -> __mkroute_output() ->
ip_check_mc_rcu(). This attempts to acquire &im->lock while &pmc->lock
is already held on the same CPU, triggering a lockdep recursive locking
warning / deadlock.

Fix this by converting IPv4 struct ip_sf_list to RCU, mirroring the
IPv6 implementation in net/ipv6/mcast.c:

1. Add struct rcu_head to struct ip_sf_list and annotate sf_next,
   sources, and tomb as __rcu pointers.
2. Use rcu_assign_pointer() and kfree_rcu() for list updates and
   deletions.
3. Remove spin_lock_bh(&im->lock) from ip_check_mc_rcu() and traverse
   im->sources locklessly with for_each_psf_rcu(), reading and writing
   counter fields with READ_ONCE() and WRITE_ONCE().

Note: RCU conversion of /proc/net/mcfilter will be done in a
separate patch.

## Which versions are affected by CVE-2026-98103?


## How do I fix CVE-2026-98103?

Apply the vendor's patch.

## Is CVE-2026-98103 being exploited?

There is no known exploitation of CVE-2026-98103 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98103 and fix it if it is affected.

What it is: CVE-2026-98103 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-2026-98103 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-2026-98103/ (Markdown: https://insights.sbomify.com/CVE-2026-98103/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98103
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98103
- OSV: https://osv.dev/vulnerability/CVE-2026-98103

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98103/
Summaries are generated and may contain mistakes; the official description above is the record.