# CVE-2026-99074: ipv4: stop PMTU walk when nexthop group shrinks


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 23 Oct 2026
- **Updated:** 23 Oct 2026
- **Also known as:** BELL-CVE-2026-99074, DEBIAN-CVE-2026-99074, GHSA-j9w6-p85x-r6r9, UBUNTU-CVE-2026-99074

## What is CVE-2026-99074?

CVE-2026-99074 is a vulnerability in Linux, published on 23 Oct 2026. 

### Official description

In the Linux kernel, the following vulnerability has been resolved:

ipv4: stop PMTU walk when nexthop group shrinks

Commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if
multipath enabled") made __ip_rt_update_pmtu() update every path.

For nexthop objects, fib_info_num_path() and fib_info_nhc()
independently load nh->nh_grp.  RCU protects each group's lifetime but
does not make the loads observe the same group.  If replacement shrinks
the group after the loop accepts an index, fib_info_nhc() returns NULL
and update_or_create_fnhe() dereferences it.

A deterministic probe only widened the existing window between the real
operations.  On v7.2, an RTM_NEWNEXTHOP replacement published a
one-member group after the PMTU reader accepted index 1 from a two-member
group, producing:

  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: update_or_create_fnhe+0x45/0x15b0

Stop when the indexed path is absent.  Groups are dense, so no later
index exists in that snapshot.  The same real-writer window completed
114,423 PMTU iterations without an oops with this check.  A reproducer
is available on request.

Replacement requires CAP_NET_ADMIN in the network namespace.  Where
unprivileged user namespaces are permitted, a local user can obtain it
in a new user and network namespace.

## Which versions are affected by CVE-2026-99074?


## How do I fix CVE-2026-99074?

Upgrade to a fixed version.

## Is CVE-2026-99074 being exploited?

There is no known exploitation of CVE-2026-99074 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99074 and fix it if it is affected.

What it is: CVE-2026-99074 is a vulnerability in Linux, published on 23 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99074 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-2026-99074/ (Markdown: https://insights.sbomify.com/CVE-2026-99074/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99074
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99074
- OSV: https://osv.dev/vulnerability/CVE-2026-99074

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99074/
Summaries are generated and may contain mistakes; the official description above is the record.