# CVE-2026-74657: ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops

> A local attacker can trigger a kernel panic by causing a route notification size mismatch, leading to denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Aug 2026
- **Updated:** 27 Aug 2026
- **Also known as:** AZL-97278, BELL-CVE-2026-74657, DEBIAN-CVE-2026-74657, GHSA-9gv9-cg8w-rc25, UBUNTU-CVE-2026-74657

## What is CVE-2026-74657?

CVE-2026-74657 is a denial of service vulnerability in Linux, published on 22 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

ipv4: Fix fib_nlmsg_size() for RTA_VIA nexthops

fib_nlmsg_size() still estimates nexthop space as if every gateway is
encoded as an IPv4 RTA_GATEWAY attribute. IPv4 routes can also carry an
IPv6 gateway, which fib_nexthop_info() dumps as RTA_VIA.

As a result, route notifications can allocate an skb that is too small.
fib_dump_info() then fails with -EMSGSIZE and rtmsg_fib() hits the
WARN_ON() that marks such failures as a fib_nlmsg_size() bug. With
panic_on_warn set, this becomes a kernel panic.

Mirror the actual nexthop dump layout in fib_nlmsg_size(): account for
IPv6 nexthop gateways dumped as RTA_VIA, for the no-header rtnexthop
layout used inside RTA_MULTIPATH, and for RTA_FLOW only when it is
actually present.

## Which versions are affected by CVE-2026-74657?


## How do I fix CVE-2026-74657?

Apply the vendor's patch.

## Is CVE-2026-74657 being exploited?

There is no known exploitation of CVE-2026-74657 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74657 and fix it if it is affected.

What it is: CVE-2026-74657 is a denial of service vulnerability in Linux, published on 22 Aug 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-74657 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-2026-74657/ (Markdown: https://insights.sbomify.com/CVE-2026-74657/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74657
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74657
- OSV: https://osv.dev/vulnerability/CVE-2026-74657

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74657/
Summaries are generated and may contain mistakes; the official description above is the record.