# CVE-2026-74656: ipv4: fix use-after-free in fib_nhc_update_mtu()

> A use-after-free vulnerability in the Linux kernel's IPv4 routing code allows attackers to cause denial of service or potentially execute code via race conditions.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **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:** 16 Oct 2026
- **Also known as:** AZL-97206, BELL-CVE-2026-74656, DEBIAN-CVE-2026-74656, GHSA-4hv8-gc9j-4wjf, UBUNTU-CVE-2026-74656

## What is CVE-2026-74656?

CVE-2026-74656 is a high-severity use after free 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 use-after-free in fib_nhc_update_mtu()

fib_nhc_update_mtu() walks the nexthop exception table under RTNL, but
RTNL does not serialize this walk with PMTU exception updates. The walk
uses rcu_dereference_protected() with a constant true condition without
holding fnhe_lock.

The following interleaving can therefore occur:

  CPU 0                              CPU 1
  fib_nhc_update_mtu()               update_or_create_fnhe()
    load fnhe                          spin_lock_bh(&fnhe_lock)
                                       fnhe_remove_oldest()
                                         unlink fnhe
                                         kfree_rcu(fnhe, rcu)
    <quiescent state>
    access fnhe after grace period

KASAN reported:

  BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410
  Read of size 8 at addr ffff888107d49000 by task poc/90
  Call Trace:
   fib_nhc_update_mtu+0x3df/0x410
   fib_sync_mtu+0x7a/0xd0
   fib_netdev_event+0x229/0x3f0
   netif_set_mtu_ext+0x33a/0x570
   dev_set_mtu+0x88/0x120

The same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a
pair and other writers serialize them with fnhe_lock. RCU alone prevents
reclamation, but would still allow concurrent writers to leave a mixed
pair.

Walk the table under RCU and acquire fnhe_lock only while updating each
exception. RCU keeps the current entry alive while the short critical
section serializes its paired PMTU fields. This avoids holding the global
lock while scanning all 2048 buckets for every nexthop.

## Which versions are affected by CVE-2026-74656?


## How do I fix CVE-2026-74656?

Apply the vendor's patch.

## Is CVE-2026-74656 being exploited?

There is no known exploitation of CVE-2026-74656 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74656 and fix it if it is affected.

What it is: CVE-2026-74656 is a high-severity use after free 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-2026-74656 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-2026-74656/ (Markdown: https://insights.sbomify.com/CVE-2026-74656/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74656
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74656
- OSV: https://osv.dev/vulnerability/CVE-2026-74656

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74656/
Summaries are generated and may contain mistakes; the official description above is the record.