# CVE-2026-72493: net: serialize netif_running() check in enqueue_to_backlog()

> A race condition in the Linux kernel networking stack allows a use-after-free, potentially leading to code execution or denial of service.

- **Severity:** Critical 9.9 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-96054, BELL-CVE-2026-72493, DEBIAN-CVE-2026-72493, GHSA-hhj5-7x2f-mw3r, UBUNTU-CVE-2026-72493

## What is CVE-2026-72493?

CVE-2026-72493 is a critical use after free vulnerability in Linux, published on 15 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

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

net: serialize netif_running() check in enqueue_to_backlog()

Syzbot reported a KASAN slab-use-after-free in fib_rules_lookup().

The root cause is a race condition where packets can escape the backlog
flushing during device unregistration (e.g., during netns exit).

Commit e9e4dd3267d0 ("net: do not process device backlog during unregistration")
introduced a lockless netif_running() check in enqueue_to_backlog() to
prevent queuing packets to an unregistering device.

However, this creates a TOCTOU race window.

A lockless transmitter (like veth_xmit) can pass
the check before dev_close() clears IFF_UP. If the transmitter is then
delayed, flush_all_backlogs() can run and finish before the transmitter
grabs the backlog lock and queues the packet. The packet then escapes
the flush and triggers UAF later when processed.

Fix this by moving the netif_running() check inside the backlog lock.
This serializes the check with the flush work (which also grabs the lock).
We then either queue the packet before the flush runs (so it gets flushed),
or check netif_running() after the flush/close completes (so it gets dropped).

## Which versions are affected by CVE-2026-72493?


## How do I fix CVE-2026-72493?

Apply the vendor's patch.

## Is CVE-2026-72493 being exploited?

There is no known exploitation of CVE-2026-72493 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72493 and fix it if it is affected.

What it is: CVE-2026-72493 is a critical use after free vulnerability in Linux, published on 15 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-72493 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-2026-72493/ (Markdown: https://insights.sbomify.com/CVE-2026-72493/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72493
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72493
- OSV: https://osv.dev/vulnerability/CVE-2026-72493

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72493/
Summaries are generated and may contain mistakes; the official description above is the record.