# CVE-2026-74478: um: vector: fix use-after-free in vector_mmsg_rx()

> A use-after-free in the Linux kernel's vector driver allows remote attackers to cause denial of service or execute code via crafted GRE or L2TPv3 packets.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Aug 2026
- **Updated:** 19 Aug 2026
- **Also known as:** BELL-CVE-2026-74478, DEBIAN-CVE-2026-74478, GHSA-mhmm-jqvj-99w7, UBUNTU-CVE-2026-74478

## What is CVE-2026-74478?

CVE-2026-74478 is a critical use after free vulnerability in Linux, published on 15 Aug 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:

um: vector: fix use-after-free in vector_mmsg_rx()

When vector_mmsg_rx() discards a packet whose overlay header fails
verify_header(), it frees the skb and continues the loop:

	if (header_check < 0) {
		dev_kfree_skb_irq(skb);
		vp->estats.rx_encaps_errors++;
		continue;
	}

The normal and short-packet paths fall through to the bottom of the
loop body, which clears the consumed slot and advances the cursors:

	(*skbuff_vector) = NULL;
	mmsg_vector++;
	skbuff_vector++;

The verify_header() < 0 path skips that via continue, so the freed skb
is left in skbuff_vector[] and the cursors do not advance. The next
iteration reads the same slot, gets the freed skb, and frees it again,
producing a refcount underflow / use-after-free in the RX path.

Discard the slot the same way the other paths do before continuing.

Only transports whose verify_header() can return negative are affected:
GRE and L2TPv3 do so on a cookie/session-id mismatch (raw/tap do not),
so any peer on such a transport can trigger it without authentication.

## Which versions are affected by CVE-2026-74478?


## How do I fix CVE-2026-74478?

Apply the vendor's patch.

## Is CVE-2026-74478 being exploited?

There is no known exploitation of CVE-2026-74478 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74478 and fix it if it is affected.

What it is: CVE-2026-74478 is a critical use after free vulnerability in Linux, published on 15 Aug 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-2026-74478 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-2026-74478/ (Markdown: https://insights.sbomify.com/CVE-2026-74478/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74478
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74478
- OSV: https://osv.dev/vulnerability/CVE-2026-74478

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74478/
Summaries are generated and may contain mistakes; the official description above is the record.