# CVE-2026-99133: net/iucv: fix the recvmsg window update


- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 28 Oct 2026
- **Updated:** 28 Oct 2026
- **Also known as:** AZL-111251, BELL-CVE-2026-99133, DEBIAN-CVE-2026-99133, GHSA-j8jx-fcfm-v8gr, UBUNTU-CVE-2026-99133

## What is CVE-2026-99133?

CVE-2026-99133 is a vulnerability in Linux, published on 28 Oct 2026. 

### Official description

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

net/iucv: fix the recvmsg window update

iucv_sock_recvmsg() sends the HiperSockets-only AF_IUCV_FLAG_WIN without
testing the transport, so on a classic z/VM socket iucv_send_ctrl() sizes
the skb through a NULL iucv->hs_dev. SO_MSGLIMIT accepts 1, so msglimit / 2
is zero and one recvmsg() on its own socket is enough for an unprivileged
process to take a spurious disconnect.

It also calls iucv_send_ctrl() under spin_lock_bh(&message_q.lock), which
allocates GFP_KERNEL inside a section the code treats as atomic. Sending
outside that lock lets two recvmsg() reach afiucv_hs_send() at once, where
msg_recv is sampled for the advertised window and subtracted after
dev_queue_xmit() -- and sendmsg reaches that counter under lock_sock()
while recvmsg holds no socket lock, so both can subtract the same value,
the counter goes negative and the credit reaches the peer twice.

Test the transport, claim the credit with atomic_xchg() after the last
error exit and hand it back if the transmit fails, and send once the lock
is dropped.

## Which versions are affected by CVE-2026-99133?


## How do I fix CVE-2026-99133?

Upgrade to a fixed version.

## Is CVE-2026-99133 being exploited?

There is no known exploitation of CVE-2026-99133 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-99133 and fix it if it is affected.

What it is: CVE-2026-99133 is a vulnerability in Linux, published on 28 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-99133 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-2026-99133/ (Markdown: https://insights.sbomify.com/CVE-2026-99133/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-99133
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-99133
- OSV: https://osv.dev/vulnerability/CVE-2026-99133

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-99133/
Summaries are generated and may contain mistakes; the official description above is the record.