# CVE-2026-43244: kcm: fix zero-frag skb in frag_list on partial sendmsg error

> A local attacker can trigger a kernel warning and potential denial of service by sending partial messages via KCM sockets.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/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:** 6 May 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-85857, BELL-CVE-2026-43244, DEBIAN-CVE-2026-43244, GHSA-f26p-p2hg-c5qg, ROOT-OS-DEBIAN-11-CVE-2026-43244, ROOT-OS-DEBIAN-12-CVE-2026-43244, ROOT-OS-UBUNTU-2204-CVE-2026-43244, ROOT-OS-UBUNTU-2404-CVE-2026-43244, UBUNTU-CVE-2026-43244

## What is CVE-2026-43244?

CVE-2026-43244 is a medium-severity vulnerability in Linux kernel, published on 6 May 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:

kcm: fix zero-frag skb in frag_list on partial sendmsg error

Syzkaller reported a warning in kcm_write_msgs() when processing a
message with a zero-fragment skb in the frag_list.

When kcm_sendmsg() fills MAX_SKB_FRAGS fragments in the current skb,
it allocates a new skb (tskb) and links it into the frag_list before
copying data. If the copy subsequently fails (e.g. -EFAULT from
user memory), tskb remains in the frag_list with zero fragments:

  head skb (msg being assembled, NOT yet in sk_write_queue)
  +-----------+
  | frags[17] |  (MAX_SKB_FRAGS, all filled with data)
  | frag_list-+--> tskb
  +-----------+    +----------+
                   | frags[0] |  (empty! copy failed before filling)
                   +----------+

For SOCK_SEQPACKET with partial data already copied, the error path
saves this message via partial_message for later completion. For
SOCK_SEQPACKET, sock_write_iter() automatically sets MSG_EOR, so a
subsequent zero-length write(fd, NULL, 0) completes the message and
queues it to sk_write_queue. kcm_write_msgs() then walks the
frag_list and hits:

  WARN_ON(!skb_shinfo(skb)->nr_frags)

TCP has a similar pattern where skbs are enqueued before data copy
and cleaned up on failure via tcp_remove_empty_skb(). KCM was
missing the equivalent cleanup.

Fix this by tracking the predecessor skb (frag_prev) when allocating
a new frag_list entry. On error, if the tail skb has zero frags,
use frag_prev to unlink and free it in O(1) without walking the
singly-linked frag_list. frag_prev is safe to dereference because
the entire message chain is only held locally (or in kcm->seq_skb)
and is not added to sk_write_queue until MSG_EOR, so the send path
cannot free it underneath us.

Also change the WARN_ON to WARN_ON_ONCE to avoid flooding the log
if the condition is somehow hit repeatedly.

There are currently no KCM selftests in the kernel tree; a simple
reproducer is available at [1].

[1] https://gist.github.com/mrpre/a94d431c757e8d6f168f4dd1a3749daa

Weakness: CWE-401 (Missing Release of Memory after Effective Lifetime)

## Which versions are affected by CVE-2026-43244?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-43244?

Apply the vendor's patch.

## Is CVE-2026-43244 being exploited?

There is no known exploitation of CVE-2026-43244 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-43244 and fix it if it is affected.

What it is: CVE-2026-43244 is a medium-severity vulnerability in Linux kernel, published on 6 May 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-2026-43244 in the wild and no public exploit code.

Affected product: Linux kernel. 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-2026-43244/ (Markdown: https://insights.sbomify.com/CVE-2026-43244/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-43244
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43244
- OSV: https://osv.dev/vulnerability/CVE-2026-43244

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-43244/
Summaries are generated and may contain mistakes; the official description above is the record.