# CVE-2025-38591: bpf: Reject narrower access to pointer ctx fields

> A flaw in the Linux kernel BPF verifier allows a kernel warning and potential crash when accessing pointer context fields with narrower sizes.

- **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:** 19 Aug 2025
- **Updated:** 15 Oct 2026
- **Also known as:** AZL-66440, AZL-71033, BELL-CVE-2025-38591, DEBIAN-CVE-2025-38591, GHSA-cgjw-h652-c9q2, ROOT-OS-DEBIAN-11-CVE-2025-38591, ROOT-OS-DEBIAN-12-CVE-2025-38591, ROOT-OS-DEBIAN-13-CVE-2025-38591, ROOT-OS-UBUNTU-2204-CVE-2025-38591, ROOT-OS-UBUNTU-2404-CVE-2025-38591, UBUNTU-CVE-2025-38591

## What is CVE-2025-38591?

CVE-2025-38591 is a medium-severity denial of service vulnerability in Linux kernel, published on 19 Aug 2025. 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:

bpf: Reject narrower access to pointer ctx fields

The following BPF program, simplified from a syzkaller repro, causes a
kernel warning:

    r0 = *(u8 *)(r1 + 169);
    exit;

With pointer field sk being at offset 168 in __sk_buff. This access is
detected as a narrower read in bpf_skb_is_valid_access because it
doesn't match offsetof(struct __sk_buff, sk). It is therefore allowed
and later proceeds to bpf_convert_ctx_access. Note that for the
"is_narrower_load" case in the convert_ctx_accesses(), the insn->off
is aligned, so the cnt may not be 0 because it matches the
offsetof(struct __sk_buff, sk) in the bpf_convert_ctx_access. However,
the target_size stays 0 and the verifier errors with a kernel warning:

    verifier bug: error during ctx access conversion(1)

This patch fixes that to return a proper "invalid bpf_context access
off=X size=Y" error on the load instruction.

The same issue affects multiple other fields in context structures that
allow narrow access. Some other non-affected fields (for sk_msg,
sk_lookup, and sockopt) were also changed to use bpf_ctx_range_ptr for
consistency.

Note this syzkaller crash was reported in the "Closes" link below, which
used to be about a different bug, fixed in
commit fce7bd8e385a ("bpf/verifier: Handle BPF_LOAD_ACQ instructions
in insn_def_regno()"). Because syzbot somehow confused the two bugs,
the new crash and repro didn't get reported to the mailing list.

## Which versions are affected by CVE-2025-38591?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-38591?

Apply the vendor's patch.

## Is CVE-2025-38591 being exploited?

There is no known exploitation of CVE-2025-38591 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-2025-38591 and fix it if it is affected.

What it is: CVE-2025-38591 is a medium-severity denial of service vulnerability in Linux kernel, published on 19 Aug 2025. 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-2025-38591 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-2025-38591/ (Markdown: https://insights.sbomify.com/CVE-2025-38591/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-38591
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-38591
- OSV: https://osv.dev/vulnerability/CVE-2025-38591

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-38591/
Summaries are generated and may contain mistakes; the official description above is the record.