# CVE-2025-39840: audit: fix out-of-bounds read in audit_compare_dname_path()

> An out-of-bounds read in the Linux kernel audit subsystem can cause a crash or leak memory when specific filesystem events occur.

- **Severity:** High 7.1 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/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 Sep 2025
- **Updated:** 5 Aug 2026
- **Also known as:** BELL-CVE-2025-39840, DEBIAN-CVE-2025-39840, GHSA-cwjm-393v-32x2, UBUNTU-CVE-2025-39840

## What is CVE-2025-39840?

CVE-2025-39840 is a high-severity memory corruption vulnerability in Linux kernel, published on 19 Sep 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:

audit: fix out-of-bounds read in audit_compare_dname_path()

When a watch on dir=/ is combined with an fsnotify event for a
single-character name directly under / (e.g., creating /a), an
out-of-bounds read can occur in audit_compare_dname_path().

The helper parent_len() returns 1 for "/". In audit_compare_dname_path(),
when parentlen equals the full path length (1), the code sets p = path + 1
and pathlen = 1 - 1 = 0. The subsequent loop then dereferences
p[pathlen - 1] (i.e., p[-1]), causing an out-of-bounds read.

Fix this by adding a pathlen > 0 check to the while loop condition
to prevent the out-of-bounds access.

[PM: subject tweak, sign-off email fixes]

Weakness: CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2025-39840?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-39840?

Apply the vendor's patch.

## Is CVE-2025-39840 being exploited?

There is no known exploitation of CVE-2025-39840 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-2025-39840 and fix it if it is affected.

What it is: CVE-2025-39840 is a high-severity memory corruption vulnerability in Linux kernel, published on 19 Sep 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-2025-39840 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-2025-39840/ (Markdown: https://insights.sbomify.com/CVE-2025-39840/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39840
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39840
- OSV: https://osv.dev/vulnerability/CVE-2025-39840

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39840/
Summaries are generated and may contain mistakes; the official description above is the record.