# CVE-2025-39817: efivarfs: Fix slab-out-of-bounds in efivarfs_d_compare

> A local attacker can trigger a slab-out-of-bounds read in the Linux kernel efivarfs module via parallel lookups, causing a system crash.

- **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:** 16 Sep 2025
- **Updated:** 5 Aug 2026
- **Also known as:** AZL-67398, AZL-74712, BELL-CVE-2025-39817, DEBIAN-CVE-2025-39817, GHSA-m685-84xg-fvfj, ROOT-OS-DEBIAN-11-CVE-2025-39817, ROOT-OS-DEBIAN-12-CVE-2025-39817, ROOT-OS-DEBIAN-13-CVE-2025-39817, ROOT-OS-UBUNTU-2204-CVE-2025-39817, ROOT-OS-UBUNTU-2404-CVE-2025-39817, UBUNTU-CVE-2025-39817

## What is CVE-2025-39817?

CVE-2025-39817 is a high-severity memory corruption vulnerability in Debian Debian Linux, published on 16 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:

efivarfs: Fix slab-out-of-bounds in efivarfs_d_compare

Observed on kernel 6.6 (present on master as well):

  BUG: KASAN: slab-out-of-bounds in memcmp+0x98/0xd0
  Call trace:
   kasan_check_range+0xe8/0x190
   __asan_loadN+0x1c/0x28
   memcmp+0x98/0xd0
   efivarfs_d_compare+0x68/0xd8
   __d_lookup_rcu_op_compare+0x178/0x218
   __d_lookup_rcu+0x1f8/0x228
   d_alloc_parallel+0x150/0x648
   lookup_open.isra.0+0x5f0/0x8d0
   open_last_lookups+0x264/0x828
   path_openat+0x130/0x3f8
   do_filp_open+0x114/0x248
   do_sys_openat2+0x340/0x3c0
   __arm64_sys_openat+0x120/0x1a0

If dentry->d_name.len < EFI_VARIABLE_GUID_LEN , 'guid' can become
negative, leadings to oob. The issue can be triggered by parallel
lookups using invalid filename:

  T1			T2
  lookup_open
   ->lookup
    simple_lookup
     d_add
     // invalid dentry is added to hash list

			lookup_open
			 d_alloc_parallel
			  __d_lookup_rcu
			   __d_lookup_rcu_op_compare
			    hlist_bl_for_each_entry_rcu
			    // invalid dentry can be retrieved
			     ->d_compare
			      efivarfs_d_compare
			      // oob

Fix it by checking 'guid' before cmp.

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

## Which versions are affected by CVE-2025-39817?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-39817?

Apply the vendor's patch.

## Is CVE-2025-39817 being exploited?

There is no known exploitation of CVE-2025-39817 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39817 and fix it if it is affected.

What it is: CVE-2025-39817 is a high-severity memory corruption vulnerability in Debian Debian Linux, published on 16 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-2025-39817 in the wild and no public exploit code.

Affected product: Debian Debian 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-2025-39817/ (Markdown: https://insights.sbomify.com/CVE-2025-39817/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39817
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39817
- OSV: https://osv.dev/vulnerability/CVE-2025-39817

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39817/
Summaries are generated and may contain mistakes; the official description above is the record.