CVE-2025-39817

Kernel memory disclosure and crash via GUID underflow

Published Sep 16, 2025 · Updated Sep 16, 2025

Out-of-bounds read in Linux efivarfs from 3.9 allows local users to disclose kernel memory or crash the system via parallel lookups. In efivarfs_d_compare, a filename shorter than the EFI GUID suffix makes the guid offset negative; conversion to size_t then gives memcmp an effectively unbounded length. An unprivileged local user can race invalid lookups against a mounted efivarfs instance, using comparison behavior as a kernel-memory oracle or driving an oops or panic.

CVSS severity7.1
High
EPSS probability0.16%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds read in Linux efivarfs from 3.9 allows local users to disclose kernel memory or crash the system via parallel lookups. In efivarfs_d_compare, a filename shorter than the EFI GUID suffix makes the guid offset negative; conversion to size_t then gives memcmp an effectively unbounded length. An unprivileged local user can race invalid lookups against a mounted efivarfs instance, using comparison behavior as a kernel-memory oracle or driving an oops or panic.

The record

CVE
CVE-2025-39817
Published
Sep 16, 2025
Updated
Sep 16, 2025
Vendor
The Linux Kernel Organization
Product
Linux
Classifications
T1499
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 16, 2025CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=3.9
  2. Affected versionversion=688289c4b745c018b3449b4b4c5a2030083c8eaf
  3. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <0f63fbabeaaaaaaf5b742a2f4c1b4590d50bf1f6
  4. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <568e7761279b99c6daa3002290fd6d8047ddb6d2
  5. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <71581a82f38e5a4d807d71fc1bb59aead80ccf95
  6. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <794399019301944fd6d2e0d7a51b3327e26c410e
  7. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <925599eba46045930b850a98ae594d2e3028ac40
  8. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <a6358f8cf64850f3f27857b8ed8c1b08cfc4685c
  9. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <c2925cd6207079c3f4d040d082515db78d63afbf
  10. Affected versionversion=da27a24383b2b10bf6ebd0db29b325548aafecb4 <d7f5e35e70507d10cbaff5f9e194ed54c4ee14f7

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

The Linux Kernel Organization · Linuxversion=3.9; version=688289c4b745c018b3449b4b4c5a2030083c8eaf; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <0f63fbabeaaaaaaf5b742a2f4c1b4590d50bf1f6; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <568e7761279b99c6daa3002290fd6d8047ddb6d2; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <71581a82f38e5a4d807d71fc1bb59aead80ccf95; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <794399019301944fd6d2e0d7a51b3327e26c410e; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <925599eba46045930b850a98ae594d2e3028ac40; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <a6358f8cf64850f3f27857b8ed8c1b08cfc4685c; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <c2925cd6207079c3f4d040d082515db78d63afbf; version=da27a24383b2b10bf6ebd0db29b325548aafecb4 <d7f5e35e70507d10cbaff5f9e194ed54c4ee14f7

Published CVSS scores

7.1Linux kernel CNACVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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