Vulnerability in Kernel (CVE-2025-39821)

Summary

Vulnerability in Kernel (CVE-2025-39821). Successful exploitation can lead to full system takeover. Mitigation: upgrade to 6.16.5 or later.

Original description

The description as published by the source.

In the Linux kernel, the following vulnerability has been resolved:

perf: Avoid undefined behavior from stopping/starting inactive events

Calling pmu->start()/stop() on perf events in PERF_EVENT_STATE_OFF can
leave event->hw.idx at -1. When PMU drivers later attempt to use this
negative index as a shift exponent in bitwise operations, it leads to UBSAN
shift-out-of-bounds reports.

The issue is a logical flaw in how event groups handle throttling when some
members are intentionally disabled. Based on the analysis and the
reproducer provided by Mark Rutland (this issue on both arm64 and x86-64).

The scenario unfolds as follows:

A group leader event is configured with a very aggressive sampling
period (e.g., sample_period = 1). This causes frequent interrupts and
triggers the throttling mechanism.
A child event in the same group is created in a disabled state
(.disabled = 1). This event remains in PERF_EVENT_STATE_OFF.
Since it hasn't been scheduled onto the PMU, its event->hw.idx remains
initialized at -1.
When throttling occurs, perf_event_throttle_group() and later
perf_event_unthrottle_group() iterate through all siblings, including
the disabled child event.
perf_event_throttle()/unthrottle() are called on this inactive child
event, which then call event->pmu->start()/stop().
The PMU driver receives the event with hw.idx == -1 and attempts to
use it as a shift exponent. e.g., in macros like PMCNTENSET(idx),
lea…

Source: OSV.dev (Open Source Vulnerabilities) NVD OSV

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2025-39821 has been found in Kernel.
Successful exploitation can lead to full system takeover.
What to do: upgrade Kernel to 6.16.5 or later.
If unsure, ask your IT team or search "Kernel CVE-2025-39821" on the vendor's site.

CVE-2025-39821 (Kernel)
Attack vector: local / no user interaction
Patched: 6.16.5 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (unclassified) exists in Kernel.

📍 Affected scope

Check the vendor advisory for the affected versions of Kernel.

🔥 Severity

Severity: ?. Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to 6.16.5.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'Kernel' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

1
Identify exposure identify
```
grep -r 'Kernel' . | grep -v node_modules
```
リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で Kernel を grep し、稼働しているサービス・バージョンを把握する。
2
Apply patch patch
```
Upgrade Kernel to 6.16.5
```
ステージング環境で 6.16.5 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
3
Post-deployment verification verify
```
Confirm patched version is live in production
```
パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

Linux Kernel

[{"type":"ECOSYSTEM","events":[{"introduced":"6.16.0"},{"fixed":"6.16.5"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

BellSoft Alpaquita BELL-CVE-2025-39821 linux-lts Fixed in: 6.18.35-r1
Debian DEBIAN-CVE-2025-39821 linux Fixed in: 6.16.5-1
Ubuntu UBUNTU-CVE-2025-39821 linux-hwe-edge