CVE-2026-23159

Medium · 5.5

CVSS

5.5

EPSS

0.1%

Published

Feb 14, 2026

CISA due date

—

Cloudanix analysis

Schedule a fix SSVC: Track

A flaw in Linux performance tracking could make the whole computer crash when watching programs that are shutting down.

Exploitation: Local Privilege
Attacker needs: Low Privilege
Complexity: Low
Impact: Denial Of Service
Component: OS kernel
Class: Memory Corruption
Remediation: Upgrade
Effort: Medium
Patch risk: Low

What it actually does

NULL pointer dereference in the perf subsystem when attempting user-space stack traces on a task that is exiting and has freed its mm field, despite flag checks indicating a user task.

Why it matters to the business

Unexpected kernel crashes during performance profiling on systems running vulnerable kernels, leading to downtime and potential loss of observability data in production environments with high task churn.

How an attack unfolds

An attacker with local access triggers perf sampling or stack tracing on a system with rapidly exiting tasks; the race window between flag checks and mm access allows a NULL dereference, crashing the kernel. No named threat actors or ransomware usage reported.

How to fix it

Upgrade the Linux kernel to version 6.6.123 or later, or apply commit d84a4836dc246b7dc244e46a08ff992956b68db0 from the stable git repository.

How to confirm the fix worked

Verify the presence of the is_user_task() helper and updated flag-plus-mm checks in the perf sched code after applying the fix; test perf stack tracing during task exit scenarios.

Typical attack path

Obtain local user access on a Linux system
Run or trigger perf events targeting user tasks
Force rapid task creation and exit to hit the mm-free window
Cause kernel NULL pointer dereference and panic

How to detect it

Search kernel logs and dmesg for oops or panic entries mentioning perf, sched, or NULL pointer dereference during task exit; correlate with perf process activity.

kernel: perf: NULL pointer dereference
is_user_task
PF_KTHREAD
PF_USER_WORKER

What your security team should do

Prioritize kernel upgrades on systems using perf for production monitoring
Monitor EPSS trend for any increase in exploit likelihood
Restrict perf access via capabilities or groups until patched

The record

In the Linux kernel, the following vulnerability has been resolved: perf: sched: Fix perf crash with new is_user_task() helper In order to do a user space stacktrace the current task needs to be a user task that has executed in user space. It use to be possible to test if a task is a user task or not by simply checking the task_struct mm field. If it was non NULL, it was a user task and if not it was a kernel task. But things have changed over time, and some kernel tasks now have their own mm field. An idea was made to instead test PF_KTHREAD and two functions were used to wrap this check in case it became more complex to test if a task was a user task or not[1]. But this was rejected and the C code simply checked the PF_KTHREAD directly. It was later found that not all kernel threads set PF_KTHREAD. The io-uring helpers instead set PF_USER_WORKER and this needed to be added as well. But checking the flags is still not enough. There's a very small window when a task exits that it frees its mm field and it is set back to NULL. If perf were to trigger at this moment, the flags test would say its a user space task but when perf would read the mm field it would crash with at NULL pointer dereference. Now there are flags that can be used to test if a task is exiting, but they are set in areas that perf may still want to profile the user space task (to see where it exited). The only real test is to check both the flags and the mm field. Instead of making this modification in every location, create a new is_user_task() helper function that does all the tests needed to know if it is safe to read the user space memory or not. [1] https://lore.kernel.org/all/20250425204120.639530125@goodmis.org/

Affected packages

Ecosystem	Package	Introduced	Fixed
Linux	Kernel	0	6.6.123