# CVE-2026-23127: perf: Fix refcount warning on event->mmap_count increment

> A use-after-free vulnerability in the Linux kernel perf subsystem allows local attackers to trigger a refcount warning and potential system instability.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/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:** 14 Feb 2026
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2026-23127, DEBIAN-CVE-2026-23127, GHSA-5pm5-3fx7-4f4r, UBUNTU-CVE-2026-23127

## What is CVE-2026-23127?

CVE-2026-23127 is a medium-severity use after free vulnerability in Linux kernel, published on 14 Feb 2026. 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:

perf: Fix refcount warning on event->mmap_count increment

When calling refcount_inc(&event->mmap_count) inside perf_mmap_rb(), the
following warning is triggered:

        refcount_t: addition on 0; use-after-free.
        WARNING: lib/refcount.c:25

PoC:

    struct perf_event_attr attr = {0};
    int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
    mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    int victim = syscall(__NR_perf_event_open, &attr, 0, -1, fd,
                         PERF_FLAG_FD_OUTPUT);
    mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, victim, 0);

This occurs when creating a group member event with the flag
PERF_FLAG_FD_OUTPUT. The group leader should be mmap-ed and then mmap-ing
the event triggers the warning.

Since the event has copied the output_event in perf_event_set_output(),
event->rb is set. As a result, perf_mmap_rb() calls
refcount_inc(&event->mmap_count) when event->mmap_count = 0.

Disallow the case when event->mmap_count = 0. This also prevents two
events from updating the same user_page.

## Which versions are affected by CVE-2026-23127?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-23127?

Apply the vendor's patch.

## Is CVE-2026-23127 being exploited?

There is no known exploitation of CVE-2026-23127 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-2026-23127 and fix it if it is affected.

What it is: CVE-2026-23127 is a medium-severity use after free vulnerability in Linux kernel, published on 14 Feb 2026. 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-2026-23127 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-2026-23127/ (Markdown: https://insights.sbomify.com/CVE-2026-23127/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23127
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23127
- OSV: https://osv.dev/vulnerability/CVE-2026-23127

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23127/
Summaries are generated and may contain mistakes; the official description above is the record.