# CVE-2025-39813: ftrace: Fix potential warning in trace_printk_seq during ftrace_dump

> A race condition in the Linux kernel ftrace subsystem can trigger a warning and potential denial of service when concurrent readers access trace data.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:L/AC:H/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:** 16 Sep 2025
- **Updated:** 12 May 2026
- **Also known as:** AZL-67383, AZL-74709, BELL-CVE-2025-39813, DEBIAN-CVE-2025-39813, GHSA-cfrf-hx42-xr6f, ROOT-OS-DEBIAN-11-CVE-2025-39813, ROOT-OS-UBUNTU-2204-CVE-2025-39813, ROOT-OS-UBUNTU-2404-CVE-2025-39813, UBUNTU-CVE-2025-39813

## What is CVE-2025-39813?

CVE-2025-39813 is a medium-severity vulnerability in Debian Debian Linux, published on 16 Sep 2025. 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:

ftrace: Fix potential warning in trace_printk_seq during ftrace_dump

When calling ftrace_dump_one() concurrently with reading trace_pipe,
a WARN_ON_ONCE() in trace_printk_seq() can be triggered due to a race
condition.

The issue occurs because:

CPU0 (ftrace_dump)                              CPU1 (reader)
echo z > /proc/sysrq-trigger

!trace_empty(&iter)
trace_iterator_reset(&iter) <- len = size = 0
                                                cat /sys/kernel/tracing/trace_pipe
trace_find_next_entry_inc(&iter)
  __find_next_entry
    ring_buffer_empty_cpu <- all empty
  return NULL

trace_printk_seq(&iter.seq)
  WARN_ON_ONCE(s->seq.len >= s->seq.size)

In the context between trace_empty() and trace_find_next_entry_inc()
during ftrace_dump, the ring buffer data was consumed by other readers.
This caused trace_find_next_entry_inc to return NULL, failing to populate
`iter.seq`. At this point, due to the prior trace_iterator_reset, both
`iter.seq.len` and `iter.seq.size` were set to 0. Since they are equal,
the WARN_ON_ONCE condition is triggered.

Move the trace_printk_seq() into the if block that checks to make sure the
return value of trace_find_next_entry_inc() is non-NULL in
ftrace_dump_one(), ensuring the 'iter.seq' is properly populated before
subsequent operations.

Weakness: CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization)

## Which versions are affected by CVE-2025-39813?

It affects 2 products.


Products: Debian Debian Linux, Linux kernel.

## How do I fix CVE-2025-39813?

Apply the vendor's patch.

## Is CVE-2025-39813 being exploited?

There is no known exploitation of CVE-2025-39813 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-39813 and fix it if it is affected.

What it is: CVE-2025-39813 is a medium-severity vulnerability in Debian Debian Linux, published on 16 Sep 2025. 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-2025-39813 in the wild and no public exploit code.

Affected product: Debian Debian Linux. 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-2025-39813/ (Markdown: https://insights.sbomify.com/CVE-2025-39813/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-39813
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-39813
- OSV: https://osv.dev/vulnerability/CVE-2025-39813

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-39813/
Summaries are generated and may contain mistakes; the official description above is the record.