# CVE-2025-39673: Linux Kernel race condition in ppp_fill_forward_path

Source: https://junglewise.ai/threats/cve-2025-39673-linux-kernel-race-condition-in-ppp-fill-forward-path  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2025-09-05. Updated: 2026-07-30  
License: CC BY 4.0 with a link back

CVE-2025-39673 · Severity: critical · CVSS 9.8 · Published 2025-09-05

A vulnerability exists in the Linux kernel's Point-to-Point Protocol (PPP) implementation, which is used for establishing network connections. Due to a flaw in how the system handles internal data lists, a race condition can occur that leads to a system crash (kernel panic). This could allow an attacker to disrupt network services or potentially gain unauthorized access to the system.

Technologies: [Linux Kernel](https://junglewise.ai/threats/technologies/kernel), [Siemens SIMATIC CN 4100](https://junglewise.ai/threats/technologies/simatic-cn-4100). Vendors: [Linux](https://junglewise.ai/threats/vendors/linux), [Siemens](https://junglewise.ai/threats/vendors/siemens).

## Quick answers

**Is CVE-2025-39673 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2025-39673 being exploited in the wild.

**How severe is CVE-2025-39673?** Critical severity, with a CVSS score of 9.8, published 5 September 2025.

**Which products and versions are affected?** Linux Kernel 5.13 to 5.15.190, 5.16 to 6.1.149, 6.2 to 6.6.103, 6.7 to 6.12.44, 6.13 to 6.16.4, 5.13 to 6.16.4; Siemens SIMATIC CN 4100 before V5.0.

**Is there a fix for CVE-2025-39673?** A patch was published on 2025-08-14: Initial patch submitted by Qingfang Deng Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2025-39673 attacked?** It is attacked over the network, in an operating system.

**Am I affected by CVE-2025-39673?** Check whether you run Linux Kernel or Siemens SIMATIC CN 4100 at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A race condition exists in the ppp_fill_forward_path() function within the Linux kernel's PPP implementation (drivers/net/ppp/ppp_generic.c). The vulnerability arises because the ppp->channels list can be modified between a list_empty() check and a list_first_entry() call without holding the ppp_lock, potentially leading to access of a freed entry. Additionally, pch->chan can be set to NULL during unregistration before the channel is removed from the list, leading to a NULL pointer dereference. Attackers can exploit this to trigger a kernel panic. The fix involves implementing a lockless RCU approach using list_first_or_null_rcu(), RCU list modification variants, and proper NULL checks for pch->chan.

## Affected products

- Linux Linux Kernel 5.13 to 6.16.4
- Siemens SIMATIC CN 4100 before V5.0

## Timeline

- 2025-08-14: patched: Initial patch submitted by Qingfang Deng
- 2025-09-05: disclosed: CVE-2025-39673 published

## References

- [https://git.kernel.org/stable/c/0417adf367a0af11adf7ace849af4638cfb573f7](https://git.kernel.org/stable/c/0417adf367a0af11adf7ace849af4638cfb573f7)
- [https://git.kernel.org/stable/c/0f1630be6fcca3f0c63e4b242ad202e5cde28a40](https://git.kernel.org/stable/c/0f1630be6fcca3f0c63e4b242ad202e5cde28a40)
- [https://git.kernel.org/stable/c/94731cc551e29511d85aa8dec61a6c071b1f2430](https://git.kernel.org/stable/c/94731cc551e29511d85aa8dec61a6c071b1f2430)
- [https://git.kernel.org/stable/c/9a1969fbffc1f1900d92d7594b1b7d8d72ef3dc7](https://git.kernel.org/stable/c/9a1969fbffc1f1900d92d7594b1b7d8d72ef3dc7)
- [https://git.kernel.org/stable/c/ca18d751bcc9faf5b7e82e9fae1223d103928181](https://git.kernel.org/stable/c/ca18d751bcc9faf5b7e82e9fae1223d103928181)
- [https://git.kernel.org/stable/c/f97f6475fdcb3c28ff3c55cc4b7bde632119ec08](https://git.kernel.org/stable/c/f97f6475fdcb3c28ff3c55cc4b7bde632119ec08)
- [https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html](https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html)

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