Critical Vulnerability in Linux Kernel: mptcp Path-Manager Fullmesh Flag Issue
Summary
CVE-2025-21706: In the Linux kernel, the following vulnerability has been resolved: mptcp: pm: only set fullmesh for subflow endp With the in-kernel path-manager, it is possible to change the 'fullmesh' flag. Affects linux versions < 6.1.129-1. Fixed in 6.1.129-1. How to fix ↓
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Background
The Linux kernel is the core component of many operating systems, providing essential services for hardware and software interaction. It supports various functionalities, including process management, memory management, and device control. The kernel is modular, allowing for the addition of features through loadable modules. This vulnerability specifically affects the MultiPath TCP (mptcp) implementation, which enhances TCP by allowing multiple paths for data transmission. The vulnerability arises from improper handling of the 'fullmesh' flag in the path-manager, potentially leading to unintended behavior in subflow endpoints.
Vulnerability Detail
In the Linux kernel, a vulnerability has been identified in the mptcp path-manager where the 'fullmesh' flag can be incorrectly set for implicit endpoints. This issue arises from the permissive nature of the set_flags() hook, which allows the flag to be applied inappropriately. The vulnerability was discovered by syzbot, leading to warnings during execution. If exploited, this could result in instability or unexpected behavior in network communications, particularly affecting systems relying on MultiPath TCP for data transmission.
How To Fix
Minimum fixed version
6.1.129-1
This is the first release that contains the fix.
sudo apt-get install --only-upgrade linuxTip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, users can disable MultiPath TCP functionality if it is not critical for their operations. This can be done by modifying the configuration settings in the kernel or by avoiding the use of mptcp in applications until the patch is applied.
Conclusion
The CVE-2025-21706 vulnerability in the Linux kernel's mptcp path-manager poses a significant risk by allowing improper setting of the 'fullmesh' flag. Users are advised to update their systems to the patched version of the kernel to mitigate this risk. For those unable to update immediately, disabling MultiPath TCP can serve as a temporary solution.
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FAQ
CVE-2025-21706 is a vulnerability in the Linux kernel's mptcp implementation that allows the 'fullmesh' flag to be incorrectly set for implicit endpoints, potentially leading to network instability.
You can use Vulert, which monitors open-source vulnerabilities in your software. By uploading your manifest file like package-lock.json, you can check for vulnerabilities without needing to install any software.
The vulnerability could lead to unexpected behavior in network communications, including instability and performance issues in systems using MultiPath TCP.
Yes, a patch has been released for the Linux kernel to address this vulnerability. Users are encouraged to update their kernel to the latest version.
If an immediate update is not possible, consider disabling MultiPath TCP functionality as a temporary workaround until the patch can be applied.
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