Critical MPTCP Disconnect Handling Vulnerability in Linux Kernel
Summary
CVE-2025-21705: In the Linux kernel, the following vulnerability has been resolved: mptcp: handle fastopen disconnect correctly Syzbot was able to trigger a data stream corruption: WARNING: CPU: 0 PID: 9846 at net/mptcp/protocol.c:1024... 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, responsible for managing hardware resources and providing essential services to applications. It supports various functionalities, including process management, memory management, and networking. The kernel's implementation of MultiPath TCP (MPTCP) allows for more efficient data transmission over multiple paths, enhancing performance and reliability. However, a recent vulnerability has been identified in the MPTCP protocol's handling of disconnect scenarios, particularly during connect FASTOPEN errors.
Vulnerability Detail
The vulnerability, identified as CVE-2025-21705, arises from improper handling of disconnect events generated internally by the MPTCP protocol when connect FASTOPEN errors occur. This flaw can lead to data stream corruption, as evidenced by a warning triggered in the kernel code. The root cause is the failure to increment the socket disconnect counter in these scenarios, which can prevent other threads waiting on the same socket lock from properly handling errors. This vulnerability poses significant risks, including potential data loss and system instability.
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 avoid using the MPTCP protocol until the patch is applied. This can be done by configuring the network settings to disable MPTCP, which may involve modifying system files or using specific commands depending on the distribution.
Conclusion
CVE-2025-21705 represents a critical vulnerability in the Linux kernel's MPTCP implementation, necessitating immediate attention from users and system administrators. By updating to the latest patched version and considering temporary workarounds, users can mitigate the risks associated with this vulnerability.
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FAQ
CVE-2025-21705 is a vulnerability in the Linux kernel's MPTCP implementation that can lead to data stream corruption due to improper handling of disconnect events.
You can use Vulert, which monitors and alerts for open-source vulnerabilities in real-time. By uploading your manifest file like package-lock.json, you can check if your application is affected by this vulnerability.
The risks include data loss and system instability, as the vulnerability can cause improper error handling in network communications.
Yes, a patch is available in the latest versions of the Linux kernel. Users are advised to update their systems to the patched version.
If you cannot apply the patch immediately, consider disabling MPTCP in your network settings as a temporary workaround.
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