CVE-2025-21705 is a vulnerability identified in the Linux kernel's implementation of the Multipath TCP (MPTCP) protocol, specifically related to the handling of socket disconnects during TCP Fast Open (TFO) connection attempts. The vulnerability arises from improper management of the disconnect() operation internally triggered by MPTCP when a connection attempt using TCP Fast Open fails. The root cause is that the socket disconnect counter is not incremented correctly in these failure scenarios, which leads to improper error signaling to other threads waiting on the same socket lock. This can result in data stream corruption, as demonstrated by Syzbot's ability to trigger a warning and kernel stack trace indicating memory corruption and unstable socket states. The problem manifests in the function __mptcp_clean_una within the MPTCP protocol code, where the cleanup of unacknowledged packets is mishandled during disconnect events caused by Fast Open errors. The vulnerability affects multiple Linux kernel versions, including release candidates and stable branches, as indicated by the affected commit hashes. The fix involves increasing the socket disconnect counter even when disconnects are generated internally due to Fast Open errors, ensuring that waiting threads receive proper error notifications and preventing data corruption or kernel instability. No known exploits are reported in the wild at this time, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a risk primarily to systems running Linux kernels with MPTCP enabled and utilizing TCP Fast Open, which is increasingly adopted for performance improvements in network communications. Potential impacts include data stream corruption leading to application crashes, kernel panics, or denial of service conditions on affected hosts. This can disrupt critical services, especially in environments relying on high availability and stable network connections such as cloud infrastructure, telecommunications, financial services, and industrial control systems. Confidentiality and integrity could be indirectly affected if corrupted streams lead to unexpected behavior or system crashes that could be exploited further. The vulnerability does not appear to allow direct remote code execution or privilege escalation but can degrade system reliability and availability. Given the widespread use of Linux in servers, cloud platforms, and embedded devices across Europe, the impact could be significant if unpatched systems are exposed to network conditions triggering this flaw. However, exploitation requires triggering specific Fast Open disconnect scenarios, which may limit the ease of exploitation.

European organizations should take the following specific mitigation steps: 1) Identify all Linux systems running kernel versions affected by this vulnerability, especially those with MPTCP and TCP Fast Open enabled. 2) Apply the official Linux kernel patches or upgrade to a kernel version where this vulnerability is resolved as soon as they become available. 3) Temporarily disable TCP Fast Open on critical systems if patching is not immediately feasible, to prevent triggering the disconnect handling flaw. 4) Monitor kernel logs for warnings or errors related to MPTCP and TCP Fast Open disconnects that could indicate attempted exploitation or instability. 5) Conduct network traffic analysis to detect abnormal connection attempts that may exploit this vulnerability. 6) Coordinate with cloud and infrastructure providers to ensure underlying platforms are patched. 7) Implement robust incident response plans to quickly address any system crashes or service disruptions potentially linked to this vulnerability. These steps go beyond generic advice by focusing on the specific protocol features and kernel components involved.