CVE-2025-11677 is a Use After Free (CWE-416) vulnerability found in the warmcat libwebsockets library, version 4.0. The flaw exists in the WebSocket server handshake function lws_handshake_server, specifically when the user provides a callback function that handles the LWS_CALLBACK_HTTP_CONFIRM_UPGRADE event. During the upgrade confirmation phase of the WebSocket handshake, improper memory management leads to a Use After Free condition, where the program attempts to access memory that has already been freed. This can cause the server process to crash or behave unpredictably, resulting in a denial of service (DoS). The vulnerability requires no authentication or user interaction but has a high attack complexity, meaning exploitation is non-trivial and depends on specific configurations and callback implementations. The CVSS 4.0 base score is 6.3, reflecting a medium severity level. No patches have been officially released yet, and no exploits are known to be active in the wild. The vulnerability primarily impacts applications embedding libwebsockets for WebSocket server functionality, especially those that customize the HTTP upgrade confirmation callback. This includes IoT devices, embedded systems, and certain telecom infrastructure components that rely on libwebsockets for real-time communication.

For European organizations, the primary impact of CVE-2025-11677 is the potential for denial of service attacks against WebSocket servers using libwebsockets version 4.0 with custom upgrade confirmation callbacks. This can disrupt real-time communication services, impacting availability and potentially causing service outages. Industries such as telecommunications, industrial control systems, and IoT device manufacturers in Europe that embed libwebsockets in their products or infrastructure could experience operational disruptions. The vulnerability does not directly compromise confidentiality or integrity but can degrade service reliability and availability, which may have downstream effects on business continuity and critical infrastructure operations. Given the medium severity and high attack complexity, widespread exploitation is less likely but targeted attacks against critical systems remain a concern.

European organizations should immediately audit their use of libwebsockets version 4.0, focusing on configurations where the LWS_CALLBACK_HTTP_CONFIRM_UPGRADE callback is implemented. Until official patches are released, developers should review and harden callback implementations to ensure no unsafe memory operations occur during the handshake. Employing runtime memory safety tools and fuzz testing on the handshake code paths can help identify and mitigate unsafe memory access. Network-level mitigations such as Web Application Firewalls (WAFs) can be tuned to detect and block suspicious WebSocket upgrade requests that might trigger the vulnerability. Organizations should also monitor vendor advisories for patches and plan timely updates. For embedded and IoT devices, firmware updates incorporating patched libwebsockets versions should be prioritized. Additionally, implementing redundancy and failover mechanisms can reduce the impact of potential DoS conditions.