Denial of Service Vulnerability in Undertow WebSocket Implementation

CVE-2026-5680 is a high-severity vulnerability identified in the Undertow web server. The flaw exists within the implementation of the WebSocket permessage-deflate extension, which is responsible for compressing messages to improve network performance. An unauthenticated remote attacker can exploit this by sending specially crafted WebSocket messages that force the PerMessageDeflateFunction.largerBuffer() method to perform recursive, exponential memory allocation. This behavior leads to rapid memory exhaustion on the server, resulting in a Denial of Service (DoS) for the affected application. Because the exhaustion occurs at the application level through standard protocol features, it is particularly difficult to distinguish from legitimate high-traffic volume without specific inspection of WebSocket frame metadata and compression negotiation parameters.

Impact

Successful exploitation results in an application-level Denial of Service, causing the affected service to crash or become unresponsive to legitimate user requests. This vulnerability impacts any system utilizing Undertow with WebSocket support enabled where the permessage-deflate extension is active. Given the common usage of Undertow within various Java-based application frameworks, the potential for service disruption is significant for enterprise environments relying on these components for real-time communication.

Recommendation

Prioritized, concrete actions for detection engineering teams:

• Patch the Undertow library to the vendor-provided security update that addresses CVE-2026-5680.
• Monitor web server application logs for spikes in memory usage or frequent service restarts associated with WebSocket endpoints.
• Evaluate current WebSocket configuration and disable the permessage-deflate extension if it is not strictly required for application functionality until patching is complete.