High [CVE-2026-15709] WebSocket permessage-deflate Unbounded Decompression Remote Denial of Service

A flaw was found in libsoup's WebSocket implementation when using the permessage-deflate extension. The extension's decompression loop (inflate()) processes data in chunks without enforcing an upper boundary limit on the output buffer size.

While libsoup limits the incoming compressed frame size via max_incoming_payload_size, it fails to track or limit memory allocation during decompression. A separate check for decompressed size (max_total_message_size) exists but executes only after inflation is complete, and it is entirely disabled by default for client connections.

A remote, unauthenticated attacker can exploit this by sending a small, highly compressed payload (a decompression bomb), causing unbounded memory allocation that triggers an Out-of-Memory (OOM) crash and a Denial of Service (DoS). This vulnerability poses a high availability risk to both clients and servers utilizing libsoup for WebSocket communication.

Because the memory exhaustion occurs during the extraction process before payload size validations are applied, standard post-decompression limits fail to mitigate the threat.

A network-based attacker requires no privileges or user interaction to deliberately crash vulnerable applications or desktop services that establish connections using the default permessage-deflate configuration.