A remote attacker who has not logged in can crash many HTTP/3 web servers with a single connection, security firm FoxIO disclosed, releasing working proof-of-concept code.

By sending a small batch of compressed HTTP headers that expands in memory to a much larger set, the attacker can force the server to use thousands of times more memory than it received, enough to run a default server out of memory.

The flaws cause only denial-of-service attacks, with no evidence of data theft or code execution and no public reports of attacks in the wild. Fixes are out for quic-go, Cloudflare quiche, lsquic and Wireshark, but Bun, the h3 Rust library and aioquic were still unpatched at disclosure.