Plug.Cowboy vulnerable to unauthenticated remote DoS via HTTP/2 `:scheme` atom-table exhaustion

Summary

An unauthenticated remote denial-of-service vulnerability in Plug.Cowboy.Conn allows any attacker who can reach an HTTPS Plug.Cowboy listener via HTTP/2 to permanently exhaust the BEAM atom table and crash the entire Erlang VM.

Am I Affected?

All users running plug_cowboy with HTTP/2 may be affected, this includes Phoenix applications. If another HTTP adapter such as Bandit is used, then the consuming project is not affected. If the HTTP/2 endpoint is exposed directly (without a proxy) then the project will be affected. If a proxy is in use then it depends on the proxy configuration. Many proxies use HTTP/1.1 internally, and would be unaffected.

Impact

The vulnerability will allow crashing the Erlang VM (BEAM) via atom exhaustion.

Mitigation

Users are advised to update to plug_cowboy v2.8.1 to mitigate this issue.

Credits

Plug.Cowboy thanks Peter Ullrich for finding and responsibly disclosing this vulnerability.