WebSocket permessage-deflate inflate has no output-size cap in bandit

Allocation of Resources Without Limits or Throttling vulnerability in mtrudel bandit allows unauthenticated remote denial of service via memory exhaustion when WebSocket permessage-deflate compression is enabled.

'Elixir.Bandit.WebSocket.PerMessageDeflate':inflate/2 in lib/bandit/websocket/permessage_deflate.ex calls :zlib.inflate/2 with no output-size cap, then materializes the entire decompressed payload as a single binary via IO.iodata_to_binary/1. The websocket_options.max_frame_size option only bounds the on-the-wire (compressed) frame size, not the decompressed output. A high-ratio compressed frame (e.g. uniform data at ~1024:1 ratio) can stay well under any wire-size limit while forcing GiB-scale heap allocations in the connection process before any application code runs.

An unauthenticated attacker who can open a WebSocket connection can send a single such frame to exhaust the BEAM node's memory and trigger an OOM kill.

This vulnerability requires both Bandit's server-level websocket_options.compress and the per-upgrade compress: true option passed to WebSockAdapter.upgrade/4 to be enabled. Stock Phoenix and LiveView applications are not affected as they default to compress: false.

This issue affects bandit: from 0.5.9 before 1.11.0.

Do not pass compress: true to WebSockAdapter.upgrade/4. Omitting this option (or setting it to false) prevents permessage-deflate from being negotiated, so the inflate path is never reached.

The vulnerability is only reachable when both of the following conditions are true:

• Bandit's server-level websocket_options.compress is enabled (it defaults to true).
• The per-upgrade compress: true option is passed to WebSockAdapter.upgrade/4 (it defaults to false; Phoenix's default is also false).

Stock Phoenix and LiveView applications are not affected because compress: false is their default.