EEF-CVE-2026-39804

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

## Summary 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. ## Workarounds 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. ## Configurations 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.