{"id":"CVE-2026-39804","aliases":["EEF-CVE-2026-39804","GHSA-frh3-6pv6-rc8j"],"url":"https://o3.security/vulnerability/CVE-2026-39804","summary":"WebSocket permessage-deflate inflate has no output-size cap in bandit","details":"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.\n\n'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.\n\nAn 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.\n\nThis 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.\n\nThis issue affects bandit: from 0.5.9 before 1.11.0.","published":"2026-05-01T20:34:24.604Z","modified":"2026-08-07T11:51:08.583667104Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Hex","name":"bandit","fixedVersion":"1.11.0"}],"fix":{"url":"https://github.com/mtrudel/bandit/commit/8156921a51e684a951221da7bc30a70a022f722e","label":"mtrudel/bandit@8156921"},"references":[{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-39804.html"},{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-39804"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/39xxx/CVE-2026-39804.json"},{"type":"ADVISORY","url":"https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-39804"},{"type":"FIX","url":"https://github.com/mtrudel/bandit/commit/8156921a51e684a951221da7bc30a70a022f722e"},{"type":"PACKAGE","url":"https://github.com/mtrudel/bandit"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-07T11:51:08.583667104Z"}}