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CVE-2026-39804 is an unauthenticated remote denial-of-service vulnerability in mtrudel Bandit affecting versions from 0.5.9 before 1.11.0. The flaw is in Elixir.Bandit.WebSocket.PerMessageDeflate:inflate/2 (lib/bandit/websocket/permessage_deflate.ex), which invokes :zlib.inflate/2 without enforcing a decompressed output-size limit and then materializes the full decompressed result into a single binary via IO.iodata_to_binary/1. As a result, Bandit can accept a small on-the-wire compressed WebSocket frame that expands at a very high ratio during permessage-deflate processing. Because websocket_options.max_frame_size constrains only the compressed frame size and not the decompressed payload size, an attacker can send a highly compressible frame that remains within configured wire-size limits while forcing GiB-scale allocations in the connection process before any application code executes.
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websocket_options.compress to be enabled and the per-upgrade compress: true option passed to WebSockAdapter.upgrade/4. Deployments that leave compression disabled are not affected; the provided context specifically notes that stock Phoenix and LiveView applications default to compress: false and are therefore not affected by default. Reducing only websocket_options.max_frame_size is not sufficient because that limit applies to compressed wire size, not decompressed output size.Patch, then assume compromise.
8156921a51e684a951221da7bc30a70a022f722e) and GitHub security advisory (GHSA-frh3-6pv6-rc8j).No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
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Cross-references every affected SKU, including bundled OEM variants.
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