Bug 2464589 (CVE-2026-39804)

Summary: CVE-2026-39804 bandit: Bandit: Denial of Service due to memory exhaustion via WebSocket permessage-deflate compression
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: jwong, omaciel, teagle, ttakamiy
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in bandit. An unauthenticated attacker who can open a WebSocket connection can exploit a vulnerability when WebSocket permessage-deflate compression is enabled. This flaw allows for memory exhaustion by sending a highly compressed frame that, when decompressed, forces large memory allocations. This can lead to a denial of service (DoS) by exhausting the system's memory and causing the application to crash.
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Bug Depends On: 2477947    
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Description OSIDB Bzimport 2026-05-01 21:01:45 UTC 
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.