CVE-2026-39804 (GCVE-0-2026-39804)
Vulnerability from cvelistv5 – Published: 2026-05-01 20:34 – Updated: 2026-05-04 17:11
VLAI?
Title
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.
Severity ?
CWE
- CWE-770 - Allocation of Resources Without Limits or Throttling
Assigner
References
| URL | Tags | |
|---|---|---|
Impacted products
Credits
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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\u0027s memory and trigger an OOM kill.\\n\\nThis vulnerability requires both Bandit\u0027s 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.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"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.\u003cp\u003e\u003ctt\u003e\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027:inflate/2\u003c/tt\u003e in \u003ctt\u003elib/bandit/websocket/permessage_deflate.ex\u003c/tt\u003e calls \u003ctt\u003e:zlib.inflate/2\u003c/tt\u003e with no output-size cap, then materializes the entire decompressed payload as a single binary via \u003ctt\u003eIO.iodata_to_binary/1\u003c/tt\u003e. The \u003ctt\u003ewebsocket_options.max_frame_size\u003c/tt\u003e 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.\u003c/p\u003e\u003cp\u003eAn unauthenticated attacker who can open a WebSocket connection can send a single such frame to exhaust the BEAM node\u0027s memory and trigger an OOM kill.\u003c/p\u003e\u003cp\u003eThis vulnerability requires both Bandit\u0027s server-level \u003ctt\u003ewebsocket_options.compress\u003c/tt\u003e and the per-upgrade \u003ctt\u003ecompress: true\u003c/tt\u003e option passed to \u003ctt\u003eWebSockAdapter.upgrade/4\u003c/tt\u003e to be enabled. Stock Phoenix and LiveView applications are not affected as they default to \u003ctt\u003ecompress: false\u003c/tt\u003e.\u003c/p\u003e\u003cp\u003eThis issue affects bandit: from 0.5.9 before 1.11.0.\u003c/p\u003e\", \"base64\": false}]}], \"problemTypes\": [{\"descriptions\": [{\"lang\": \"en\", \"type\": \"CWE\", \"cweId\": \"CWE-770\", \"description\": \"CWE-770 Allocation of Resources Without Limits or Throttling\"}]}], \"configurations\": [{\"lang\": \"en\", \"value\": \"The vulnerability is only reachable when both of the following conditions are true:\\n- Bandit\u0027s server-level websocket_options.compress is enabled (it defaults to true).\\n- The per-upgrade compress: true option is passed to WebSockAdapter.upgrade/4 (it defaults to false; Phoenix\u0027s default is also false).\\n\\nStock Phoenix and LiveView applications are not affected because compress: false is their default.\", \"supportingMedia\": [{\"type\": \"text/html\", \"value\": \"The vulnerability is only reachable when both of the following conditions are true:\u003cul\u003e\u003cli\u003eBandit\u0027s server-level \u003ctt\u003ewebsocket_options.compress\u003c/tt\u003e is enabled (it defaults to \u003ctt\u003etrue\u003c/tt\u003e).\u003c/li\u003e\u003cli\u003eThe per-upgrade \u003ctt\u003ecompress: true\u003c/tt\u003e option is passed to \u003ctt\u003eWebSockAdapter.upgrade/4\u003c/tt\u003e (it defaults to \u003ctt\u003efalse\u003c/tt\u003e; Phoenix\u0027s default is also \u003ctt\u003efalse\u003c/tt\u003e).\u003c/li\u003e\u003c/ul\u003eStock Phoenix and LiveView applications are not affected because \u003ctt\u003ecompress: false\u003c/tt\u003e is their default.\", \"base64\": false}]}], \"cpeApplicability\": [{\"nodes\": [{\"negate\": false, \"cpeMatch\": [{\"criteria\": \"cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*\", \"vulnerable\": true, \"versionEndExcluding\": \"1.11.0\", \"versionStartIncluding\": \"0.5.9\"}], \"operator\": \"OR\"}], \"operator\": \"AND\"}], \"providerMetadata\": {\"orgId\": \"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\", \"shortName\": \"EEF\", \"dateUpdated\": \"2026-05-04T17:11:39.276Z\"}}}",
"cveMetadata": "{\"cveId\": \"CVE-2026-39804\", \"state\": \"PUBLISHED\", \"dateUpdated\": \"2026-05-04T17:11:39.276Z\", \"dateReserved\": \"2026-04-07T12:28:54.916Z\", \"assignerOrgId\": \"6b3ad84c-e1a6-4bf7-a703-f496b71e49db\", \"datePublished\": \"2026-05-01T20:34:24.604Z\", \"assignerShortName\": \"EEF\"}",
"dataType": "CVE_RECORD",
"dataVersion": "5.2"
}
}
}
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Experimental. This forecast is provided for visualization only and may change without notice. Do not use it for operational decisions.
Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.
Sightings
| Author | Source | Type | Date | Other |
|---|
Nomenclature
- Seen: The vulnerability was mentioned, discussed, or observed by the user.
- Confirmed: The vulnerability has been validated from an analyst's perspective.
- Published Proof of Concept: A public proof of concept is available for this vulnerability.
- Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
- Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
- Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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