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High

WebSocket fragmented message reassembly unbounded in bandit

Type: advisorySources: OSV

Allocation of Resources Without Limits or Throttling vulnerability in mtrudel bandit allows unauthenticated remote denial of service via memory exhaustion. The fragment reassembly path in 'Elixir.Bandit.WebSocket.Connection':handle_frame/3 in lib/bandit/websocket/connection.ex appends every incoming Continuation{fin: false} frame's payload to a per-connection iolist with no cumulative size cap. The existing max_frame_size option only bounds individual frames; a peer that streams an unbounded number of continuation frames without ever setting fin=1 grows BEAM heap linearly until the OS or a supervisor kills the process. Because the accumulation happens before WebSock.handle_in/2 is called, the application has no opportunity to interpose a size check. Phoenix Channels and LiveView both run over WebSock on Bandit, so a stock Phoenix application exposes this surface as soon as it accepts socket connections. This issue affects bandit: from 0.5.0 before 1.11.0.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2026-42786
Confirmed fixed CVEs
Not specified
Updated in this feed
4 Nov 2026, 23:52 UTC · Revision 1

Source evidence

WebSocket fragmented message reassembly unbounded in bandit

core-osv · Original source

Published 1 May 2026, 20:34 UTC

Allocation of Resources Without Limits or Throttling vulnerability in mtrudel bandit allows unauthenticated remote denial of service via memory exhaustion. The fragment reassembly path in 'Elixir.Bandit.WebSocket.Connection':handle_frame/3 in lib/bandit/websocket/connection.ex appends every incoming Continuation{fin: false} frame's payload to a per-connection iolist with no cumulative size cap. The existing max_frame_size option only bounds individual frames; a peer that streams an unbounded number of continuation frames without ever setting fin=1 grows BEAM heap linearly until the OS or a supervisor kills the process. Because the accumulation happens before WebSock.handle_in/2 is called, the application has no opportunity to interpose a size check. Phoenix Channels and LiveView both run over WebSock on Bandit, so a stock Phoenix application exposes this surface as soon as it accepts socket connections. This issue affects bandit: from 0.5.0 before 1.11.0.

osv:CVE-2026-42786
Complete source record
{
  "id": "629a83c5207b8c85d89bf00aa312ee5970057f12de6e47069ddebbf4e8d2a542",
  "url": "https://osv.dev/vulnerability/CVE-2026-42786",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "WebSocket fragmented message reassembly unbounded in bandit",
    "summary": "Allocation of Resources Without Limits or Throttling vulnerability in mtrudel bandit allows unauthenticated remote denial of service via memory exhaustion. The fragment reassembly path in 'Elixir.Bandit.WebSocket.Connection':handle_frame/3 in lib/bandit/websocket/connection.ex appends every incoming Continuation{fin: false} frame's payload to a per-connection iolist with no cumulative size cap. The existing max_frame_size option only bounds individual frames; a peer that streams an unbounded number of continuation frames without ever setting fin=1 grows BEAM heap linearly until the OS or a supervisor kills the process. Because the accumulation happens before WebSock.handle_in/2 is called, the application has no opportunity to interpose a size check. Phoenix Channels and LiveView both run over WebSock on Bandit, so a stock Phoenix application exposes this surface as soon as it accepts socket connections. This issue affects bandit: from 0.5.0 before 1.11.0.",
    "evidence": "osv:CVE-2026-42786",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-42786",
      "aliases": [
        "CVE-2026-42786",
        "EEF-CVE-2026-42786",
        "GHSA-pf94-94m9-536p"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/mtrudel/bandit",
              "type": "GIT",
              "events": [
                {
                  "introduced": "8909391f486d42138c5308410bc5ea49a65f4d46"
                },
                {
                  "fixed": "21612c7c7b1ce43eccd36d3af3a2299d23513667"
                },
                {
                  "introduced": "102adfd56bd4aacd39469a5582104377618abc9f"
                },
                {
                  "fixed": "e62619895d0d2584d7ffa57d43ef6f72437dfaff"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "8909391f486d42138c5308410bc5ea49a65f4d46"
                      },
                      {
                        "fixed": "21612c7c7b1ce43eccd36d3af3a2299d23513667"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  },
                  {
                    "range": [
                      {
                        "introduced": "0.5.0"
                      },
                      {
                        "fixed": "1.11.0"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  },
                  {
                    "range": [
                      {
                        "fixed": "21612c7c7b1ce43eccd36d3af3a2299d23513667"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  }
                ]
              }
            }
          ],
          "versions": [
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            "1.6.0",
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            "1.5.6",
            "1.5.5",
            "1.5.4",
            "1.5.3",
            "1.5.2",
            "1.5.1",
            "1.5.0",
            "1.4.2",
            "1.4.1",
            "1.4.0",
            "1.3.0",
            "1.2.3",
            "1.2.0",
            "1.1.3",
            "1.1.2",
            "1.1.1",
            "1.1.0",
            "1.0.0",
            "1.0.0-pre.18",
            "1.0.0-pre.17",
            "1.0.0-pre.16",
            "1.0.0-pre.15",
            "1.0.0-pre.14",
            "1.0.0-pre.13",
            "1.0.0-pre.12",
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            "1.0.0-pre.10",
            "1.0.0-pre.9",
            "1.0.0-pre.8",
            "1.0.0-pre.7",
            "1.0.0-pre.6",
            "1.0.0-pre.5",
            "1.0.0-pre.4",
            "1.0.0-pre.3",
            "1.0.0-pre.2",
            "1.0.0-pre.1",
            "0.7.7",
            "0.7.6",
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            "0.7.4",
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            "0.7.2",
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            "0.6.2",
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            "0.5.3",
            "0.5.2",
            "0.5.1",
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          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-42786.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:40.523938417Z",
      "provider": "osv",
      "references": [
        "https://cna.erlef.org/cves/CVE-2026-42786.html",
        "https://github.com",
        "https://osv.dev/vulnerability/EEF-CVE-2026-42786",
        "https://repo.hex.pm",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42786.json",
        "https://github.com/mtrudel/bandit/security/advisories/GHSA-pf94-94m9-536p",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-42786",
        "https://github.com/mtrudel/bandit/commit/21612c7c7b1ce43eccd36d3af3a2299d23513667",
        "https://github.com/mtrudel/bandit"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-770"
        ],
        "cna_assigner": "EEF",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/42xxx/CVE-2026-42786.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-42786",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 508070,
  "published_at": "2026-05-01T20:34:17.014+00:00"
}