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python-engineio has unbound thread allocation that can cause denial of service

Type: advisorySources: OSV

python-engineio is a Python implementation of the Engine.IO realtime client and server. Prior to version 4.13.2, an attacker can cause the creation of unnecessary background threads in the python-engineio server by exploiting the heartbeat mechanism, which launches a thread when a new connection is received, and when the client sends a PONG packet. This issue primarily affects synchronous servers. Asynchronous servers allocate background tasks instead of physical threads, which are lightweight and less likely to cause denial of service. However, the fix that was implemented was also applied to the asynchronous case. Version 4.13.2 addresses this issue as follows: The initial background thread (or async task( for heartbeat management is only launched if a client passes authentication in the `connect` handler; and the server now ensures that there is only one background heatbeat thread (or async task) per client at a given point in time. Out of sequence PONG packets are now discarded when an active heartbeat thread is already running.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2026-48802
Confirmed fixed CVEs
Not specified
Updated in this feed
4 Nov 2026, 05:19 UTC · Revision 1

Source evidence

python-engineio has unbound thread allocation that can cause denial of service

core-osv · Original source

Published 11 Aug 2026, 18:27 UTC

python-engineio is a Python implementation of the Engine.IO realtime client and server. Prior to version 4.13.2, an attacker can cause the creation of unnecessary background threads in the python-engineio server by exploiting the heartbeat mechanism, which launches a thread when a new connection is received, and when the client sends a PONG packet. This issue primarily affects synchronous servers. Asynchronous servers allocate background tasks instead of physical threads, which are lightweight and less likely to cause denial of service. However, the fix that was implemented was also applied to the asynchronous case. Version 4.13.2 addresses this issue as follows: The initial background thread (or async task( for heartbeat management is only launched if a client passes authentication in the `connect` handler; and the server now ensures that there is only one background heatbeat thread (or async task) per client at a given point in time. Out of sequence PONG packets are now discarded when an active heartbeat thread is already running.

osv:CVE-2026-48802
Complete source record
{
  "id": "3927a6ed9391c5e7ea27e57396eb3c82656f4ce13387bcec18d9889362613ecb",
  "url": "https://osv.dev/vulnerability/CVE-2026-48802",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "python-engineio has unbound thread allocation that can cause denial of service",
    "summary": "python-engineio is a Python implementation of the Engine.IO realtime client and server. Prior to version 4.13.2, an attacker can cause the creation of unnecessary background threads in the python-engineio server by exploiting the heartbeat mechanism, which launches a thread when a new connection is received, and when the client sends a PONG packet. This issue primarily affects synchronous servers. Asynchronous servers allocate background tasks instead of physical threads, which are lightweight and less likely to cause denial of service. However, the fix that was implemented was also applied to the asynchronous case. Version 4.13.2 addresses this issue as follows: The initial background thread (or async task( for heartbeat management is only launched if a client passes authentication in the `connect` handler; and the server now ensures that there is only one background heatbeat thread (or async task) per client at a given point in time. Out of sequence PONG packets are now discarded when an active heartbeat thread is already running.",
    "evidence": "osv:CVE-2026-48802",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-48802",
      "aliases": [
        "CVE-2026-48802",
        "GHSA-cgwc-pv48-fhj5",
        "PYSEC-2026-3032"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/miguelgrinberg/python-engineio",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "b698159e56883352f0e74852a596d5b4b5d6252a"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
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                      {
                        "fixed": "4.13.2"
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                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "introduced": "0"
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                      {
                        "fixed": "4.13.2"
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                    ],
                    "source": "DESCRIPTION"
                  }
                ]
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          ],
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          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-48802.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:33.165560405Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48802.json",
        "https://github.com/miguelgrinberg/python-engineio/security/advisories/GHSA-cgwc-pv48-fhj5",
        "https://github.com/pypa/advisory-database/tree/main/vulns/python-engineio/PYSEC-2026-3032.yaml",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-48802"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-770"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/48xxx/CVE-2026-48802.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-48802",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 500480,
  "published_at": "2026-08-11T18:27:50.703+00:00"
}