tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)

{
  "id": "8421cb857d3b248254c3432ede66aafa369ec241c72b3f27f2cb3513c57ef5a9",
  "url": "https://osv.dev/vulnerability/CVE-2026-49855",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "tornado AsyncHTTPClient accumulates decompressed chunks without size limit (gzip bomb)",
    "summary": "Tornado is a Python web framework and asynchronous networking library. Prior to 6.5.6, Tornado gzip decompression routines processed limited-size chunks but did not enforce an overall limit on accumulated decompressed chunks, allowing a malicious server accessed by SimpleAsyncHTTPClient or an HTTPServer configured with decompress_request=True to consume effectively unlimited memory. This issue is fixed in version 6.5.6.",
    "evidence": "osv:CVE-2026-49855",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-49855",
      "aliases": [
        "CVE-2026-49855",
        "GHSA-mgf9-4vpg-hj56",
        "PYSEC-2026-3389"
      ],
      "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/tornadoweb/tornado",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "aba2569f7ed7a6bdbef816658fb6b7182531b751"
                },
                {
                  "fixed": "ff808b33adc52d89a549376a5e3628e92abbc8ff"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "6.5.6"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "ff808b33adc52d89a549376a5e3628e92abbc8ff"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v6.5.5",
            "v6.5.4",
            "v6.5.3",
            "v6.5.2",
            "v6.5.1",
            "v6.5.0",
            "v6.5.0b1",
            "v6.4.1",
            "v6.4.0",
            "v6.4.0b1",
            "v6.3.1",
            "v6.3.0",
            "v6.3.0b1",
            "v6.2.0",
            "v6.2.0b2",
            "v6.2.0b1",
            "v6.1.0",
            "v6.1.0b2",
            "v6.1.0b1",
            "v6.0.0",
            "v6.0.0b1",
            "v5.1.0",
            "v5.0.0",
            "v4.5.0",
            "v4.5.1",
            "v4.4.1",
            "v4.4.0",
            "v4.4.0b1",
            "v4.3.0",
            "v4.3.0b2",
            "v4.3.0b1",
            "v4.2.0",
            "v4.2.0b1",
            "v4.1.0",
            "v4.1.0b2",
            "v4.1.0b1",
            "v4.0.0",
            "v4.0.0b3",
            "v4.0.0b2",
            "v4.0.0b1",
            "v3.2.0",
            "v3.2.0b2",
            "v3.2.0b1",
            "v3.1.0",
            "v3.0.1",
            "v3.0.0",
            "v2.1.1",
            "v2.3.0",
            "v2.2.0",
            "v2.1.0",
            "v2.0.0",
            "v1.2.0",
            "v1.1.0",
            "v1.0.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-49855.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:40.483948173Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49855.json",
        "https://github.com/tornadoweb/tornado/security/advisories/GHSA-mgf9-4vpg-hj56",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-49855",
        "https://github.com/tornadoweb/tornado/commit/ff808b33adc52d89a549376a5e3628e92abbc8ff",
        "https://github.com/tornadoweb/tornado/pull/3626"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-409"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49855.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-49855",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 508211,
  "published_at": "2026-07-14T20:45:02.982+00:00"
}