High

Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser

Type: advisorySources: OSV

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.

Products: Not specified
Affected versions: Not specified
Fixed versions: Not specified
CVEs: CVE-2026-34827
Confirmed fixed CVEs: Not specified
Updated in this feed: 7 Nov 2026, 14:58 UTC · Revision 1

Source evidence

Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser

core-osv · Original source

Published 2 Apr 2026, 17:07 UTC

osv:CVE-2026-34827

Complete source record

{
  "id": "c6fd4744fbb419dac873b02b64fcfcde66d94fee8266b399901018b8d83b2afb",
  "url": "https://osv.dev/vulnerability/CVE-2026-34827",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser",
    "summary": "Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name=\"...\" using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.",
    "evidence": "osv:CVE-2026-34827",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-34827",
      "aliases": [
        "CVE-2026-34827",
        "GHSA-v6x5-cg8r-vv6x"
      ],
      "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/rack/rack",
              "type": "GIT",
              "events": [
                {
                  "introduced": "52901caf09ebcda879512a8605059963a49df55d"
                },
                {
                  "fixed": "ae8431120e66e92d1885ab8ec0a553d9cad5ec13"
                },
                {
                  "introduced": "b68251c03788ff39d4a4b25424df7360426e4afd"
                },
                {
                  "fixed": "e1f22fdbe99afd2126b6fbf05bb12399359574b7"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "cpe": "cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*",
                    "range": [
                      {
                        "introduced": "3.0.0"
                      },
                      {
                        "fixed": "3.1.21"
                      }
                    ],
                    "source": "CPE_RANGE"
                  },
                  {
                    "cpe": "cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*",
                    "range": [
                      {
                        "introduced": "3.2.0"
                      },
                      {
                        "fixed": "3.2.6"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v3.2.5",
            "v3.1.20",
            "v3.1.19",
            "v3.2.4",
            "v3.2.3",
            "v3.1.18",
            "v3.1.17",
            "v3.2.2",
            "v3.2.1",
            "v3.2.0",
            "v3.1.16",
            "v3.1.15",
            "v3.1.14",
            "v3.1.13",
            "v3.1.12",
            "v3.1.11",
            "v3.1.10",
            "v3.1.9",
            "v3.1.8",
            "v3.1.7",
            "v3.1.6",
            "v3.1.5",
            "v3.1.4",
            "v3.1.3",
            "v3.1.2",
            "v3.1.0",
            "3.0.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-34827.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:51.312745895Z",
      "provider": "osv",
      "references": [
        "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-34827.json",
        "https://access.redhat.com/security/cve/CVE-2026-34827",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/34xxx/CVE-2026-34827.json",
        "https://github.com/rack/rack/security/advisories/GHSA-v6x5-cg8r-vv6x",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-34827",
        "https://bugzilla.redhat.com/show_bug.cgi?id=2454501"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-400",
          "CWE-407",
          "CWE-770"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/34xxx/CVE-2026-34827.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-34827",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 519702,
  "published_at": "2026-04-02T17:07:48.279+00:00"
}