Vulnerability-Lookup

CVE-2026-53539 (GCVE-0-2026-53539)

Vulnerability from cvelistv5 – Published: 2026-06-22 16:55 – Updated: 2026-06-23 16:08

Title

Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

Summary

Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30.

Severity

7.5 (High)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-400 - Uncontrolled Resource Consumption
CWE-407 - Inefficient Algorithmic Complexity

Assigner

GitHub_M

References

1 reference

URL	Tags
https://github.com/Kludex/python-multipart/securi…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version
Kludex	python-multipart	Affected: < 0.0.30

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2026-53539",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2026-06-23T14:05:50.901666Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2026-06-23T16:08:36.474Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "python-multipart",
          "vendor": "Kludex",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.0.30"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for \u0026, and only when no \u0026 existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no \u0026, every field iteration performed a full failed \u0026 scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "HIGH",
            "baseScore": 7.5,
            "baseSeverity": "HIGH",
            "confidentialityImpact": "NONE",
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
            "version": "3.1"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-400",
              "description": "CWE-400: Uncontrolled Resource Consumption",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-407",
              "description": "CWE-407: Inefficient Algorithmic Complexity",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-06-22T16:55:42.787Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf"
        }
      ],
      "source": {
        "advisory": "GHSA-5rvq-cxj2-64vf",
        "discovery": "UNKNOWN"
      },
      "title": "Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-53539",
    "datePublished": "2026-06-22T16:55:42.787Z",
    "dateReserved": "2026-06-09T18:13:07.263Z",
    "dateUpdated": "2026-06-23T16:08:36.474Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-53539",
      "date": "2026-06-23",
      "epss": "0.00263",
      "percentile": "0.17423"
    },
    "vulnrichment": {
      "containers": "{\"cna\": {\"title\": \"Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service\", \"problemTypes\": [{\"descriptions\": [{\"cweId\": \"CWE-400\", \"lang\": \"en\", \"description\": \"CWE-400: Uncontrolled Resource Consumption\", \"type\": \"CWE\"}]}, {\"descriptions\": [{\"cweId\": \"CWE-407\", \"lang\": \"en\", \"description\": \"CWE-407: Inefficient Algorithmic Complexity\", \"type\": \"CWE\"}]}], \"metrics\": [{\"cvssV3_1\": {\"attackComplexity\": \"LOW\", \"attackVector\": \"NETWORK\", \"availabilityImpact\": \"HIGH\", \"baseScore\": 7.5, \"baseSeverity\": \"HIGH\", \"confidentialityImpact\": \"NONE\", \"integrityImpact\": \"NONE\", \"privilegesRequired\": \"NONE\", \"scope\": \"UNCHANGED\", \"userInteraction\": \"NONE\", \"vectorString\": \"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H\", \"version\": \"3.1\"}}], \"references\": [{\"name\": \"https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf\", \"tags\": [\"x_refsource_CONFIRM\"], \"url\": \"https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf\"}], \"affected\": [{\"vendor\": \"Kludex\", \"product\": \"python-multipart\", \"versions\": [{\"version\": \"\u003c 0.0.30\", \"status\": \"affected\"}]}], \"providerMetadata\": {\"orgId\": \"a0819718-46f1-4df5-94e2-005712e83aaa\", \"shortName\": \"GitHub_M\", \"dateUpdated\": \"2026-06-22T16:55:42.787Z\"}, \"descriptions\": [{\"lang\": \"en\", \"value\": \"Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for \u0026, and only when no \u0026 existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no \u0026, every field iteration performed a full failed \u0026 scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30.\"}], \"source\": {\"advisory\": \"GHSA-5rvq-cxj2-64vf\", \"discovery\": \"UNKNOWN\"}}, \"adp\": [{\"title\": \"CISA ADP Vulnrichment\", \"metrics\": [{\"other\": {\"type\": \"ssvc\", \"content\": {\"id\": \"CVE-2026-53539\", \"role\": \"CISA Coordinator\", \"options\": [{\"Exploitation\": \"none\"}, {\"Automatable\": \"yes\"}, {\"Technical Impact\": \"partial\"}], \"version\": \"2.0.3\", \"timestamp\": \"2026-06-23T14:05:50.901666Z\"}}}], \"providerMetadata\": {\"orgId\": \"134c704f-9b21-4f2e-91b3-4a467353bcc0\", \"shortName\": \"CISA-ADP\", \"dateUpdated\": \"2026-06-23T14:06:05.397Z\"}}]}",
      "cveMetadata": "{\"cveId\": \"CVE-2026-53539\", \"assignerOrgId\": \"a0819718-46f1-4df5-94e2-005712e83aaa\", \"state\": \"PUBLISHED\", \"assignerShortName\": \"GitHub_M\", \"dateReserved\": \"2026-06-09T18:13:07.263Z\", \"datePublished\": \"2026-06-22T16:55:42.787Z\", \"dateUpdated\": \"2026-06-23T16:08:36.474Z\"}",
      "dataType": "CVE_RECORD",
      "dataVersion": "5.2"
    }
  }
}

FKIE_CVE-2026-53539

Vulnerability from fkie_nvd - Published: 2026-06-22 18:16 - Updated: 2026-06-23 15:50

Severity

7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "python-multipart",
          "vendor": "Kludex",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 0.0.30"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for \u0026, and only when no \u0026 existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no \u0026, every field iteration performed a full failed \u0026 scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. This vulnerability is fixed in 0.0.30."
    }
  ],
  "id": "CVE-2026-53539",
  "lastModified": "2026-06-23T15:50:26.920",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "HIGH",
          "baseScore": 7.5,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 3.6,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-53539",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "yes"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-06-23T14:05:50.901666Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-06-22T18:16:44.360",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Undergoing Analysis",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-400"
        },
        {
          "lang": "en",
          "value": "CWE-407"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-5RVQ-CXJ2-64VF

Vulnerability from github – Published: 2026-06-15 20:24 – Updated: 2026-06-15 20:24

Summary

python-multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service

Details

Summary

When parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk.

An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes.

Details

In python_multipart/multipart.py, both the FIELD_NAME and FIELD_DATA states located the next separator like this:

sep_pos = data.find(b"&", i)
if sep_pos == -1:
    sep_pos = data.find(b";", i)

data.find(b"&", i) scans from i to the end of the buffer and returns -1 only when there is no & anywhere in the remainder. For a ; separated body with no &, this failed full buffer scan repeats once per field, making parsing quadratic in the body length.

For example, a 1 MiB url encoded body consisting of a; repeated ~500,000 times, submitted with Content-Type: application/x-www-form-urlencoded, causes the parser to perform on the order of 10^11 byte comparisons, consuming several seconds of CPU for a single request. Cost scales quadratically with chunk size.

The parser is reachable through the public QuerystringParser class and through the high level FormParser, create_form_parser, and parse_form APIs for url encoded bodies. It is also the parser Starlette and FastAPI use for application/x-www-form-urlencoded request bodies via request.form().

Impact

Uncontrolled CPU consumption (denial of service). Parsing is synchronous, so a single small crafted form body occupies the handling worker for seconds, blocking any other work on that worker until parsing finishes. Sustained concurrent requests keep workers continuously busy, degrading or denying service.

Mitigation

Upgrade to python-multipart 0.0.30 or later, which treats only & as a field separator (per the WHATWG URL standard) using a single bounded scan, making parsing linear in the body length.

Severity

7.5 (High)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "python-multipart"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.0.30"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-53539"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400",
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-15T20:24:09Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n\nWhen parsing `application/x-www-form-urlencoded` bodies, `QuerystringParser` located the field separator with a two step lookup: it first scanned the entire remaining buffer for `\u0026`, and only when no `\u0026` existed anywhere ahead did it fall back to scanning for `;`. For a body that uses `;` as the separator and contains no `\u0026`, every field iteration performed a full failed `\u0026` scan over the entire remaining buffer before locating the nearby `;`. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk.\n\nAn attacker can submit a small crafted body of the form `a;a;a;...` and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes.\n\n### Details\n\nIn `python_multipart/multipart.py`, both the `FIELD_NAME` and `FIELD_DATA` states located the next separator like this:\n\n```python\nsep_pos = data.find(b\"\u0026\", i)\nif sep_pos == -1:\n    sep_pos = data.find(b\";\", i)\n```\n\n`data.find(b\"\u0026\", i)` scans from `i` to the end of the buffer and returns `-1` only when there is no `\u0026` anywhere in the remainder. For a `;` separated body with no `\u0026`, this failed full buffer scan repeats once per field, making parsing quadratic in the body length.\n\nFor example, a 1 MiB url encoded body consisting of `a;` repeated ~500,000 times, submitted with `Content-Type: application/x-www-form-urlencoded`, causes the parser to perform on the order of 10^11 byte comparisons, consuming several seconds of CPU for a single request. Cost scales quadratically with chunk size.\n\nThe parser is reachable through the public `QuerystringParser` class and through the high level `FormParser`, `create_form_parser`, and `parse_form` APIs for url encoded bodies. It is also the parser Starlette and FastAPI use for `application/x-www-form-urlencoded` request bodies via `request.form()`.\n\n### Impact\n\nUncontrolled CPU consumption (denial of service). Parsing is synchronous, so a single small crafted form body occupies the handling worker for seconds, blocking any other work on that worker until parsing finishes. Sustained concurrent requests keep workers continuously busy, degrading or denying service.\n\n### Mitigation\n\nUpgrade to `python-multipart` `0.0.30` or later, which treats only `\u0026` as a field separator (per the [WHATWG URL standard](https://url.spec.whatwg.org/#urlencoded-parsing)) using a single bounded scan, making parsing linear in the body length.",
  "id": "GHSA-5rvq-cxj2-64vf",
  "modified": "2026-06-15T20:24:09Z",
  "published": "2026-06-15T20:24:09Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Kludex/python-multipart/security/advisories/GHSA-5rvq-cxj2-64vf"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Kludex/python-multipart"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "python-multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
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.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-53539 (GCVE-0-2026-53539)

FKIE_CVE-2026-53539

GHSA-5RVQ-CXJ2-64VF

Summary

Details

Impact

Mitigation

Tags

Sightings

Nomenclature