Vulnerability-Lookup

CVE-2026-54283 (GCVE-0-2026-54283)

Vulnerability from cvelistv5 – Published: 2026-06-22 16:46 – Updated: 2026-06-23 16:10

Title

Starlette: request.form() limits silently ignored for application/x-www-form-urlencoded enable DoS

Summary

Starlette is a lightweight ASGI framework/toolkit. From 0.4.1 until 1.3.1, request.form() accepts max_fields and max_part_size to bound resource consumption while parsing form data. These limits are enforced for multipart/form-data, but silently ignored for application/x-www-form-urlencoded. An unauthenticated attacker can therefore send a urlencoded body with an arbitrarily large number of fields or an arbitrarily large field, even when the application configured limits it believed would apply. This vulnerability is fixed in 1.3.1.

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-770 - Allocation of Resources Without Limits or Throttling

Assigner

GitHub_M

References

1 reference

URL	Tags
https://github.com/Kludex/starlette/security/advi…	x_refsource_CONFIRM

Impacted products

1 product

Vendor	Product	Version
Kludex	starlette	Affected: >= 0.4.1, < 1.3.1

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GHSA-82W8-QH3P-5JFQ

Vulnerability from github – Published: 2026-06-15 20:39 – Updated: 2026-06-15 20:39

Summary

Starlette: request.form() limits silently ignored for application/x-www-form-urlencoded enable DoS

Details

Summary

request.form() accepts max_fields and max_part_size to bound resource consumption while parsing form data. These limits are enforced for multipart/form-data, but silently ignored for application/x-www-form-urlencoded. An unauthenticated attacker can therefore send a urlencoded body with an arbitrarily large number of fields or an arbitrarily large field, even when the application configured limits it believed would apply.

Details

request.form() dispatches to a different parser depending on the Content-Type. For multipart/form-data the max_files, max_fields, and max_part_size limits are forwarded to the parser, but for application/x-www-form-urlencoded the parser is constructed without them. It has no max_fields or max_part_size parameter to receive them, and it appends every field with no count check and accumulates each field's name and value with no size check. The configured limits are therefore both unreachable and unenforced for url-encoded bodies.

Because the url-encoded parser does its work synchronously between stream reads, the two attack shapes have different effects:

Field count drives CPU and event-loop blocking. A body of ~1,000,000 fields (a sub-10MB payload such as f0=v&f1=v&...) blocks the worker's event loop for several seconds while parsing, during which the worker serves no other request.
Field size drives memory. A single large field value (e.g. a 50MB value) is buffered in full to build the FormData, forcing memory allocation proportional to the request body.

The equivalent multipart/form-data request is correctly rejected with 400 Too many fields / 400 Field exceeded maximum size.

Impact

This Denial of service (DoS) vulnerability affects all applications built with Starlette (or FastAPI) that call request.form() on application/x-www-form-urlencoded requests. A single request with a very large number of fields blocks the event loop for several seconds, and a single request with a very large field forces unbounded memory allocation; in either case, parallel requests can render the service unusable. A reverse proxy that enforces a request body size limit reduces but does not eliminate the exposure, since a sub-10MB body is already enough to block the event loop.

Mitigation

Upgrade to a patched version, which forwards max_fields and max_part_size to the url-encoded parser and enforces them while parsing, raising before the oversized field or excess fields are accumulated. The defaults match multipart/form-data (max_fields=1000, max_part_size=1MB) and can be customized via request.form(max_fields=..., max_part_size=...).

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": "starlette"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0.4.1"
            },
            {
              "fixed": "1.3.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-54283"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-770"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-15T20:39:53Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n`request.form()` accepts `max_fields` and `max_part_size` to bound resource consumption while parsing form data. These limits are enforced for `multipart/form-data`, but silently ignored for `application/x-www-form-urlencoded`. An unauthenticated attacker can therefore send a urlencoded body with an arbitrarily large number of fields or an arbitrarily large field, even when the application configured limits it believed would apply.\n\n### Details\n`request.form()` dispatches to a different parser depending on the `Content-Type`. For `multipart/form-data` the `max_files`, `max_fields`, and `max_part_size` limits are forwarded to the parser, but for `application/x-www-form-urlencoded` the parser is constructed without them. It has no `max_fields` or `max_part_size` parameter to receive them, and it appends every field with no count check and accumulates each field\u0027s name and value with no size check. The configured limits are therefore both unreachable and unenforced for url-encoded bodies.\n\nBecause the url-encoded parser does its work synchronously between stream reads, the two attack shapes have different effects:\n\n- **Field count** drives CPU and event-loop blocking. A body of ~1,000,000 fields (a sub-10MB payload such as `f0=v\u0026f1=v\u0026...`) blocks the worker\u0027s event loop for several seconds while parsing, during which the worker serves no other request.\n- **Field size** drives memory. A single large field value (e.g. a 50MB value) is buffered in full to build the `FormData`, forcing memory allocation proportional to the request body.\n\nThe equivalent `multipart/form-data` request is correctly rejected with `400 Too many fields` / `400 Field exceeded maximum size`.\n\n### Impact\nThis Denial of service (DoS) vulnerability affects all applications built with Starlette (or FastAPI) that call `request.form()` on `application/x-www-form-urlencoded` requests. A single request with a very large number of fields blocks the event loop for several seconds, and a single request with a very large field forces unbounded memory allocation; in either case, parallel requests can render the service unusable. A reverse proxy that enforces a request body size limit reduces but does not eliminate the exposure, since a sub-10MB body is already enough to block the event loop.\n\n### Mitigation\nUpgrade to a patched version, which forwards `max_fields` and `max_part_size` to the url-encoded parser and enforces them while parsing, raising before the oversized field or excess fields are accumulated. The defaults match `multipart/form-data` (`max_fields=1000`, `max_part_size=1MB`) and can be customized via `request.form(max_fields=..., max_part_size=...)`.",
  "id": "GHSA-82w8-qh3p-5jfq",
  "modified": "2026-06-15T20:39:53Z",
  "published": "2026-06-15T20:39:53Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/Kludex/starlette/security/advisories/GHSA-82w8-qh3p-5jfq"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/Kludex/starlette"
    }
  ],
  "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": "Starlette: request.form() limits silently ignored for application/x-www-form-urlencoded enable DoS"
}

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-2026-54283 (GCVE-0-2026-54283)

GHSA-82W8-QH3P-5JFQ

Summary

Details

Impact

Mitigation

Tags

Sightings

Nomenclature