GHSA-W9PC-FMGC-VXVW

Vulnerability from github – Published: 2025-10-07 17:27 – Updated: 2025-10-13 15:29
VLAI?
Summary
Rack: Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)
Details

Summary

Rack::Multipart::Parser stores non-file form fields (parts without a filename) entirely in memory as Ruby String objects. A single large text field in a multipart/form-data request (hundreds of megabytes or more) can consume equivalent process memory, potentially leading to out-of-memory (OOM) conditions and denial of service (DoS).

Details

During multipart parsing, file parts are streamed to temporary files, but non-file parts are buffered into memory:

body = String.new  # non-file → in-RAM buffer
@mime_parts[mime_index].body << content

There is no size limit on these in-memory buffers. As a result, any large text field—while technically valid—will be loaded fully into process memory before being added to params.

Impact

Attackers can send large non-file fields to trigger excessive memory usage. Impact scales with request size and concurrency, potentially leading to worker crashes or severe garbage-collection overhead. All Rack applications processing multipart form submissions are affected.

Mitigation

  • Upgrade: Use a patched version of Rack that enforces a reasonable size cap for non-file fields (e.g., 2 MiB).
  • Workarounds:
  • Restrict maximum request body size at the web-server or proxy layer (e.g., Nginx client_max_body_size).
  • Validate and reject unusually large form fields at the application level.
Severity ?
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
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{
  "affected": [
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "rack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.2.19"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "rack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.1"
            },
            {
              "fixed": "3.1.17"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "RubyGems",
        "name": "rack"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.2"
            },
            {
              "fixed": "3.2.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-61771"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-10-07T17:27:07Z",
    "nvd_published_at": "2025-10-07T15:16:03Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\n`Rack::Multipart::Parser` stores non-file form fields (parts without a `filename`) entirely in memory as Ruby `String` objects. A single large text field in a multipart/form-data request (hundreds of megabytes or more) can consume equivalent process memory, potentially leading to out-of-memory (OOM) conditions and denial of service (DoS).\n\n## Details\n\nDuring multipart parsing, file parts are streamed to temporary files, but non-file parts are buffered into memory:\n\n```ruby\nbody = String.new  # non-file \u2192 in-RAM buffer\n@mime_parts[mime_index].body \u003c\u003c content\n```\n\nThere is no size limit on these in-memory buffers. As a result, any large text field\u2014while technically valid\u2014will be loaded fully into process memory before being added to `params`.\n\n## Impact\n\nAttackers can send large non-file fields to trigger excessive memory usage. Impact scales with request size and concurrency, potentially leading to worker crashes or severe garbage-collection overhead. All Rack applications processing multipart form submissions are affected.\n\n## Mitigation\n\n* **Upgrade:** Use a patched version of Rack that enforces a reasonable size cap for non-file fields (e.g., 2 MiB).\n* **Workarounds:**\n  * Restrict maximum request body size at the web-server or proxy layer (e.g., Nginx `client_max_body_size`).\n  * Validate and reject unusually large form fields at the application level.",
  "id": "GHSA-w9pc-fmgc-vxvw",
  "modified": "2025-10-13T15:29:51Z",
  "published": "2025-10-07T17:27:07Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/rack/rack/security/advisories/GHSA-w9pc-fmgc-vxvw"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-61771"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rack/rack/commit/589127f4ac8b5cf11cf88fb0cd116ffed4d2181e"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rack/rack/commit/d869fed663b113b95a74ad53e1b5cae6ab31f29e"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rack/rack/commit/e08f78c656c9394d6737c022bde087e0f33336fd"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/rack/rack"
    },
    {
      "type": "WEB",
      "url": "https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2025-61771.yml"
    }
  ],
  "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": "Rack: Multipart parser buffers large non\u2011file fields entirely in memory, enabling DoS (memory exhaustion)"
}


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