CVE-2025-61771: Rack Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-w9pc-fmgc-vxvw",
  "modified" : "2025-10-13T15:29:51Z",
  "published" : "2025-10-07T17:27:07Z",
  "aliases" : [ "CVE-2025-61771" ],
  "summary" : "Rack: Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)",
  "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 → in-RAM buffer\n@mime_parts[mime_index].body << content\n```\n\nThere 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`.\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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "rack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.2.19"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "rack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.1"
      }, {
        "fixed" : "3.1.17"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "RubyGems",
      "name" : "rack"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.2"
      }, {
        "fixed" : "3.2.2"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2025-10-07T17:27:07Z",
    "nvd_published_at" : "2025-10-07T15:16:03Z"
  }
}