{"id":"CVE-2025-61771","aliases":["GHSA-w9pc-fmgc-vxvw"],"url":"https://o3.security/vulnerability/CVE-2025-61771","summary":"Rack's 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.","published":"2025-10-07T14:42:53.366Z","modified":"2026-10-28T02:31:07.863495096Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"RubyGems","name":"rack","fixedVersion":"2.2.19"},{"ecosystem":"RubyGems","name":"rack","fixedVersion":"3.1.17"},{"ecosystem":"RubyGems","name":"rack","fixedVersion":"3.2.2"}],"fix":{"url":"https://github.com/rack/rack/commit/589127f4ac8b5cf11cf88fb0cd116ffed4d2181e","label":"rack/rack@589127f"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/61xxx/CVE-2025-61771.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/rack/rack/commit/589127f4ac8b5cf11cf88fb0cd116ffed4d2181e"},{"type":"FIX","url":"https://github.com/rack/rack/commit/d869fed663b113b95a74ad53e1b5cae6ab31f29e"},{"type":"FIX","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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:31:07.863495096Z"}}