Summary
Rack::Multipart::Parser only wraps the request body in a BoundedIO when CONTENT_LENGTH is present. When a multipart/form-data request is sent without a Content-Length header, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit.
For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space.
This results in a denial of service condition for Rack applications that accept multipart form data.
Details
Rack::Multipart::Parser.parse applies BoundedIO only when content_length is not nil:
io = BoundedIO.new(io, content_length) if content_length
When CONTENT_LENGTH is absent, the parser reads the multipart body until EOF without a global byte limit.
Although Rack enforces BUFFERED_UPLOAD_BYTESIZE_LIMIT for retained non-file parts, file uploads are handled differently. When a multipart part includes a filename, the body is streamed to a Tempfile, and the retained-size accounting is not applied to that file content. As a result, file parts are not subject to the same upload size bound.
An attacker can exploit this by sending a chunked multipart/form-data request containing a file part and continuously streaming data without declaring a Content-Length. Rack will continue writing the uploaded data to disk until the client stops or the server exhausts available storage.
Impact
Any Rack application that accepts multipart/form-data uploads may be affected if no upstream component enforces a request body size limit.
An unauthenticated attacker can send a large chunked file upload to consume disk space on the application host. This may cause request failures, application instability, or broader service disruption if the host runs out of available storage.
The practical impact depends on deployment architecture. Reverse proxies or application servers that enforce upload limits may reduce or eliminate exploitability, but Rack itself does not impose a total multipart upload limit in this code path when CONTENT_LENGTH is absent.
Mitigation
- Update to a patched version of Rack that enforces a total multipart upload size limit even when
CONTENT_LENGTH is absent.
- Enforce request body size limits at the reverse proxy or application server.
- Isolate temporary upload storage and monitor disk consumption for multipart endpoints.
References
th4s1s Reporter
jeremyevans Remediation reviewer
ioquatix Coordinator
Summary
Rack::Multipart::Parseronly wraps the request body in aBoundedIOwhenCONTENT_LENGTHis present. When amultipart/form-datarequest is sent without aContent-Lengthheader, such as with HTTP chunked transfer encoding, multipart parsing continues until end-of-stream with no total size limit.For file parts, the uploaded body is written directly to a temporary file on disk rather than being constrained by the buffered in-memory upload limit. An unauthenticated attacker can therefore stream an arbitrarily large multipart file upload and consume unbounded disk space.
This results in a denial of service condition for Rack applications that accept multipart form data.
Details
Rack::Multipart::Parser.parseappliesBoundedIOonly whencontent_lengthis notnil:When
CONTENT_LENGTHis absent, the parser reads the multipart body until EOF without a global byte limit.Although Rack enforces
BUFFERED_UPLOAD_BYTESIZE_LIMITfor retained non-file parts, file uploads are handled differently. When a multipart part includes a filename, the body is streamed to aTempfile, and the retained-size accounting is not applied to that file content. As a result, file parts are not subject to the same upload size bound.An attacker can exploit this by sending a chunked
multipart/form-datarequest containing a file part and continuously streaming data without declaring aContent-Length. Rack will continue writing the uploaded data to disk until the client stops or the server exhausts available storage.Impact
Any Rack application that accepts
multipart/form-datauploads may be affected if no upstream component enforces a request body size limit.An unauthenticated attacker can send a large chunked file upload to consume disk space on the application host. This may cause request failures, application instability, or broader service disruption if the host runs out of available storage.
The practical impact depends on deployment architecture. Reverse proxies or application servers that enforce upload limits may reduce or eliminate exploitability, but Rack itself does not impose a total multipart upload limit in this code path when
CONTENT_LENGTHis absent.Mitigation
CONTENT_LENGTHis absent.References