Rack's multipart parser buffers unbounded per-part headers, enabling DoS (memory exhaustion)

Vulnerability Description

Rack is a modular Ruby web server interface. In versions prior to 2.2.19, 3.1.17, and 3.2.2, `Rack::Multipart::Parser` can accumulate unbounded data when a multipart part’s header block never terminates with the required blank line (`CRLFCRLF`). The parser keeps appending incoming bytes to memory without a size cap, allowing a remote attacker to exhaust memory and cause a denial of service (DoS). Attackers can send incomplete multipart headers to trigger high memory use, leading to process termination (OOM) or severe slowdown. The effect scales with request size limits and concurrency. All applications handling multipart uploads may be affected. Versions 2.2.19, 3.1.17, and 3.2.2 cap per-part header size (e.g., 64 KiB). As a workaround, restrict maximum request sizes at the proxy or web server layer (e.g., Nginx `client_max_body_size`).

References

• https://github.com/rack/rack/security/advisories/GHSA-wpv5-97wm-hp9c
• https://github.com/rack/rack/commit/589127f4ac8b5cf11cf88fb0cd116ffed4d2181e
• https://github.com/rack/rack/commit/d869fed663b113b95a74ad53e1b5cae6ab31f29e
• https://github.com/rack/rack/commit/e08f78c656c9394d6737c022bde087e0f33336fd