Rack's multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)

Summary

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` 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). 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. Versions 2.2.19, 3.1.17, and 3.2.2 enforce a reasonable size cap for non-file fields (e.g., 2 MiB). Workarounds include restricting maximum request body size at the web-server or proxy layer (e.g., Nginx `client_max_body_size`) and validating and rejecting unusually large form fields at the application level.

Timeline

1. Oct 7, 2025, 2:42 PM UTCCVE published
2. Oct 7, 2025, 5:51 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Oct 7, 2025, 5:52 PM UTCRecord last updated

Affected versions

• rack rack

  fixed in 2.2.19; From 3.1 · fixed in 3.1.17; From 3.2 · fixed in 3.2.2

• rackRubyGems

  fixed in 2.2.19

• rackRubyGems

  From 3.1 · fixed in 3.1.17

• rackRubyGems

  From 3.2 · fixed in 3.2.2

References 4