Rack: Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)CVE-2025-61771

## Summary `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). ## Details During multipart parsing, file parts are streamed to temporary files, but non-file parts are buffered into memory: ```ruby body = String.new # non-file → in-RAM buffer @mime_parts[mime_index].body << content ``` There 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`. ## Impact 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. ## Mitigation * **Upgrade:** Use a patched version of Rack that enforces a reasonable size cap for non-file fields (e.g., 2 MiB). * **Workarounds:** * Restrict maximum request body size at the web-server or proxy layer (e.g., Nginx `client_max_body_size`). * Validate and reject unusually large form fields at the application level.

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