GHSA-w9pc-fmgc-vxvw

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GHSA-w9pc-fmgc-vxvw

Summary: Rack: Multipart parser buffers large non‑file fields entirely in memory, enabling DoS (memory exhaustion)

Details: ## 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.

References: https://github.com/rack/rack/security/advisories/GHSA-w9pc-fmgc-vxvw, https://nvd.nist.gov/vuln/detail/CVE-2025-61771, https://github.com/rack/rack/commit/589127f4ac8b5cf11cf88fb0cd116ffed4d2181e, https://github.com/rack/rack/commit/d869fed663b113b95a74ad53e1b5cae6ab31f29e, https://github.com/rack/rack/commit/e08f78c656c9394d6737c022bde087e0f33336fd, https://github.com/rack/rack, https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2025-61771.yml