CVE-2025-61771: 
Ruby vulnerability analysis and mitigation

CVE-2025-61771 affects Rack, a modular Ruby web server interface, in versions prior to 2.2.19, 3.1.17, and 3.2.2. The vulnerability was discovered and disclosed on October 7, 2025, and involves the Rack::Multipart::Parser component which stores non-file form fields entirely in memory as Ruby String objects, potentially leading to memory exhaustion (GitHub Advisory).

The vulnerability exists in the Rack::Multipart::Parser component where non-file form fields (parts without a filename) are stored entirely in memory as Ruby String objects. When processing multipart/form-data requests, file parts are streamed to temporary files, but non-file parts are buffered into memory without size limits. The vulnerability has been assigned a CVSS v3.1 score of 7.5 (High) with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H, indicating it can be exploited remotely with low complexity and requires no privileges or user interaction (GitHub Advisory).

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). The impact scales with request size and concurrency, potentially causing worker crashes or severe garbage-collection overhead. All Rack applications processing multipart form submissions are affected (GitHub Advisory).

The vulnerability has been patched in versions 2.2.19, 3.1.17, and 3.2.2, which enforce a reasonable size cap (2 MiB) for non-file fields. For users unable to upgrade immediately, workarounds include restricting maximum request body size at the web-server or proxy layer (e.g., using Nginx client_max_body_size) and validating and rejecting unusually large form fields at the application level (GitHub Advisory).