GHSA-625h-95r8-8xpm Rack has an unsafe default in Rack::QueryParser allows params_limit bypass via semicolon-separated parameters

• RubyGems rack: < 2.2.18 → fixed in 2.2.18

Summary `Rack::QueryParser` in version `< 2.2.18` enforces its `params_limit` only for parameters separated by `&`, while still splitting on both `&` and `;`. As a result, attackers could use `;` separators to bypass the parameter count limit and submit more parameters than intended. Details The issue arises because `Rack::QueryParser#check_query_string` counts only `&` characters when determining the number of parameters, but the default separator regex `DEFAULT_SEP = /[&;] */n` splits on both `&` and `;`. This mismatch means that queries using `;` separators were not included in the parameter count, allowing `params_limit` to be bypassed. Other safeguards (`bytesize_limit` and `key_space_limit`) still applied, but did not prevent this particular bypass. Impact Applications or middleware that directly invoke `Rack::QueryParser` with its default configuration (no explicit delimiter) could be exposed to increased CPU and memory consumption. This can be abused as a limited denial-of-service vector. `Rack::Request`, the primary entry point for typical Rack applications, uses `QueryParser` in a safe way and does not appear vulnerable by default. As such, the severity is considered low, with the impact limited to edge cases where `QueryParser` is used directly. Mitigation • Upgrade to a patched version of Rack where both `&` and `;` are counted consistently toward `params_limit`. • If upgrading is not immediately possible, configure `QueryParser` with an explicit delimiter (e.g., `&`) to avoid the mismatch. • As a general precaution, enforce query string and request size limits at the web server or proxy layer (e.g., Nginx, Apache, or a CDN) to mitigate excessive parsing overhead.