Rack has an unsafe default in Rack::QueryParser allows params_limit bypass via semicolon-separated parametersCVE-2025-59830

## 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.

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