Affected: Homebrew: mailcatcher

## 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…

Affected: rack

Rack is a modular Ruby web server interface. Prior to version 2.2.18, Rack::QueryParser 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. 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. This issue has been patched in version 2.2.18.