Crass is a Ruby CSS parser that's fully compliant with the CSS Syntax Level 3 specification. Prior to 1.0.7, Crass recursively parses "CSS simple blocks and functions" without a depth guard. An attacker-controlled CSS value containing deeply nested blocks or functions can recurse until Ruby raises "SystemStackError": stack level too deep, or cause excessive memory usage. Version 1.0.7 adds a configurable depth limit with a conservative default. This issue is fixed in version 1.0.7.