The NLTK RecursiveDescentParser and SteppingRecursiveDescentParser components are vulnerable to uncontrolled resource consumption when parsing crafted ambiguous or left-recursive grammars. This can cause unbounded CPU usage or exhaustion of the Python recursion stack, resulting in a denial of service. The vulnerability affects versions from 2.0.0 up to but not including 2.0.0_5. A fix is available in version 2.0.0_5.