A denial of service vulnerability exists in the NLTK library's FeatStructReader parser due to uncontrolled recursion when processing deeply nested feature-structure strings. The parser lacks a maximum recursion depth limit, causing Python's RecursionError to be raised and crash applications that parse untrusted input. This affects NLP tools and pipelines using FeatStruct or FeatureGrammar.fromstring(). The issue is not related to memory safety or code execution but results in application crashes. A patch is available to address this issue.