CVE-2021-43854

NLTK (Natural Language Toolkit) is a suite of open source Python modules,
data sets, and tutorials supporting research and development in Natural
Language Processing. Versions prior to 3.6.5 are vulnerable to regular
expression denial of service (ReDoS) attacks. The vulnerability is present
in PunktSentenceTokenizer, sent_tokenize and word_tokenize. Any users of
this class, or these two functions, are vulnerable to the ReDoS attack. In
short, a specifically crafted long input to any of these vulnerable
functions will cause them to take a significant amount of execution time.
If your program relies on any of the vulnerable functions for tokenizing
unpredictable user input, then we would strongly recommend upgrading to a
version of NLTK without the vulnerability. For users unable to upgrade the
execution time can be bounded by limiting the maximum length of an input to
any of the vulnerable functions. Our recommendation is to implement such a
limit.