Inefficient Regular Expression Complexity in nltk (word_tokenize, sent_tokenize)
ImpactThe vulnerability is present in [`PunktSentenceTokenizer`](https://www.nltk.org/api/nltk.tokenize.punkt.html#nltk.tokenize.punkt.PunktSentenceTokenizer), [`sent_tokenize`](https://www.nltk.org/api/nltk.tokenize.html#nltk.tokenize.sent_tokenize)  and [`word_tokenize`](https://www.nltk.org/api/nltk.tokenize.html#nltk.tokenize.word_tokenize). Any users of this class, or these two functions, are vulnerable to a Regular Expression Denial of Service (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. The effect of this vulnerability is noticeable with the following example:Which gave the following output during testing:I canceled the execution of the program after running it for several hours.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, or applying the workaround described below.PatchesThe problem has been patched in NLTK 3.6.6. After the fix, running the above program gives the following result:This output shows a linear relationship in execution time versus input length, which is desirable for regular expressions.
We recommend updating to NLTK 3.6.6+ if possible.WorkaroundsThe execution time of the vulnerable functions is exponential to the length of a malicious input. With other words, 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.ReferencesThe issue showcasing the vulnerability: https://github.com/nltk/nltk/issues/2866The pull request containing considerably more information on the vulnerability, and the fix: https://github.com/nltk/nltk/pull/2869The commit containing the fix: 1405aad979c6b8080dbbc8e0858f89b2e3690341Information on CWE-1333: Inefficient Regular Expression Complexity: https://cwe.mitre.org/data/definitions/1333.htmlFor more informationIf you have any questions or comments about this advisory:Open an issue in [github.com/nltk/nltk](https://github.com/nltk/nltk)Email us at [nltk.team@gmail.com](mailto:nltk.team@gmail.com)