CVE-2021-43854: high-severity vulnerability in nltk
Inefficient Regular Expression Complexity in nltk
Published · Updated
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21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 7.5epss 2.8%
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.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Affected products
nltk · nltkRelated CVEs — nltk
In the same product, most dangerous first.
CVE-2026-79657CRITICALNLTK before 3.10.3 Remote Code Execution via Unsafe Pickle DeserializationEPSS 1.3%CVE-2026-33231HIGHNLTK has unauthenticated remote shutdown in nltk.app.wordnet_appEPSS 1.3%CVE-2026-71513HIGHNLTK 3.10.0 through 3.10.2 Remote Code Execution via AllowlistUnpickler Dotted-Name BypassEPSS 1.1%CVE-2026-79675CRITICALNLTK before 3.10.3 JVM Argument Injection via Per-Call OptionsEPSS 0.8%CVE-2026-72818HIGHNLTK TweetTokenizer URL Pattern Backtracks Catastrophically on Naked-Domain-Like InputEPSS 0.7%CVE-2026-80205HIGHNLTK before 3.10.0 ReDoS via Text.findall() unvalidated regexEPSS 0.7%