CVE-2026-80205

NLTK (Natural Language Toolkit) contains a regular expression denial-of-service flaw (ReDoS, CWE-1333) in the nltk.text module: Text.findall() and TokenSearcher.findall() pass caller-supplied regular expressions to Python's re engine with no validation or timeout, after preprocessing that does not prevent catastrophic backtracking. When an application feeds an attacker-controlled pattern (for example, a nested-quantifier pattern that fails to match) into these methods against a crafted token string, re.findall can backtrack indefinitely and the call never returns. An unauthenticated attacker who can influence the regex or the searched tokens can saturate the CPU of the Python process with a single request, denying service to all other users of that process; confidentiality and integrity are unaffected. Affected parties are Python applications installed via pip that expose these nltk.text search functions to external input; NLTK is extremely widely installed, but only services that pass untrusted regular expressions to findall() are actually exploitable. The issue was resolved upstream in commit d8e4753 and no in-the-wild exploitation is known: it is not in CISA KEV, no public PoC is known, EPSS estimates roughly a 0.5% probability of exploitation within 30 days (40th percentile), and CVSS 4.0 scores it 8.7 High.

What to do: Update NLTK to a release that includes fix commit d8e4753; the advisory does not name a version number, so verify the fix is present in your installed version rather than assuming any current release is safe. Until then, do not pass user-controlled regular expressions to Text.findall()/TokenSearcher.findall(); if unavoidable, validate and length-limit patterns (avoid nested quantifiers) and run matching under a timeout, for example in a worker thread with a deadline. Audit your codebase and dependencies for calls to these methods with external input to determine whether you are exposed at all.