CVE-2026-80206: Nltk

Medium severity, CVSS 5.9. EPSS: 0.4% chance of exploitation in the next 30 days.

NLTK before 3.10.3 contains a regular expression denial of service (ReDoS) vulnerability in the tgrep module. The _tgrep_node_action function compiles user-supplied regular expressions embedded in /regex/ pattern nodes and executes them via re.search against tree node labels without any validation or timeout. An attacker who controls the tgrep pattern (e.g., via tgrep_positions() or tgrep_compile() exposed to external input) can supply a pattern that triggers catastrophic backtracking, causing indefinite CPU saturation that blocks the Python process.

Affected products

• Nltk Nltk: before 3.10.3 (fixed in 3.10.3)

Published 2026-08-26. Last modified 2026-09-01.