Vulnerability in Nltk
CVE-2026-80206
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.
Vulnerability class: ReDoS (Regular Expression Denial of Service)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.9 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.004 (36.3th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 36th percentile — 36.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.004 · change: +0.002
Affected products
Weakness classification (CWE)
References
- github.com/nltk/nltk/security/advisories/GHSA-w3v8-gmh9-3wv7 (Exploit, vendor-advisory, Vendor Advisory)
- www.vulncheck.com/advisories/nltk-3.10.2-regular-expression-denial-of-service-v… (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-80206?
- CVE-2026-80206 is a medium-severity vulnerability in Nltk, classified under Inefficient Regular Expression Complexity. CVSS score: 5.9/10. Published 2026-08-26.
- How severe is CVE-2026-80206?
- Medium severity. CVSS v3 base score is 5.9 out of 10.