Vulnerability · Published Aug 26, 2026 · Assigned by VulnCheck

CVE-2026-80206NLTK 3.10.2 Regular Expression Denial of Service via tgrep

Medium 5.9

In plain English

A medium-severity regular-expression denial of service in nltk from nltk. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 38% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.9Medium

Exploit odds (30 days)0.4%top 61.5% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

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 packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	nltk	no fix listed	DEBIAN-CVE-2026-80206
Debian:13	nltk	no fix listed	DEBIAN-CVE-2026-80206
Debian:14	nltk	3.10.3-1	DEBIAN-CVE-2026-80206
PyPI	nltk	3.10.3	GHSA-w3v8-gmh9-3wv7
PyPI	nltk	3.10.3	PYSEC-2026-3751

Affected products (as reported)

Vendor	Product	Versions
nltk	nltk	0 – before 3.10.3

Weakness: CWE-1333 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

References

GitHub Security Advisory (GHSA-w3v8-gmh9-3wv7) vendor-advisory
VulnCheck Advisory: NLTK 3.10.2 Regular Expression Denial of Service via tgrep third-party-advisory