Vulnerability · Published Aug 27, 2026 · Assigned by VulnCheck

CVE-2026-81725NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader

Low 3.7

In plain English

A low-severity uncontrolled resource consumption in nltk from nltk. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 31% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)3.7Low

Exploit odds (30 days)0.4%top 69.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

NLTK before 3.10.3 contains a regular expression denial of service vulnerability in Pl196xCorpusReader that allows attackers to cause quadratic CPU consumption by supplying malformed TEI blocks with many unmatched opening tags. Attackers can exploit lazy regex patterns in the read_block method through public APIs like words() and tagged_words() to force repeated rescans and achieve near-quadratic runtime growth.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	nltk	no fix listed	DEBIAN-CVE-2026-81725
Debian:13	nltk	no fix listed	DEBIAN-CVE-2026-81725
Debian:14	nltk	3.10.3-1	DEBIAN-CVE-2026-81725
PyPI	nltk	3.10.3	GHSA-8mpw-7fpc-4gqj
PyPI	nltk	3.10.3	PYSEC-2026-3752

Affected products (as reported)

Vendor	Product	Versions
nltk	nltk	0 – before 3.10.3

Weakness: CWE-400 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

References

GitHub Security Advisory (GHSA-8mpw-7fpc-4gqj) vendor-advisory
VulnCheck Advisory: NLTK before 3.10.3 Regular Expression Denial of Service via Pl196xCorpusReader third-party-advisory