Vulnerability · Published Aug 27, 2026 · Assigned by VulnCheck

CVE-2026-81722nltk PorterStemmer before 3.10.3 Quadratic-time DoS

High 7.5

In plain English

A high-severity security flaw in nltk from nltk. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 44% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High

Exploit odds (30 days)0.5%top 55.6% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

nltk PorterStemmer in versions <= 3.10.2 (fixed in 3.10.3) contains an inefficient-algorithmic-complexity denial of service in PorterStemmer.stem(). The _is_consonant() helper walks backward over the entire run of trailing 'y' characters on every call, and _measure() invokes it for each stem position, causing O(n^2) behavior. A single ~20-50 KB untrusted token consisting of a long run of the letter 'y' followed by a matching suffix (e.g., 'ness') can pin a CPU core for seconds to minutes, causing availability impact.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	nltk	no fix listed	DEBIAN-CVE-2026-81722
Debian:13	nltk	no fix listed	DEBIAN-CVE-2026-81722
Debian:14	nltk	3.10.3-1	DEBIAN-CVE-2026-81722
PyPI	nltk	3.10.3	GHSA-ww6m-cw3f-q94g
PyPI	nltk	3.10.3	PYSEC-2026-3738

Affected products (as reported)

Vendor	Product	Versions
nltk	nltk	0 – before 3.10.3

Weakness: CWE-407 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

GitHub Security Advisory (GHSA-ww6m-cw3f-q94g) vendor-advisory
VulnCheck Advisory: nltk PorterStemmer before 3.10.3 Quadratic-time DoS third-party-advisory