CVE-2026-81722: nltk Inefficient Algorithmic Complexity

Vulnerability summary

CVE-2026-81722 is a high (CVSS 8.7) Inefficient Algorithmic Complexity vulnerability in nltk. EPSS estimates a 0.52% chance of exploitation in the next 30 days, higher than 43.3% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 27-08-2026
   PublishedVulnerability first disclosed.
2. 27-08-2026
   Last modifiedVulnerability information updated.

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.

• CWE-407Inefficient Algorithmic ComplexityAn algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.