CVE-2026-81722

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-407

• https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94gExploit, Vendor Advisory
• https://www.vulncheck.com/advisories/nltk-porterstemmer-before-3.10.3-quadratic-time-dosThird Party Advisory
• https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94gExploit, Vendor Advisory

Published

Aug 27, 2026

Last Modified

Aug 31, 2026

Status

Analyzed

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