What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-08-27 · updated 2026-08-31
• Weakness: CWE-407

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 products

• nltk: before 3.10.3 — fixed in 3.10.3

How to fix

Upgrade to a fixed version: nltk 3.10.3. Check the vendor advisory for mitigations.

References

• https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g
• https://www.vulncheck.com/advisories/nltk-porterstemmer-before-3.10.3-quadratic-time-dos
• https://github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g

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