Vulnerability in Nltk
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.005 (42.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 43rd percentile — 42.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.002
Affected products
Weakness classification (CWE)
References
- github.com/nltk/nltk/security/advisories/GHSA-ww6m-cw3f-q94g (Exploit, vendor-advisory, Vendor Advisory)
- www.vulncheck.com/advisories/nltk-porterstemmer-before-3.10.3-quadratic-time-dos (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-81722?
- CVE-2026-81722 is a high-severity vulnerability in Nltk, classified under Inefficient Algorithmic Complexity. CVSS score: 7.5/10. Published 2026-08-27.
- How severe is CVE-2026-81722?
- High severity. CVSS v3 base score is 7.5 out of 10.