Resource exhaustion in Nltk
CVE-2026-81725
NLTK before 3.10.3 contains a regular expression denial of service vulnerability in Pl196xCorpusReader that allows attackers to cause quadratic CPU consumption by supplying malformed TEI blocks with many unmatched opening tags. Attackers can exploit lazy regex patterns in the read_block method through public APIs like words() and tagged_words() to force repeated rescans and achieve near-quadratic runtime growth.
Vulnerability class: DoS (Denial of Service)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 3.7 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L.
EPSS exploit prediction
EPSS: 0.004 (28.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 29th percentile — 28.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.002 · newest: 0.004 · change: +0.001
Affected products
Weakness classification (CWE)
References
- github.com/nltk/nltk/security/advisories/GHSA-8mpw-7fpc-4gqj (Exploit, vendor-advisory, Vendor Advisory)
- www.vulncheck.com/advisories/nltk-before-3.10.3-regular-expression-denial-of-se… (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-81725?
- CVE-2026-81725 is a low-severity vulnerability in Nltk, classified under Uncontrolled Resource Consumption. CVSS score: 3.7/10. Published 2026-08-27.
- How severe is CVE-2026-81725?
- Low severity. CVSS v3 base score is 3.7 out of 10.