Vulnerability in Nltk
CVE-2026-81724
NLTK before 3.10.3 contains an uncontrolled recursion vulnerability in nltk.featstruct.FeatStructReader that allows unauthenticated attackers to cause a denial of service by supplying deeply nested feature-structure input. Attackers can craft trivial payloads with nested brackets that exceed Python's recursion limit and raise an unhandled RecursionError, crashing applications that parse user-supplied feature structures or feature grammars.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L.
EPSS exploit prediction
EPSS: 0.005 (37.7th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 38th percentile — 37.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.005 · change: +0.002
Affected products
Weakness classification (CWE)
References
- github.com/nltk/nltk/security/advisories/GHSA-cw6x-m8jw-qmrh (Exploit, vendor-advisory, Vendor Advisory)
- www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrol… (Third Party Advisory, third-party-advisory)
Frequently asked questions
- What is CVE-2026-81724?
- CVE-2026-81724 is a medium-severity vulnerability in Nltk, classified under Uncontrolled Recursion. CVSS score: 5.3/10. Published 2026-08-27.
- How severe is CVE-2026-81724?
- Medium severity. CVSS v3 base score is 5.3 out of 10.