Machine-written summary
Summary written by Claude Haiku from the source advisory. Not reviewed by a person.
NLTK versions before 3.10.3 are affected by a denial-of-service issue in the feature-structure parser (nltk.featstruct.FeatStructReader, used by FeatStruct(str) and FeatureGrammar.fromstring()). Deeply nested bracket input causes an unhandled RecursionError instead of the library's normal parse error, so an unauthenticated attacker who can supply the text can crash the process that parses it. Upgrade to NLTK 3.10.3 or later. Applications can also limit the nesting depth or reject untrusted feature-structure text before parsing.
Not analysed yet
We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.
Severity and exploitation
| Score | Rating | CVSS | Vector | Source |
|---|---|---|---|---|
| 6.9 | Medium | 4.0 | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N | CNA (VulnCheck) |
- CISA KEV Not listed.
- EPSS 0.0046, the 40th percentile, on 14 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.
Timeline
- 31 Oct 2026, 14:10 UTC First seen by agentprime
- 31 Oct 2026, 14:10 UTC Confirmed by OSV
Sources
- GitHub Advisory Database: GHSA-cw6x-m8jw-qmrh
- OSV: GHSA-cw6x-m8jw-qmrh
- github.com/nltk/nltk/security/advisories/GHSA-cw6x-m8jw-qmrh
- nvd.nist.gov/vuln/detail/CVE-2026-81724
- github.com/nltk/nltk/commit/43c7b78cc8ea37e5cd3a129e27e32c415ea21cf1
- github.com/nltk/nltk
- github.com/nltk/nltk/releases/tag/v3.10.3
- github.com/pypa/advisory-database/tree/main/vulns/nltk/PYSEC-2026-3739.yaml
- www.vulncheck.com/advisories/nltk-before-3.10.3-denial-of-service-via-uncontrolled-recursion