Vulnerability · Published Aug 27, 2026 · Assigned by VulnCheck

CVE-2026-81724NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion

Medium 5.3

In plain English

A medium-severity security flaw in nltk from nltk. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 40% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.3Medium

Exploit odds (30 days)0.5%top 60.3% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	nltk	no fix listed	DEBIAN-CVE-2026-81724
Debian:13	nltk	no fix listed	DEBIAN-CVE-2026-81724
Debian:14	nltk	3.10.3-1	DEBIAN-CVE-2026-81724
PyPI	nltk	3.10.3	GHSA-cw6x-m8jw-qmrh
PyPI	nltk	3.10.3	PYSEC-2026-3739

Affected products (as reported)

Vendor	Product	Versions
nltk	nltk	0 – before 3.10.3

Weakness: CWE-674 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

References

GitHub Security Advisory (GHSA-cw6x-m8jw-qmrh) vendor-advisory
VulnCheck Advisory: NLTK before 3.10.3 Denial of Service via Uncontrolled Recursion third-party-advisory