Uncontrolled Recursion in Natural Language Toolkit - CVE-2026-12876
Published: August 15, 2026
Vulnerability identifier: #VU142773
CSH Severity: Medium
CVSS v4: 8.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-12876
CWE-ID: CWE-674
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to cause a denial of service.
The vulnerability exists due to uncontrolled recursion in RecursiveDescentParser and SteppingRecursiveDescentParser when parsing crafted ambiguous or left-recursive grammars. A remote attacker can provide a crafted grammar or input to cause a denial of service.
The issue can result in unbounded CPU consumption or exhaustion of the Python recursion stack, affecting availability of a single process.
Affected software
Natural Language Toolkit
How to mitigate CVE-2026-12876
Install security update from vendor's website.
Natural Language Toolkit - update to 3.9.3