Server-Side Request Forgery (SSRF) in Natural Language Toolkit - CVE-2026-63311
Published: August 7, 2026 / Updated: September 14, 2026
Vulnerability identifier: #VU141250
CSH Severity: Medium
CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-63311
CWE-ID: CWE-918
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to perform server-side request forgery.
The vulnerability exists due to improper control of outbound network requests in validate_network_url() in nltk/pathsec.py when handling URLs whose hostname resolution fails. A remote attacker can supply a crafted URL to perform server-side request forgery.
The issue is caused by a fail-open condition when DNS resolution returns an error and _resolve_hostname() returns an empty list, which can also enable bypass after DNS rebinding once the cache entry expires.
Affected software
Natural Language Toolkit
How to mitigate CVE-2026-63311
Install security update from vendor's website.
Natural Language Toolkit - update to 3.10.0