CVE-2026-63311

NLTK before 3.10.0 (affected versions <= 3.9.4) contains a server-side request forgery (SSRF) vulnerability in the validate_network_url() function in nltk/pathsec.py. The _resolve_hostname() helper catches OSError and ValueError during socket.getaddrinfo() and returns an empty list; when DNS resolution fails, the validation loop executes no IP checks and the function fails open, allowing urlopen() to proceed without validation. An attacker who can trigger DNS resolution failures or use DNS rebinding can bypass SSRF protections and reach restricted network resources, including cloud metadata endpoints (e.g., 169.254.169.254).

• CWE-918

• https://github.com/nltk/nltk/security/advisories/GHSA-3gqm-fcw5-w839Exploit, Mitigation, Vendor Advisory
• https://www.vulncheck.com/advisories/nltk-before-ssrf-via-dns-resolution-failureThird Party Advisory

Published

Aug 22, 2026

Last Modified

Aug 27, 2026

Status

Analyzed

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