CVE-2026-12372 - Server-Side Request Forgery (SSRF) in nltk/nltk

CVE-2026-12372 - Server-Side Request Forgery (SSRF) in nltk/nltk

A Server-Side Request Forgery (SSRF) vulnerability exists in nltk/nltk versions 3.9.4 and the current develop branch. The nltk.pathsec.validate_network_url() function, intended to prevent SSRF by rejecting internal network addresses, fails to reject IPs in the RFC 6598 shared address space (100.64.0.0/10). This occurs because Python’s ipaddress module does not classify such addresses as is_private or is_global, and the current guard only checks is_private and a few explicit categories.

An attacker who can influence a URL passed to NLTK’s network-loading helpers can exploit this vulnerability to make a strict-mode application send requests to shared-address-space hosts, potentially exposing non-public infrastructure reachable from the application host. The impact is limited to SSRF-style confidentiality exposure, with no code execution claimed.

Recommendations

• Update NLTK to patch the SSRF vulnerability by properly validating network URLs.
• Review network access controls for NLTK.
• Limit user control over URLs passed to NLTK.

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