CVE-2026-63311: nltk Server-Side Request Forgery (SSRF)

Vulnerability summary

CVE-2026-63311 is a medium (CVSS 6.9) Server-Side Request Forgery (SSRF) vulnerability in nltk. EPSS estimates a 0.33% chance of exploitation in the next 30 days, higher than 25.3% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 22-08-2026
   PublishedVulnerability first disclosed.
2. 26-08-2026
   Last modifiedVulnerability information updated.

Description

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-918Server-Side Request Forgery (SSRF)The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.