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).

Published 22 Aug 2026, 15:16 UTC Last modified 22 Aug 2026, 15:16 UTC

6.9

CVSS 4.0 MEDIUM base score

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:L/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Attack profile

The conditions required to exploit this vulnerability and its potential impact.

Attack vector Network

Attack complexity Low

Privileges required None

User interaction None

Scope Not assessed

Confidentiality Not assessed

Integrity Not assessed

Availability Not assessed

Affected products

Product applicability statements supplied with the NVD record.

NVD has not published structured affected-product data for this record.

Weakness classification

CWE categories help security teams group the underlying software weakness.

CWECWE-918

Source references

External advisories, patches and technical reports attached to this CVE record.

NVD National Vulnerability Database recordnvd.nist.gov 01 Technical reference disclosure@vulncheck.com 02 Technical reference disclosure@vulncheck.com