Critical Vulnerability Information Vulnerability Overview CVE ID: CVE-2025-15104 CVSS v4.0 Base Score: 6.9 Vulnerability Name: Nu Html Checker (validator.nu) - Bypass Restriction Vulnerability, Allowing Local SSRF Discoverer: Oscar Uribe, Fluid Attacks Disclosure Date: January 13, 2026 Vulnerability Description Impact: This vulnerability allows remote attackers to bypass restrictions, enabling the server to perform arbitrary HTTP/HTTPS requests to internal resources, including local host services. Attack Vectors: 1. Direct SSRF via the URL parameter. 2. SSRF via XML External Entities (when using the parser). Insufficient Security Controls: - Hostname-based local host blocking: Only blocks strings like and , but does not block domains that resolve via DNS to private/loopback IP addresses. - Port filtering: Only allows ports 80, 81, 443, and ports above 1024, but many modern services run on high-numbered ports. Evidence and Validation Proof of Concept (PoC): Additional Information System Information: - Affected Software: Nu Html Checker (vnu) - Version: Latest version - Operating System: Any system