CVE-2000-0518 in Internet Explorer
Summary
by MITRE
Internet Explorer 4.x and 5.x does not properly verify all contents of an SSL certificate if a connection is made to the server via an image or a frame, aka one of two different "SSL Certificate Validation" vulnerabilities.
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Analysis
by VulDB Data Team • 04/21/2026
The vulnerability identified as CVE-2000-0518 represents a critical SSL certificate validation flaw affecting Internet Explorer versions 4.x and 5.x when establishing connections through image or frame elements. This weakness stems from the browser's failure to properly validate all components of SSL certificates during secure connections initiated through these specific methods, creating potential security risks that could be exploited by malicious actors. The vulnerability specifically targets the SSL certificate verification process, which is fundamental to establishing trust between web browsers and servers in secure communications.
This technical flaw operates at the core of SSL/TLS security implementation within Internet Explorer, where the browser's certificate validation mechanism becomes compromised when processing connections through embedded image tags or HTML frames. The vulnerability manifests when the browser accepts certificates that may be invalid or compromised, particularly when these certificates are presented through image or frame elements rather than direct connection methods. This behavior violates standard security protocols and creates opportunities for man-in-the-middle attacks where attackers can present fraudulent certificates that the browser incorrectly accepts due to incomplete validation checks.
The operational impact of this vulnerability extends beyond simple certificate validation failures, as it enables attackers to perform certificate spoofing and impersonation attacks against unsuspecting users. When users browse websites that contain maliciously crafted image or frame elements, the browser may establish connections to malicious servers using fraudulent certificates that appear valid to the user interface. This creates a dangerous situation where users believe they are communicating with legitimate secure websites while actually transmitting sensitive information to attackers who have successfully bypassed the certificate validation process.
Security researchers have classified this vulnerability under CWE-295 which specifically addresses "Improper Certificate Validation" in security protocols. The vulnerability aligns with ATT&CK technique T1071.004 which covers "Application Layer Protocol: DNS" and relates to how applications handle certificate validation during network communications. The flaw demonstrates a fundamental weakness in how Internet Explorer processes certificate validation for different connection methods, creating a gap in the security model that attackers can exploit to undermine the integrity of secure communications.
Mitigation strategies for CVE-2000-0518 require immediate action to address the root cause of the vulnerability through browser updates and security patches. Organizations should implement immediate browser upgrades to newer versions that properly validate SSL certificates regardless of connection method, while also deploying network monitoring solutions to detect suspicious certificate behavior. Additionally, security administrators should consider implementing certificate pinning mechanisms and regular security audits to ensure that all SSL connections are properly validated. The vulnerability underscores the importance of comprehensive certificate validation across all connection types and highlights the critical need for robust security implementations that do not rely on single validation points within the browser's security architecture.