CVE-1999-0858 in Internet Explorer
Summary
by MITRE
Internet Explorer 5 allows a remote attacker to modify the IE client's proxy configuration via a malicious Web Proxy Auto-Discovery (WPAD) server.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-0858 represents a significant security flaw in Internet Explorer 5 that exploited the Web Proxy Auto-Discovery protocol to manipulate client proxy settings remotely. This issue falls under the category of man-in-the-middle attacks and proxy configuration manipulation, where a malicious actor could potentially redirect network traffic through unauthorized proxy servers. The vulnerability specifically targeted the WPAD functionality that was designed to automatically discover proxy settings for web browsers, creating an attack vector that could compromise network security and user privacy.
The technical flaw in Internet Explorer 5 stemmed from insufficient validation of proxy configuration data received from WPAD servers. When a user accessed the internet, IE would automatically attempt to locate a WPAD server to configure proxy settings, but the browser failed to properly authenticate or validate the legitimacy of the proxy configuration data received. This weakness allowed attackers to host a malicious WPAD server that would respond to client requests with forged proxy configuration data, effectively redirecting the user's web traffic through the attacker's proxy server without any user awareness or consent. The vulnerability exploited the trust relationship that browsers establish with WPAD servers, which was intended to simplify network configuration but became a security liability.
The operational impact of this vulnerability was substantial as it enabled attackers to intercept, modify, or redirect all web traffic originating from affected IE 5 clients. This capability could be used for various malicious activities including credential theft, data interception, malware distribution, and network reconnaissance. The vulnerability was particularly dangerous because it could be exploited without requiring any user interaction or specialized knowledge, making it an attractive target for automated attacks. Additionally, since the proxy configuration changes occurred at the client level, they could bypass traditional network security controls and monitoring systems that might not detect the traffic redirection.
Organizations and users affected by this vulnerability needed to implement immediate mitigations to protect their systems. The most effective approaches included disabling WPAD functionality in Internet Explorer, implementing proper network segmentation to isolate WPAD servers, and deploying network monitoring tools to detect unauthorized proxy configuration changes. Security professionals should have also considered updating to newer versions of Internet Explorer that addressed this vulnerability, as well as implementing network-level controls to prevent access to unauthorized WPAD servers. This vulnerability highlighted the importance of validating automatic configuration protocols and demonstrated how seemingly benign network features could become security risks when not properly secured.
The vulnerability aligns with several CWE categories including CWE-284 for improper access control and CWE-345 for insufficient validation of data. From an ATT&CK framework perspective, this issue maps to techniques involving proxy configuration manipulation and credential access through network traffic interception. The vulnerability also demonstrates the broader security principle that automatic configuration protocols, while convenient, must include proper authentication and validation mechanisms to prevent exploitation. This case study became a foundational example of how legacy browser security features could introduce significant risks when not properly implemented with security in mind, influencing subsequent development of more secure proxy configuration protocols and network security practices.