CVE-2001-0904 in Internet Explorer
Summary
by MITRE
Internet Explorer 5.5 and 6 with the Q312461 (MS01-055) patch modifies the HTTP_USER_AGENT (UserAgent) information that indicates that the patch has been installed, which could allow remote malicious web sites to more easily identify and exploit vulnerable clients.
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Analysis
by VulDB Data Team • 04/07/2017
This vulnerability in Internet Explorer 5.5 and 6 represents a significant security issue that emerged from the Microsoft security patch MS01-055 designed to address a critical vulnerability in the browser. The flaw occurs when the Q312461 patch is applied to affected versions of Internet Explorer, creating a situation where the browser's UserAgent string is modified in a way that reveals the presence of the security update. This modification effectively creates a fingerprint that malicious actors can use to identify which systems are running patched versions of Internet Explorer, thereby enabling more targeted exploitation attempts. The vulnerability falls under the category of information disclosure, as it inadvertently exposes system configuration details that should remain hidden from remote attackers. According to CWE-200, this represents an information exposure vulnerability where sensitive system information is revealed through the modification of HTTP headers, specifically the UserAgent string that browsers send to web servers during HTTP communication.
The technical implementation of this flaw occurs at the application layer where Internet Explorer's HTTP client component is modified to include patch identification information within the UserAgent header. When a patched browser makes an HTTP request to a remote server, the modified UserAgent string contains specific indicators that reveal the presence of the Q312461 patch, typically through version number modifications or additional identifiers that reference the security update. This behavior creates a predictable pattern that malicious websites can easily detect and exploit by implementing simple user agent string detection mechanisms. The vulnerability is particularly concerning because it undermines the principle of least information disclosure, where browsers should not reveal unnecessary system details that could aid in attack targeting. The flaw creates a situation where security hardening efforts actually become counterproductive by providing attackers with information that helps them craft more effective attacks against vulnerable systems.
The operational impact of this vulnerability extends beyond simple information disclosure, as it directly enables more sophisticated attack vectors that could lead to complete system compromise. Malicious web sites can now implement detection logic that identifies patched Internet Explorer versions and automatically serves exploit code tailored specifically to those configurations, potentially bypassing other security measures that might otherwise protect against attacks. This creates a scenario where the very security update intended to protect users becomes a tool for attackers to more effectively target vulnerable systems, effectively turning the security patch into an attack vector. The vulnerability affects organizations that deploy Internet Explorer in enterprise environments where attackers may have access to web resources that could be used to perform reconnaissance and attack targeting. From an ATT&CK framework perspective, this vulnerability enables techniques such as credential access through web-based attacks and defense evasion by allowing attackers to bypass security controls that might otherwise be effective against unpatched systems.
Organizations facing this vulnerability should implement immediate mitigations that include network-level filtering to prevent access to malicious websites that could exploit this information disclosure. The recommended approach involves configuring web proxies and firewalls to monitor and block suspicious user agent patterns that indicate the presence of patched systems, while also implementing additional security controls such as browser hardening measures that limit the information exposed through HTTP headers. The most effective long-term solution involves migrating away from Internet Explorer to more modern browsers that do not exhibit this behavior, as well as implementing comprehensive network monitoring to detect and respond to exploitation attempts that leverage this vulnerability. Security teams should also consider implementing endpoint detection and response solutions that can monitor for unusual network traffic patterns that might indicate exploitation attempts targeting this specific vulnerability, while ensuring that all systems are properly patched against the underlying vulnerability that the Q312461 patch was designed to address, rather than relying on the patch's modification of the UserAgent string as a security measure.