CVE-2009-2069 in Internet Explorer
Summary
by MITRE
Microsoft Internet Explorer before 8 displays a cached certificate for a (1) 4xx or (2) 5xx CONNECT response page returned by a proxy server, which allows man-in-the-middle attackers to spoof an arbitrary https site by letting a browser obtain a valid certificate from this site during one request, and then sending the browser a crafted 502 response page upon a subsequent request.
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Analysis
by VulDB Data Team • 08/11/2021
This vulnerability exists in Microsoft Internet Explorer versions prior to 8 and represents a significant security flaw in the browser's handling of SSL/TLS certificate validation during proxy server interactions. The issue stems from how Internet Explorer processes cached certificates when encountering specific HTTP status codes from proxy servers, creating a window of opportunity for sophisticated man-in-the-middle attacks. The vulnerability specifically affects scenarios where proxy servers return 4xx or 5xx status codes during CONNECT requests, which are typically used for establishing secure connections through proxy servers. When a browser encounters such responses, it may cache a certificate from a legitimate HTTPS site that was accessed during a previous request, then use this cached certificate to validate subsequent malicious responses from a compromised proxy server.
The technical flaw exploits the certificate caching mechanism within Internet Explorer's SSL/TLS implementation, where the browser maintains certificate information for sites that have been previously accessed. When a proxy server returns a 502 response code or similar error condition, the browser may incorrectly validate the connection using a cached certificate from a legitimate site rather than properly validating the certificate against the current connection attempt. This behavior creates a path for attackers to perform certificate spoofing attacks by first establishing a legitimate connection to a target HTTPS site, allowing the browser to cache the valid certificate, and then subsequently intercepting subsequent requests to that same site and responding with a crafted 502 error page that contains a malicious certificate. This vulnerability falls under the CWE-295 category of "Improper Certificate Validation" and aligns with ATT&CK technique T1573.002 for "Encrypted Channel: Proxy", as it exploits the trust relationship between browsers and proxy servers.
The operational impact of this vulnerability is substantial as it allows attackers to bypass standard SSL/TLS security mechanisms that are designed to prevent certificate spoofing and man-in-the-middle attacks. An attacker positioned between a victim and a proxy server can successfully impersonate legitimate HTTPS sites, potentially capturing sensitive data such as login credentials, personal information, or financial data transmitted over the compromised connection. The vulnerability is particularly dangerous in corporate environments where proxy servers are commonly used for web filtering and security monitoring, as it can enable attackers to bypass network security controls while maintaining the appearance of legitimate secure connections to users. The attack requires minimal privileges and can be executed through standard network interception techniques, making it an attractive target for cybercriminals seeking to conduct large-scale credential theft or data exfiltration operations.
Organizations affected by this vulnerability should implement immediate mitigations including updating Internet Explorer to version 8 or later, which contains fixes for this specific certificate caching issue. Additionally, network administrators should consider implementing additional security controls such as explicit certificate pinning policies, enhanced proxy server configurations that properly handle error responses, and network monitoring solutions that can detect anomalous certificate validation patterns. The use of modern browsers with more robust certificate validation mechanisms and the implementation of certificate transparency protocols can provide additional defense-in-depth measures. Organizations should also review their proxy server configurations to ensure that error responses do not inadvertently expose cached certificates, and consider implementing network segmentation to limit the impact of potential certificate spoofing attacks. This vulnerability demonstrates the critical importance of proper certificate validation in secure communication protocols and the potential consequences of inadequate handling of cached security information in web browsers.