CVE-2000-0439 in Internet Explorer
Summary
by MITRE
Internet Explorer 4.0 and 5.0 allows a malicious web site to obtain client cookies from another domain by including that domain name and escaped characters in a URL, aka the "Unauthorized Cookie Access" vulnerability.
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Analysis
by VulDB Data Team • 04/21/2026
The CVE-2000-0439 vulnerability represents a critical security flaw in Internet Explorer versions 4.0 and 5.0 that exploited the browser's improper handling of cross-domain cookie access. This vulnerability falls under the CWE-200 category, specifically addressing "Information Exposure Through Sent Data" and demonstrates how web browsers can inadvertently leak sensitive authentication tokens across domain boundaries. The flaw emerged from Internet Explorer's implementation of the Same-Origin Policy, which should have prevented scripts from accessing cookies from different domains but failed to properly enforce these restrictions in specific URL construction scenarios.
The technical exploitation of this vulnerability occurred when malicious websites constructed URLs containing domain names and escaped characters that would cause Internet Explorer to improperly interpret cookie access requests. Attackers could craft specially formatted URLs that would trick the browser into sending cookies from one domain to another domain, effectively enabling unauthorized access to user sessions and authentication tokens. The vulnerability specifically targeted the cookie handling mechanism within Internet Explorer's HTTP request processing, where the browser would incorrectly parse URLs containing escaped domain names and forward cookies associated with those domains even when the requesting site was not authorized to access them.
The operational impact of this vulnerability was significant as it allowed attackers to perform session hijacking and authentication bypass attacks across different domains. An attacker could create a malicious webpage that would automatically send cookies from a user's session with a banking website to their own server, enabling unauthorized access to financial accounts. This vulnerability particularly affected users who maintained multiple authenticated sessions across different domains, as it provided a mechanism for stealing authentication tokens without requiring additional user interaction or complex attack vectors. The exploit could be delivered through various means including malicious websites, email attachments, or compromised web pages that users might visit during normal browsing activities.
This vulnerability aligns with several tactics described in the MITRE ATT&CK framework, particularly those related to credential access and privilege escalation through web-based attacks. The attack vector corresponds to techniques involving the exploitation of web browser vulnerabilities to gain unauthorized access to user sessions and authentication tokens. Organizations affected by this vulnerability needed to implement immediate mitigations including browser updates, network-level restrictions, and user education about avoiding suspicious websites. The remediation process required users to upgrade to newer versions of Internet Explorer where the Same-Origin Policy was properly enforced, along with implementing additional security measures such as secure cookie flags and proper HTTP headers to prevent similar cross-domain access issues in other web applications.