CVE-1999-1094 in Internet Explorer
Summary
by MITRE
Buffer overflow in Internet Explorer 4.01 and earlier allows remote attackers to execute arbitrary commands via a long URL with the "mk:" protocol, aka the "MK Overrun security issue."
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Analysis
by VulDB Data Team • 04/20/2026
The CVE-1999-1094 vulnerability represents a critical buffer overflow flaw in Microsoft Internet Explorer 4.01 and earlier versions that specifically targets the handling of URLs with the "mk:" protocol. This vulnerability falls under the CWE-121 category of buffer overflow conditions, where insufficient bounds checking allows attackers to write beyond the allocated memory buffer. The flaw occurs when Internet Explorer processes a maliciously crafted URL containing the mk: protocol identifier, which triggers an improper memory allocation during URL parsing operations. The vulnerability is particularly dangerous because it allows remote attackers to execute arbitrary code on vulnerable systems without requiring any user interaction beyond visiting a malicious webpage or clicking a link.
The technical implementation of this buffer overflow stems from the browser's inadequate validation of URL components when processing the mk: protocol, which is typically used for Microsoft's Multiple Key Protocol for handling various network operations. When a specially crafted URL containing an excessively long string following the mk: prefix is processed, the application fails to properly check the length of the input data against the allocated buffer size. This allows attackers to overflow the buffer and overwrite adjacent memory locations, potentially corrupting the program's execution flow and enabling code injection attacks. The vulnerability is classified as a classic stack-based buffer overflow, where the overflow occurs in the stack memory region, making it particularly susceptible to exploitation techniques that manipulate the instruction pointer.
The operational impact of this vulnerability is severe as it provides remote code execution capabilities to attackers who can leverage this flaw to gain complete control over vulnerable systems. The attack vector requires only that a user visits a malicious webpage or clicks on a malicious link containing the crafted URL, making it highly exploitable in phishing campaigns and drive-by download scenarios. Successful exploitation can result in unauthorized access to system resources, data theft, privilege escalation, and potential establishment of persistent backdoors. The vulnerability affects not only individual user systems but also enterprise networks where infected machines can serve as entry points for broader attacks, making it particularly dangerous in corporate environments. This type of vulnerability is categorized under the ATT&CK technique T1059.007 for command and scripting interpreter and T1068 for exploit for privilege escalation.
Mitigation strategies for this vulnerability involve immediate patching of affected Internet Explorer versions, as Microsoft released security updates specifically addressing this buffer overflow issue. Organizations should implement network-level protections such as URL filtering and content validation to prevent users from accessing malicious URLs containing the mk: protocol. Browser security configurations should be hardened by disabling unnecessary protocols and implementing strict input validation measures. Additionally, security awareness training for users to recognize potentially malicious links and websites is crucial in preventing exploitation. The vulnerability highlights the importance of proper input validation and bounds checking in software development, as outlined in the OWASP Top 10 security principles and the CERT/CC secure coding guidelines. System administrators should also monitor network traffic for suspicious URL patterns and implement intrusion detection systems to identify potential exploitation attempts. Given the age of this vulnerability, modern systems should not be running unpatched versions of Internet Explorer, as this represents a significant security risk that could be exploited by contemporary attack frameworks.