CVE-1999-1093 in Internet Explorer
Summary
by MITRE
Buffer overflow in the Window.External function in the JScript Scripting Engine in Internet Explorer 4.01 SP1 and earlier allows remote attackers to execute arbitrary commands via a malicious web page.
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Analysis
by VulDB Data Team • 04/20/2026
The vulnerability identified as CVE-1999-1093 represents a critical buffer overflow flaw within the JScript Scripting Engine of Internet Explorer 4.01 SP1 and earlier versions. This issue specifically affects the Window.External function which serves as a bridge between JavaScript and the Windows Scripting Host environment. The flaw stems from inadequate bounds checking in the handling of string data within this particular scripting interface, creating a condition where maliciously crafted input can exceed the allocated buffer space and overwrite adjacent memory regions. This vulnerability falls under the Common Weakness Enumeration category CWE-121, which specifically addresses stack-based buffer overflow conditions that occur when a program writes data beyond the boundaries of a fixed-length buffer.
The technical exploitation of this vulnerability occurs when a remote attacker crafts a malicious web page containing specially formatted JavaScript code that invokes the Window.External function with excessive input parameters. When Internet Explorer processes this malicious code, the buffer overflow allows the attacker to overwrite critical memory locations including return addresses and function pointers within the execution stack. This memory corruption can be leveraged to redirect program execution flow to malicious code injected by the attacker, effectively enabling remote code execution with the privileges of the user running the vulnerable browser. The attack vector operates entirely through web-based delivery mechanisms, making it particularly dangerous as users can be compromised simply by visiting malicious websites without any additional user interaction required.
The operational impact of this vulnerability extends beyond individual user compromise to encompass broader security implications for enterprise environments and web infrastructure. Organizations running Internet Explorer 4.01 SP1 or earlier versions face significant risk of unauthorized access and potential system compromise when users browse the internet. The vulnerability's exploitation does not require any special user privileges or complex attack scenarios, making it particularly attractive to attackers seeking widespread compromise. This weakness directly aligns with tactics described in the MITRE ATT&CK framework under the T1059.007 technique for 'Scripting' and T1078.004 for 'Valid Accounts' as the attack can leverage legitimate scripting capabilities to achieve malicious objectives. The vulnerability also represents a critical failure in the application's memory management practices, demonstrating the importance of proper input validation and bounds checking in security-critical components.
Mitigation strategies for CVE-1999-1093 should focus on immediate remediation through software updates and patches provided by Microsoft. The most effective solution involves upgrading to Internet Explorer 5.0 or later versions where this vulnerability has been addressed through improved buffer management and input validation mechanisms. Organizations should implement comprehensive patch management policies to ensure all systems running vulnerable versions receive updates promptly. Additionally, network-level protections such as web application firewalls and content filtering systems can provide temporary defense while patches are deployed. Security configurations should include disabling unnecessary scripting capabilities where possible and implementing browser hardening measures. The vulnerability underscores the importance of maintaining up-to-date software versions and demonstrates how seemingly minor flaws in scripting engines can lead to complete system compromise, emphasizing the need for robust security practices and regular vulnerability assessments.