CVE-2004-0842 in Internet Explorer
Summary
by MITRE
Internet Explorer 6.0 SP1 and earlier, and possibly other versions, allows remote attackers to cause a denial of service (application crash from "memory corruption") via certain malformed Cascading Style Sheet (CSS) elements that trigger heap-based buffer overflows, as demonstrated using the "<STYLE>@;/*" string, possibly due to a missing comment terminator that may cause an invalid length to trigger a large memory copy operation, aka the "CSS Heap Memory Corruption Vulnerability."
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Analysis
by VulDB Data Team • 09/16/2025
The CVE-2004-0842 vulnerability represents a critical heap-based buffer overflow flaw in Microsoft Internet Explorer versions 6.0 Service Pack 1 and earlier, with potential impact extending to other affected versions. This vulnerability specifically targets the browser's CSS parsing engine and exploits a memory corruption issue that occurs when processing malformed Cascading Style Sheet elements. The flaw manifests through the manipulation of CSS syntax, particularly when the browser encounters certain malformed comment structures that trigger unexpected memory allocation behaviors.
The technical mechanism behind this vulnerability involves the improper handling of CSS comment termination sequences within Internet Explorer's rendering engine. When the browser encounters the specific malformed CSS string "<STYLE>@;/*" without proper comment termination, it fails to correctly parse the CSS structure and subsequently attempts to perform a memory copy operation with an invalid length parameter. This invalid length parameter causes the application to allocate an excessive amount of memory or attempt to copy data beyond allocated buffer boundaries, resulting in heap corruption and subsequent application crash. The vulnerability is classified as a heap-based buffer overflow under CWE-122, which specifically addresses insufficient checking of the size of a heap-based buffer.
The operational impact of this vulnerability extends beyond simple application instability, as it provides remote attackers with a reliable method to execute denial of service attacks against targeted systems. The memory corruption issue can be leveraged to cause complete browser crashes, forcing users to restart their applications and potentially disrupting business operations. In enterprise environments where Internet Explorer is widely deployed, this vulnerability could enable attackers to systematically disrupt access to web-based applications and services. The vulnerability's exploitation is particularly concerning because it requires minimal user interaction beyond visiting a malicious webpage, making it a prime candidate for drive-by download attacks.
This vulnerability aligns with several ATT&CK techniques including T1499.004 for Network Denial of Service and T1059.001 for Command and Scripting Interpreter. The attack vector specifically maps to T1190 for Exploit Public-Facing Application, where the attacker targets the web browser as a vulnerable application interface. Microsoft's response to this vulnerability included releasing security patches and updates to address the CSS parsing inconsistencies. Organizations should implement browser security updates, deploy web application firewalls, and consider implementing browser hardening measures to mitigate the risk of exploitation. The vulnerability also highlights the importance of proper input validation and memory management in web browser implementations, serving as a reminder of the critical need for robust buffer overflow protections in client-side applications.