CVE-2006-6956 in Internet Explorer
Summary
by MITRE
Microsoft Internet Explorer allows remote attackers to cause a denial of service (crash) via a web page that contains a large number of nested marquee tags, a related issue to CVE-2006-2723.
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Analysis
by VulDB Data Team • 07/13/2021
Microsoft Internet Explorer suffered from a denial of service vulnerability that emerged in 2006, specifically identified as CVE-2006-6956. This flaw manifested when Internet Explorer encountered web pages containing an excessive number of nested marquee tags, leading to application instability and potential system crashes. The vulnerability represented a classic example of resource exhaustion through malformed HTML elements, where the browser's rendering engine failed to properly handle deeply nested markup structures. The issue was closely related to CVE-2006-2723, indicating a broader class of vulnerabilities within Internet Explorer's handling of dynamic HTML elements. When processing pages with excessive nesting levels, the browser's memory management and rendering algorithms would consume increasing amounts of system resources until the application became unresponsive or terminated unexpectedly.
The technical nature of this vulnerability stemmed from Internet Explorer's inadequate bounds checking and recursive processing mechanisms when encountering nested marquee elements. Marquee tags, designed for scrolling text displays, when nested excessively, created a scenario where the browser's internal parsing and rendering processes would recurse through multiple levels of markup without proper termination conditions. This behavior aligns with CWE-121, which addresses stack-based buffer overflow conditions, and CWE-122, which covers heap-based buffer overflow conditions, as the browser's memory allocation patterns became increasingly strained. The vulnerability exploited the browser's lack of proper depth limiting mechanisms in its HTML parser, allowing attackers to craft malicious web pages that would consume system resources until the application crashed.
The operational impact of CVE-2006-6956 extended beyond simple application instability, potentially affecting user productivity and system availability. Users accessing compromised web pages would experience unexpected browser crashes, forcing them to restart their applications and potentially lose unsaved work. In enterprise environments, this vulnerability could be exploited to disrupt normal business operations through targeted attacks against specific users or departments. The vulnerability's remote exploitation capability meant that attackers could deliver malicious payloads through standard web browsing activities, requiring no local system access or privileges. This characteristic placed the vulnerability within the ATT&CK framework under the T1499 category for Network Denial of Service, specifically targeting application availability through web-based attack vectors.
Mitigation strategies for CVE-2006-6956 required both immediate and long-term approaches to address the underlying browser vulnerability. Microsoft recommended applying the relevant security updates and patches released as part of their regular security bulletin cycle, which would modify the browser's HTML parsing logic to include proper nesting depth limits. Organizations should implement network-based filtering solutions to block access to known malicious websites and establish browser security policies that restricted the execution of potentially harmful HTML elements. Additionally, user education programs became crucial to ensure that personnel understood the risks associated with visiting untrusted websites and the importance of keeping browser software updated. The vulnerability highlighted the importance of robust input validation and resource management in web browser implementations, emphasizing the need for defensive programming practices that prevent recursive processing from consuming excessive system resources.