CVE-2011-1961 in Internet Explorer
Summary
by MITRE
The telnet URI handler in Microsoft Internet Explorer 6 through 9 does not properly launch the handler application, which allows remote attackers to execute arbitrary programs via a crafted web site, aka "Telnet Handler Remote Code Execution Vulnerability."
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Analysis
by VulDB Data Team • 11/17/2021
The vulnerability identified as CVE-2011-1961 represents a critical remote code execution flaw in Microsoft Internet Explorer versions 6 through 9, specifically within the telnet URI handler component. This vulnerability exploits the improper handling of telnet protocol requests when processed through the web browser, creating a pathway for malicious actors to execute arbitrary code on affected systems. The issue stems from the browser's failure to properly validate and sanitize telnet URI schemes before initiating the associated handler application, which typically points to the system's telnet client or a custom handler implementation. The vulnerability is categorized under CWE-74 as a 'Improper Neutralization of Special Elements in Output Used by a Downstream Component' and aligns with ATT&CK technique T1059.007 for command and scripting interpreter execution.
The technical exploitation of this vulnerability occurs when a malicious website presents a specially crafted telnet URI that, when clicked or automatically triggered by the browser, causes Internet Explorer to invoke the telnet handler with potentially malicious parameters. This flaw allows attackers to bypass normal security boundaries and execute code with the privileges of the user running the vulnerable browser. The vulnerability is particularly dangerous because it leverages the browser's built-in URI handling mechanisms, making it difficult to detect through traditional web application firewalls or security controls. Attackers can construct malicious URLs that appear legitimate while secretly triggering harmful actions through the telnet client, potentially leading to full system compromise. The vulnerability's impact is amplified by the fact that many enterprise environments still maintain legacy Internet Explorer installations, and the telnet client is often present on Windows systems by default.
The operational impact of CVE-2011-1961 extends beyond simple remote code execution, as it represents a fundamental flaw in how web browsers handle protocol handlers and can be exploited in various attack scenarios. The vulnerability enables attackers to perform actions such as establishing unauthorized network connections, executing local programs, and potentially escalating privileges on compromised systems. Security researchers have noted that this vulnerability can be combined with other exploits to create more sophisticated attack chains, particularly in targeted attacks against enterprise networks where legacy systems are prevalent. The vulnerability's exploitation typically requires user interaction through social engineering techniques, as users must navigate to a malicious website containing the crafted telnet URI. However, the ease of crafting such attacks combined with the prevalence of legacy Internet Explorer installations makes this vulnerability particularly dangerous in enterprise environments where browser updates may be delayed or restricted.
Mitigation strategies for CVE-2011-1961 focus primarily on disabling or removing the telnet URI handler from affected Internet Explorer installations, implementing strict browser security policies, and ensuring comprehensive patch management across all systems. Organizations should consider disabling the telnet protocol handler through registry modifications or group policy settings to prevent automatic execution of telnet URIs. Additionally, implementing network-based controls such as firewall rules that block telnet traffic and web application firewalls that can detect and block malicious telnet URI patterns provides additional layers of protection. Microsoft released security updates for this vulnerability through their regular patching cycles, but the long lifespan of this vulnerability in legacy systems requires ongoing vigilance and proactive security measures. The remediation process must also include user education regarding the dangers of clicking on suspicious links and the importance of maintaining up-to-date security patches across all browser versions. Security professionals should also consider implementing monitoring solutions that can detect attempts to exploit this vulnerability through abnormal network behavior or unusual process execution patterns.