CVE-2006-3808 in Firefox
Summary
by MITRE
Mozilla Firefox before 1.5.0.5 and SeaMonkey before 1.0.3 allows remote Proxy AutoConfig (PAC) servers to execute code with elevated privileges via a PAC script that sets the FindProxyForURL function to an eval method on a privileged object.
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Analysis
by VulDB Data Team • 03/13/2021
This vulnerability exists in Mozilla Firefox versions prior to 1.5.0.5 and SeaMonkey versions prior to 1.0.3, representing a critical privilege escalation flaw that allows remote attackers to execute arbitrary code with elevated privileges. The vulnerability stems from the browser's handling of Proxy AutoConfig scripts, which are designed to automatically configure proxy settings for web browsing. When a malicious PAC script is loaded, it can manipulate the browser's execution environment by directly assigning the FindProxyForURL function to an eval method that operates on privileged objects within the browser's security context. This flaw specifically exploits the trust model between the browser and PAC scripts, where legitimate PAC scripts are expected to be safe and non-malicious.
The technical implementation of this vulnerability involves a sophisticated manipulation of JavaScript execution contexts within the browser's privileged environment. When a PAC script attempts to set the FindProxyForURL function to an eval method operating on privileged objects, it effectively bypasses the normal security boundaries that separate user-level JavaScript execution from privileged browser operations. This creates a scenario where untrusted code can leverage the browser's internal APIs and execute with the same privileges as the browser itself. The vulnerability is classified under CWE-94, which describes "Improper Control of Generation of Code" and specifically relates to situations where code is generated or executed in a manner that allows for privilege escalation. The flaw demonstrates a fundamental breakdown in the browser's sandboxing mechanism for handling external configuration scripts.
The operational impact of this vulnerability is severe and far-reaching, as it enables attackers to execute arbitrary code with elevated privileges on vulnerable systems. An attacker can craft a malicious PAC script that, when loaded by a victim's browser, would allow remote code execution with the same permissions as the browser process itself. This means that attackers could potentially access sensitive user data, install malware, modify browser configurations, or even escalate to full system compromise depending on the victim's privileges and system configuration. The vulnerability is particularly dangerous because it leverages the legitimate proxy configuration functionality that users often trust and enable without questioning the security implications of external PAC scripts. According to ATT&CK framework category T1059, this represents a code injection technique where the malicious code is injected through a trusted configuration mechanism rather than direct exploit delivery.
The exploitation of this vulnerability typically occurs through a man-in-the-middle attack or by compromising a PAC server that users trust and automatically load. The attack vector requires the victim to either be on a network where the attacker controls the PAC server or to manually configure their browser to use a malicious PAC file. Once loaded, the malicious PAC script executes the FindProxyForURL function through the eval method on privileged objects, effectively granting the attacker full control over the browser's execution environment. This vulnerability highlights the importance of proper input validation and the principle of least privilege in browser security architecture, where even trusted configuration mechanisms must be protected against malicious manipulation. Organizations should implement network-level protections such as PAC script content filtering and ensure that only trusted sources are allowed to provide proxy configuration scripts to prevent exploitation of this vulnerability.
The remediation for this vulnerability required updating to Firefox 1.5.0.5 or later and SeaMonkey 1.0.3 or later, which implemented proper sandboxing and privilege separation for PAC script execution. The fix addressed the core issue by ensuring that PAC scripts cannot directly access or manipulate privileged objects through eval methods, thereby preventing the privilege escalation that enabled remote code execution. This vulnerability serves as a critical lesson in the importance of maintaining up-to-date browser software and implementing proper security controls for network configuration mechanisms. The incident also demonstrated the need for comprehensive security testing of browser features that interact with external configuration data, particularly those that operate in privileged contexts and can potentially be manipulated by untrusted sources.