CVE-2018-18506 in Firefox
Summary
by MITRE
When proxy auto-detection is enabled, if a web server serves a Proxy Auto-Configuration (PAC) file or if a PAC file is loaded locally, this PAC file can specify that requests to the localhost are to be sent through the proxy to another server. This behavior is disallowed by default when a proxy is manually configured, but when enabled could allow for attacks on services and tools that bind to the localhost for networked behavior if they are accessed through browsing. This vulnerability affects Firefox < 65.
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Analysis
by VulDB Data Team • 05/08/2020
This vulnerability exists in the proxy configuration handling mechanism of firefox browsers version 65 and earlier, where the browser fails to properly enforce security boundaries when processing proxy auto-configuration files. The flaw stems from an improper implementation of the proxy auto-configuration protocol which allows maliciously crafted pac files to redirect localhost traffic through external proxies, bypassing normal security restrictions that would typically prevent such redirection. The vulnerability specifically affects systems where proxy auto-detection is enabled and a PAC file is either served by a web server or loaded locally from the filesystem. When a PAC file specifies that localhost requests should be routed through a proxy to another server, this creates a security boundary violation that can be exploited by attackers to gain unauthorized access to services bound to localhost. The issue is particularly concerning because localhost-bound services often assume they are protected from external network access, making them vulnerable to attacks when proxy configurations are improperly handled. This vulnerability operates at the network layer and specifically targets the browser's proxy resolution and routing mechanisms, creating an attack vector that can be exploited through web-based delivery methods. The problem is categorized as a proxy misconfiguration vulnerability and falls under the broader category of network security boundary violations. The attack pattern aligns with techniques described in the attack tree methodology where an attacker can manipulate browser proxy settings to redirect local traffic. The vulnerability is particularly dangerous in environments where localhost services are used for administrative tasks, database access, or other sensitive operations that should remain isolated from external network access. This represents a classic case of improper input validation in proxy configuration handling, where the browser fails to validate the intended destination of redirected traffic. The security implications are significant because it allows attackers to potentially intercept or manipulate traffic that should remain isolated within the local network environment. The vulnerability demonstrates a failure in the principle of least privilege where localhost-bound services are not properly protected from external proxy redirection. The impact extends beyond simple traffic redirection to potentially enable more sophisticated attacks such as man-in-the-middle scenarios or credential interception. This issue directly relates to CWE-284 which describes improper access control in network services, and can be mapped to attack techniques in the MITRE ATT&CK framework under network infiltration and proxy manipulation categories. The vulnerability affects the core browser security model by allowing bypass of localhost security restrictions through proxy configuration manipulation.
The technical flaw manifests in the way firefox processes proxy auto-configuration files when auto-detection is enabled. When a PAC file is loaded, the browser does not properly validate whether the proxy redirection targets localhost addresses, allowing malicious redirection to occur even when such redirection would normally be blocked. The vulnerability specifically exploits the difference in behavior between manually configured proxies and auto-detected proxies, where the latter lacks the security validation that would normally be applied. This creates a scenario where an attacker can host a malicious PAC file on a web server and trick the browser into routing localhost traffic through an external proxy server. The PAC file can contain javascript code that determines proxy routing based on the target URL, and in this case it can be crafted to redirect localhost requests to external servers. The flaw exists because the browser's proxy resolution logic does not enforce the same security restrictions for auto-detected proxies as it does for manually configured ones. This inconsistency in proxy handling creates an exploitable gap in the browser's security model where localhost traffic can be redirected without proper validation. The vulnerability requires that the user be browsing with proxy auto-detection enabled and that a malicious PAC file be loaded, either through web delivery or local file access. The technical implementation fails to properly distinguish between trusted and untrusted proxy configurations when processing auto-detected proxy settings.
The operational impact of this vulnerability is significant for organizations using firefox browsers in environments where localhost-bound services are present. Attackers can exploit this vulnerability to gain unauthorized access to services that are intended to be isolated from external network access, including database servers, administrative interfaces, and development tools. The vulnerability is particularly dangerous in development environments where localhost services often lack proper authentication or encryption. When a user visits a malicious website with a crafted PAC file, the browser will automatically redirect localhost traffic through the attacker's proxy, potentially exposing sensitive data or allowing remote code execution against localhost services. The attack can be particularly effective because it leverages the user's browsing session and does not require any special privileges or direct system access. This vulnerability can be exploited to perform reconnaissance on localhost services, gather sensitive information, or even take control of services that are not properly secured. The impact is compounded by the fact that many localhost services are assumed to be secure by default and may not implement proper access controls or encryption. Organizations with internal development servers, database instances, or administrative tools bound to localhost are particularly at risk because these services can be accessed through the browser's proxy redirection mechanism. The vulnerability can also enable more advanced attacks such as credential harvesting from localhost-bound applications that rely on browser-based authentication.
The recommended mitigations for this vulnerability include upgrading to firefox version 65 or later where the issue has been resolved through proper validation of proxy redirection targets. Organizations should also implement network segmentation and access controls to limit the exposure of localhost services to external network access. Browser security policies should be configured to disable proxy auto-detection when not required, and administrators should regularly audit proxy configurations to ensure that localhost traffic is properly protected. Network monitoring should be implemented to detect unusual proxy traffic patterns that might indicate exploitation attempts. The fix implemented in firefox version 65 properly validates proxy redirection targets and ensures that localhost addresses cannot be redirected through external proxies even when auto-detection is enabled. Security awareness training should be provided to users to help them recognize potentially malicious websites that might attempt to exploit this vulnerability. Additionally, organizations should consider implementing content filtering solutions that can block access to known malicious PAC files or suspicious proxy configuration requests. The vulnerability highlights the importance of maintaining up-to-date browser software and implementing proper network security controls to protect localhost-bound services. Regular security assessments should be conducted to identify and remediate similar proxy configuration vulnerabilities across the organization's browser infrastructure. Organizations should also implement proper incident response procedures to quickly detect and respond to exploitation attempts targeting this vulnerability. The mitigation strategy should include both technical controls and administrative procedures to ensure comprehensive protection against this type of proxy-based attack vector.