CVE-2017-5384 in Firefox
Summary
by MITRE
Proxy Auto-Config (PAC) files can specify a JavaScript function called for all URL requests with the full URL path which exposes more information than would be sent to the proxy itself in the case of HTTPS. Normally the Proxy Auto-Config file is specified by the user or machine owner and presumed to be non-malicious, but if a user has enabled Web Proxy Auto Detect (WPAD) this file can be served remotely. This vulnerability affects Firefox < 51.
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Analysis
by VulDB Data Team • 11/04/2022
The vulnerability described in CVE-2017-5384 represents a significant security flaw in Firefox's handling of Proxy Auto-Config files that can lead to information disclosure and potential man-in-the-middle attacks. This issue specifically affects Firefox versions prior to 51 and stems from the way the browser processes PAC files that contain JavaScript functions designed to handle URL requests. The fundamental problem occurs when a PAC file specifies a JavaScript function that is executed for every URL request, including HTTPS connections, which can expose more information than would normally be transmitted to a proxy server. The vulnerability becomes particularly dangerous when Web Proxy Auto Detect (WPAD) is enabled, as this allows PAC files to be automatically downloaded from remote locations, potentially enabling attackers to serve malicious PAC files that can capture sensitive data.
The technical flaw in this vulnerability resides in the improper handling of PAC file JavaScript execution contexts, where the proxy configuration script can access full URL paths including sensitive information that would typically be encrypted and protected during HTTPS communication. When Firefox processes a PAC file containing a function like FindProxyForURL, it executes this function in a context that can potentially leak information about the requested URLs, including query parameters, fragments, and other metadata that should remain confidential during encrypted communications. This behavior violates the expected security boundaries between the user's browser and the network infrastructure, creating a scenario where information that should be protected by HTTPS encryption becomes accessible to the PAC file execution environment. The vulnerability is classified under CWE-200 as exposure of sensitive information and can be categorized under ATT&CK technique T1071.004 for application layer protocol manipulation.
The operational impact of this vulnerability extends beyond simple information disclosure, as it can enable sophisticated attacks including credential harvesting, session tracking, and targeted data exfiltration. When WPAD is enabled, attackers can remotely serve malicious PAC files that contain JavaScript code designed to capture and transmit sensitive information from users' browsing activities. This makes the vulnerability particularly dangerous in enterprise environments where WPAD is commonly enabled for network management purposes, as it can allow attackers to gain access to information that would normally be protected by HTTPS encryption. The attack surface is significantly expanded because the PAC file execution environment can access the complete URL structure, including paths, query strings, and other metadata that might contain authentication tokens, personal information, or business-sensitive data. This vulnerability essentially undermines the security assumptions that users and administrators make about encrypted communications, as the PAC file execution context can bypass normal security boundaries that should protect HTTPS traffic.
Mitigation strategies for CVE-2017-5384 focus primarily on updating Firefox to version 51 or later, where the vulnerability has been addressed through improved PAC file handling and execution context restrictions. Organizations should disable WPAD functionality when possible, as it significantly increases the attack surface by allowing remote PAC file delivery. Network administrators should implement strict controls over PAC file distribution, ensuring that only trusted, locally-hosted PAC files are used in enterprise environments. Additional protective measures include monitoring for suspicious PAC file content, implementing network segmentation to limit access to potentially malicious PAC servers, and educating users about the risks associated with enabling WPAD functionality. The vulnerability highlights the importance of sandboxing and privilege separation in browser security implementations, as the PAC file execution should not have access to the full URL information that could compromise user privacy and security. Organizations should also consider implementing network monitoring solutions that can detect anomalous PAC file behavior and alert security teams to potential exploitation attempts.