CVE-2016-5134 in Chrome
Summary
by MITRE
net/proxy/proxy_service.cc in the Proxy Auto-Config (PAC) feature in Google Chrome before 52.0.2743.82 does not ensure that URL information is restricted to a scheme, host, and port, which allows remote attackers to discover credentials by operating a server with a PAC script, a related issue to CVE-2016-3763.
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Analysis
by VulDB Data Team • 12/05/2024
The vulnerability identified as CVE-2016-5134 resides within the Proxy Auto-Config (PAC) implementation in Google Chrome versions prior to 52.0.2743.82. This issue specifically affects the net/proxy/proxy_service.cc component that handles PAC script execution and URL parsing. The flaw represents a security oversight in how Chrome processes URL information within PAC scripts, creating a potential information disclosure channel that could be exploited by remote attackers. The vulnerability is categorized under CWE-200, which deals with information exposure, and demonstrates a failure in proper input validation and sanitization of URL components.
The technical implementation flaw stems from insufficient validation of URL information within the PAC script execution context. When Chrome processes a PAC script, it must parse and evaluate URLs to determine appropriate proxy configurations. However, the vulnerable implementation failed to properly restrict URL information to only include scheme, host, and port components while ignoring potentially sensitive elements such as credentials, path, or query parameters. This incomplete URL parsing allows malicious servers to craft PAC scripts that can extract credential information from URLs that would normally be protected by browser security mechanisms. The vulnerability operates through a related attack pattern that mirrors the techniques described in CVE-2016-3763, which also involved proxy configuration vulnerabilities.
The operational impact of this vulnerability is significant for users who rely on PAC scripts for network proxy configuration, particularly in enterprise environments where complex proxy setups are common. Attackers can leverage this flaw by setting up malicious servers that respond with specially crafted PAC scripts designed to extract sensitive information from URLs that browsers normally handle securely. When Chrome executes these malicious PAC scripts, the incomplete URL parsing allows credential information to be exposed to the attacker. This creates a persistent threat vector that can affect users across various network environments, from corporate networks to public Wi-Fi connections where PAC scripts might be automatically configured.
The attack surface expands when considering that many organizations use PAC scripts to manage complex proxy configurations, making this vulnerability particularly dangerous in enterprise settings. Users who access potentially malicious websites or are subjected to man-in-the-middle attacks could have their credentials exposed through this mechanism. The vulnerability also aligns with ATT&CK technique T1566, which involves phishing attacks that could leverage this PAC script manipulation to extract credentials from user browsers. Organizations should consider this vulnerability as part of their broader credential protection strategy, particularly when implementing proxy configurations that rely on automated PAC script resolution.
Mitigation strategies should focus on immediate patching of affected Chrome versions to 52.0.2743.82 or later, which contains the necessary fixes for proper URL parsing within PAC script execution. Network administrators should also implement monitoring for unusual PAC script behavior and consider restricting automatic PAC script loading from untrusted sources. Additional protective measures include implementing network-level controls that can detect and block malicious PAC script content, as well as educating users about the risks of accessing untrusted websites that might deliver malicious proxy configurations. The fix implemented by Google addresses the core parsing issue by ensuring that URL information is properly restricted to scheme, host, and port components, thereby eliminating the credential exposure channel that attackers could exploit through malformed PAC scripts.