CVE-2016-3763 in Android
Summary
by MITRE
net/PacProxySelector.java in the Proxy Auto-Config (PAC) feature in Android 4.x before 4.4.4, 5.0.x before 5.0.2, 5.1.x before 5.1.1, and 6.x before 2016-07-01 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, aka internal bug 27593919.
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Analysis
by VulDB Data Team • 02/22/2019
The vulnerability described in CVE-2016-3763 resides within the Proxy Auto-Config (PAC) implementation of various Android versions, specifically affecting Android 4.x before 4.4.4, 5.0.x before 5.0.2, 5.1.x before 5.1.1, and 6.x before the 2016-07-01 security update. This issue stems from improper validation of URL information within the PacProxySelector.java component, which is responsible for handling automatic proxy configuration scripts. The flaw creates a critical security gap in how Android processes PAC scripts, potentially exposing sensitive authentication credentials to malicious actors who can manipulate the proxy selection process. The vulnerability is categorized under CWE-200, which deals with exposure of sensitive information to an unauthorized actor, and aligns with ATT&CK technique T1566.001 for credential access through phishing.
The technical root cause of this vulnerability lies in the insufficient restriction of URL components within the PAC processing logic. When Android systems process PAC scripts, they should validate that URLs contain only the essential components of scheme, host, and port to prevent unauthorized access to credential information. However, the flawed implementation fails to properly sanitize these URL elements, allowing attackers to construct malicious PAC scripts that can extract and transmit authentication credentials to external servers. This occurs because the system does not properly validate the URL format or restrict the information that can be extracted from proxy configuration scripts, creating an information disclosure vulnerability that can be exploited by remote attackers.
The operational impact of this vulnerability is severe, as it enables remote attackers to conduct credential harvesting attacks against Android devices running the affected versions. Attackers can set up malicious servers that respond to PAC script requests with carefully crafted configurations designed to capture user credentials, including username and password information. This vulnerability is particularly dangerous because it operates at the system level within the Android proxy configuration mechanism, meaning that any application or service that relies on PAC scripts for proxy configuration could potentially expose sensitive information. The vulnerability essentially allows attackers to perform man-in-the-middle attacks against proxy configurations, making it a significant threat to enterprise security and user privacy.
Mitigation strategies for this vulnerability involve applying the relevant security patches released by Google for the affected Android versions, specifically targeting the July 2016 security update cycle. Organizations should ensure that all Android devices are updated to versions that contain the fixed PacProxySelector.java implementation, which properly validates URL information and restricts access to credential data. Additionally, network administrators should consider implementing network monitoring to detect unusual PAC script requests or connections to suspicious servers. The fix addresses the underlying CWE-200 vulnerability by ensuring proper input validation and information flow control within the proxy configuration processing, aligning with ATT&CK defensive techniques that focus on input validation and access control measures. Organizations should also review their proxy configuration policies and consider implementing additional security controls around PAC script distribution and execution.