CVE-2016-4694 in macOS
Summary
by MITRE
The Apache HTTP Server in Apple OS X before 10.12 and OS X Server before 5.2 follows RFC 3875 section 4.1.18 and therefore does not protect applications from the presence of untrusted CGI client data in the HTTP_PROXY environment variable, which might allow remote attackers to redirect an application's outbound HTTP traffic to an arbitrary proxy server via a crafted Proxy header in an HTTP request, aka an "httpoxy" issue, a related issue to CVE-2016-5387.
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Analysis
by VulDB Data Team • 09/20/2022
The CVE-2016-4694 vulnerability represents a critical security flaw in the Apache HTTP Server implementation within Apple's operating systems, specifically affecting versions prior to macOS 10.12 and OS X Server prior to version 5.2. This issue stems from the server's adherence to RFC 3875 section 4.1.18, which governs CGI environment variable handling, creating an exploitable condition where untrusted client data can be injected into the HTTP_PROXY environment variable. The vulnerability is categorized under CWE-200, which deals with Information Exposure, and specifically manifests as a weakness in the server's environment variable processing mechanism. The httpoxy vulnerability, as it is commonly known, exploits the fundamental trust model that operating systems place in environment variables, particularly those that control proxy settings for outbound network communications.
The technical exploitation of this vulnerability occurs when a malicious actor crafts a specially formatted HTTP request containing a Proxy header that manipulates the HTTP_PROXY environment variable. This environment variable, when processed by the Apache server, can cause applications to route their outbound HTTP traffic through an attacker-controlled proxy server instead of the intended destination. The flaw exists because the server does not properly sanitize or validate the contents of the HTTP_PROXY variable before passing it to CGI applications, creating a path for man-in-the-middle attacks and potential data interception. The vulnerability operates at the application level where CGI scripts are executed, making it particularly dangerous as it can affect any web application running on the affected Apache server configuration. The issue is closely related to CVE-2016-5387, which also addresses proxy-related vulnerabilities in the same software ecosystem, and both vulnerabilities fall under the broader ATT&CK technique T1071.004 for application layer protocol tunneling.
The operational impact of CVE-2016-4694 is significant for organizations running affected Apple operating systems, as it creates a persistent vector for attackers to redirect network traffic through malicious proxies. This can lead to unauthorized data interception, credential theft, and potential compromise of sensitive information flowing through web applications. The vulnerability affects not only the direct web server functionality but also any CGI-based applications that rely on proper proxy handling, including authentication systems, API gateways, and enterprise web applications. Attackers can leverage this flaw to establish persistent proxy connections that can be used for extended surveillance or to redirect traffic to compromised servers that can then harvest sensitive data. The vulnerability's exploitation requires minimal technical skill, making it particularly dangerous as it can be automated and used in large-scale attacks against multiple systems. Organizations using the affected versions of macOS and OS X Server face substantial risk of data breaches, especially in environments where web applications handle sensitive user information or financial transactions.
Mitigation strategies for CVE-2016-4694 primarily involve upgrading to supported versions of macOS and OS X Server where the vulnerability has been patched, specifically macOS 10.12 and OS X Server 5.2 or later. System administrators should also implement network-level controls such as firewall rules that restrict outbound proxy connections and monitor for unusual proxy behavior in network traffic logs. Additionally, organizations can apply temporary workarounds by modifying CGI application configurations to explicitly ignore or sanitize the HTTP_PROXY environment variable, though this approach is less secure than a full system upgrade. The mitigation process should also include comprehensive security auditing of web applications to identify those that may be vulnerable due to their reliance on untrusted proxy configurations. Organizations should implement monitoring solutions that can detect anomalous proxy usage patterns and establish incident response procedures specifically addressing potential proxy-based attacks. The vulnerability's resolution through system updates demonstrates the importance of maintaining current software versions and implementing robust patch management processes to prevent exploitation of known security flaws.