CVE-2016-5387 in macOS
Summary
by MITRE
The Apache HTTP Server through 2.4.23 follows RFC 3875 section 4.1.18 and therefore does not protect applications from the presence of untrusted 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. NOTE: the vendor states "This mitigation has been assigned the identifier CVE-2016-5387"; in other words, this is not a CVE ID for a vulnerability.
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Analysis
by VulDB Data Team • 01/21/2021
The Apache HTTP Server vulnerability identified as CVE-2016-5387 represents a significant security concern related to improper handling of proxy environment variables, specifically the HTTP_PROXY variable. This issue affects Apache HTTP Server versions through 2.4.23 and stems from the server's adherence to RFC 3875 section 4.1.18, which governs the processing of CGI environment variables. The vulnerability occurs because the server does not adequately sanitize or validate the HTTP_PROXY environment variable, allowing malicious actors to manipulate this variable through crafted HTTP requests. The flaw specifically manifests when applications running on the server process the HTTP_PROXY environment variable without proper validation, creating a pathway for attackers to redirect outbound HTTP traffic through arbitrary proxy servers. This behavior constitutes a fundamental misconfiguration in how the server handles untrusted input from client requests, particularly within the context of CGI applications that may rely on proxy settings for outbound connections.
The technical exploitation of this vulnerability occurs through the manipulation of HTTP Proxy headers in client requests, where attackers craft malicious Proxy headers that get processed by the Apache server and subsequently propagated to applications running in CGI environments. When these applications attempt to establish outbound connections, they inadvertently route traffic through proxy servers controlled by the attacker rather than the intended destination servers. This creates a man-in-the-middle scenario where the attacker can intercept, modify, or redirect sensitive data flows. The vulnerability is particularly dangerous because it operates at the intersection of HTTP protocol handling and application-level proxy configuration, making it difficult to detect and mitigate through traditional security measures. The flaw essentially allows attackers to bypass normal network security controls by leveraging the legitimate HTTP_PROXY environment variable processing mechanisms that are typically used for legitimate proxy configurations.
The operational impact of CVE-2016-5387 extends beyond simple traffic redirection, as it can enable various attack vectors including data exfiltration, credential theft, and network reconnaissance. Applications that rely on outbound HTTP connections, such as web applications that communicate with external APIs, database servers, or other network services, become vulnerable to this attack. The vulnerability is particularly concerning for applications that process sensitive information and require secure network communications, as the attacker can potentially intercept communications between the application and backend services. This issue affects not only the Apache server itself but also any CGI applications or modules that may be processing HTTP_PROXY variables, creating a broad attack surface that extends throughout the application ecosystem. The vulnerability's impact is further amplified because it requires minimal privileges to exploit and can be automated through standard web scanning tools, making it attractive to both automated attacks and targeted campaigns.
Mitigation strategies for CVE-2016-5387 involve multiple layers of defense including server configuration changes, application-level protections, and network-level controls. The most effective approach involves modifying the Apache server configuration to either disable or properly sanitize the HTTP_PROXY environment variable processing, typically through the use of mod_env module directives or by setting environment variables to prevent propagation of untrusted proxy information. Organizations should implement strict input validation for Proxy headers and consider using environment variable whitelisting to prevent arbitrary proxy settings from being processed by CGI applications. Additionally, network administrators should monitor for unusual proxy traffic patterns and implement proper firewall rules to restrict outbound proxy connections to known, trusted proxy servers. This vulnerability aligns with CWE-20, which addresses improper input validation, and maps to ATT&CK technique T1071.004 for application layer protocol usage, specifically focusing on web protocols and their manipulation for attack purposes. The remediation process should include comprehensive testing to ensure that legitimate proxy configurations continue to function while eliminating the security risk associated with untrusted input processing.