CVE-2016-6287 in http-client
Summary
by MITRE
The "http-client" egg always used a HTTP_PROXY environment variable to determine whether HTTP traffic should be routed via a proxy, even when running as a CGI process. Under several web servers this would mean a user-supplied "Proxy" header could allow an attacker to direct all HTTP requests through a proxy (also known as a "httpoxy" attack). This affects all versions of http-client before 0.10.
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Analysis
by VulDB Data Team • 04/20/2025
The CVE-2016-6287 vulnerability represents a critical security flaw in the http-client library that exposed applications to man-in-the-middle attacks through improper proxy handling. This vulnerability specifically affected versions of the library prior to 0.10 and emerged from a fundamental design oversight in how the library processed proxy configuration. The flaw occurred when the http-client egg consistently checked for the presence of the HTTP_PROXY environment variable regardless of the execution context, creating a dangerous condition where CGI processes would honor user-supplied proxy headers in addition to system proxy settings. This behavior created a pathway for attackers to manipulate HTTP traffic routing through malicious proxy servers.
The technical implementation of this vulnerability stems from the library's failure to properly distinguish between different execution contexts when processing proxy configuration. In CGI environments, web servers often pass user-supplied HTTP headers to applications, including the Proxy header that should typically be ignored or filtered. The http-client library's design flaw meant it treated these user-provided headers as authoritative proxy configuration, bypassing normal security controls that would normally prevent such header manipulation. This design decision created a scenario where an attacker could inject a Proxy header into their HTTP request, causing the application to route all outbound HTTP traffic through the attacker-controlled proxy server. The vulnerability directly aligns with CWE-284, which addresses improper access control in network communications, and demonstrates poor input validation and header processing practices.
The operational impact of this vulnerability was severe across multiple web server configurations and deployment scenarios. Applications using affected versions of the http-client library became susceptible to full traffic interception and manipulation, allowing attackers to eavesdrop on sensitive communications, modify data in transit, and potentially gain access to backend systems. The vulnerability was particularly dangerous in environments where applications made outbound HTTP requests to internal services, as it could enable attackers to redirect these requests to malicious endpoints. The attack vector was straightforward and required minimal skill level, making it attractive to threat actors seeking to exploit web applications. Security researchers noted that this vulnerability was particularly prevalent in CGI-based environments and could affect a wide range of applications including web applications, API clients, and automated systems that relied on the http-client library for outbound communications.
Mitigation strategies for CVE-2016-6287 required immediate action from system administrators and developers to update to patched versions of the http-client library. The most effective solution was upgrading to version 0.10 or later, which contained proper context-aware proxy handling that would not honor user-supplied proxy headers in CGI environments. Organizations needed to conduct comprehensive inventory assessments to identify all applications using vulnerable versions of the library and implement patch management procedures to address the issue across their infrastructure. Additional protective measures included implementing strict header validation at the web server level, filtering out Proxy headers in application firewalls, and configuring proper network segmentation to limit the impact of any successful exploitation attempts. This vulnerability highlighted the importance of proper context awareness in security libraries and the critical need for thorough testing of security controls in different execution environments, particularly those involving user input processing and network communication libraries.