CVE-2016-1000108 in Yaws
Summary
by MITRE
yaws before 2.0.4 does not attempt to address RFC 3875 section 4.1.18 namespace conflicts and therefore does not protect CGI applications from the presence of untrusted client data in the HTTP_PROXY environment variable, which might allow remote attackers to redirect a CGI application's outbound HTTP traffic to an arbitrary proxy server via a crafted Proxy header in an HTTP request, aka an "httpoxy" issue.
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Analysis
by VulDB Data Team • 03/09/2024
The vulnerability identified as CVE-2016-1000108 affects the yaws web server version 2.0.4 and earlier, exposing a critical security flaw related to CGI application protection mechanisms. This issue stems from the server's failure to properly handle namespace conflicts as defined in RFC 3875 section 4.1.18, which governs CGI environment variable handling and security considerations. The vulnerability specifically impacts how yaws processes the HTTP_PROXY environment variable, creating a dangerous condition where untrusted client data can influence application behavior through crafted HTTP requests.
The technical flaw manifests when yaws does not adequately sanitize or validate the HTTP_PROXY environment variable that may be set by client requests containing a Proxy header. This occurs because the web server fails to implement proper isolation between client-provided data and server-side environment variables that control application behavior. When a CGI application running on the affected yaws server attempts to make outbound HTTP connections, it may inadvertently use the proxy server specified in the HTTP_PROXY environment variable, which could have been manipulated by an attacker through a malicious Proxy header in the HTTP request.
The operational impact of this vulnerability is significant, as it enables remote attackers to perform man-in-the-middle attacks against CGI applications running on the vulnerable yaws server. An attacker can craft HTTP requests with a Proxy header containing a malicious proxy server address, which gets processed and set as the HTTP_PROXY environment variable. This allows the attacker to redirect all outbound HTTP traffic from CGI applications to their controlled proxy server, potentially enabling data interception, manipulation, or redirection attacks. The vulnerability is particularly dangerous because it affects the fundamental communication layer of CGI applications, which are commonly used for dynamic content generation and server-side processing.
This vulnerability maps directly to CWE-284, which addresses improper access control in web applications, and relates to the broader category of insecure environment variable handling. From an ATT&CK perspective, this represents a technique for initial access and lateral movement through the exploitation of environment variable manipulation, falling under the T1059.007 execution technique for command and script interpreter usage and potentially T1090.001 for proxy usage. The httpoxy vulnerability class specifically targets the common practice of using environment variables for proxy configuration in web applications, which is a standard approach in many server-side environments and frameworks.
Organizations should immediately upgrade to yaws version 2.0.4 or later, which includes proper handling of RFC 3875 section 4.1.18 requirements and addresses the namespace conflict issue. Additionally, administrators should implement network-level restrictions to prevent untrusted clients from setting proxy headers, though this approach is less secure than proper server-side handling. The recommended mitigation strategy involves ensuring that CGI applications do not rely on HTTP_PROXY environment variables for outbound connections, or implementing strict validation and sanitization of any client-provided proxy information. Security monitoring should also be enhanced to detect unusual outbound traffic patterns that might indicate proxy redirection attempts, and regular security audits should verify that all web server configurations properly handle environment variables according to security best practices and RFC standards.