CVE-2016-5388 in Tomcat
Summary
by MITRE
Apache Tomcat through 8.5.4, when the CGI Servlet is enabled, 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 "A mitigation is planned for future releases of Tomcat, tracked as CVE-2016-5388"; in other words, this is not a CVE ID for a vulnerability.
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Analysis
by VulDB Data Team • 09/05/2022
The httpoxy vulnerability identified as CVE-2016-5388 represents a critical security flaw in Apache Tomcat versions up to 8.5.4 that stems from improper handling of HTTP proxy headers within the CGI Servlet implementation. This vulnerability specifically manifests when the CGI Servlet is enabled and the server processes requests containing Proxy headers, creating a pathway for attackers to manipulate application behavior through environment variable injection. The flaw directly violates security principles by failing to sanitize or validate client-provided proxy information, allowing malicious actors to inject untrusted data into the HTTP_PROXY environment variable that applications subsequently use for outbound connections.
The technical exploitation of this vulnerability occurs through the RFC 3875 section 4.1.18 compliance mechanism that Apache Tomcat implements for CGI processing. When a client sends a request containing a Proxy header, the server incorporates this header value into the HTTP_PROXY environment variable without proper validation or sanitization. This design choice creates a dangerous precedent where application code that relies on outbound HTTP connections can be manipulated to route traffic through attacker-controlled proxy servers. The vulnerability is particularly concerning because it operates at the application level rather than the network level, making it difficult to detect through traditional network security measures and allowing attackers to bypass standard firewall and network monitoring controls.
From an operational impact perspective, this vulnerability exposes applications running on affected Tomcat versions to significant risks including data exfiltration, man-in-the-middle attacks, and potential compromise of sensitive application data. Attackers can redirect outbound HTTP traffic through malicious proxy servers they control, enabling them to intercept, modify, or redirect application communications. This capability can be leveraged to steal session cookies, access sensitive data, perform credential harvesting, or establish persistent access points within the target network. The vulnerability affects any application that makes outbound HTTP requests and relies on the default proxy handling behavior, making it particularly dangerous for enterprise applications that frequently communicate with external services.
Security professionals should note that this vulnerability aligns with CWE-20, which covers "Improper Input Validation," and maps to ATT&CK technique T1071.004 for application layer protocol manipulation. The vulnerability demonstrates the critical importance of environment variable sanitization and proper input validation in web application security. Organizations should implement immediate mitigations including disabling the CGI Servlet when not required, implementing proper proxy header validation, and configuring application-level restrictions on outbound connections. The vendor's acknowledgment that this issue was planned for future releases indicates that the root cause lies in the server's default behavior rather than specific application code, making comprehensive system-level remediation necessary. Additionally, security teams should monitor for any applications that might be inadvertently using the CGI Servlet functionality and ensure that all outbound HTTP traffic is properly secured through network-level controls and application-level validation mechanisms.