CVE-2016-6816 in Tomcat
Summary
by MITRE
The code in Apache Tomcat 9.0.0.M1 to 9.0.0.M11, 8.5.0 to 8.5.6, 8.0.0.RC1 to 8.0.38, 7.0.0 to 7.0.72, and 6.0.0 to 6.0.47 that parsed the HTTP request line permitted invalid characters. This could be exploited, in conjunction with a proxy that also permitted the invalid characters but with a different interpretation, to inject data into the HTTP response. By manipulating the HTTP response the attacker could poison a web-cache, perform an XSS attack and/or obtain sensitive information from requests other then their own.
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Analysis
by VulDB Data Team • 01/19/2025
The vulnerability CVE-2016-6816 represents a critical HTTP request parsing flaw in Apache Tomcat versions ranging from 9.0.0.M1 through 9.0.0.M11, 8.5.0 through 8.5.6, 8.0.0.RC1 through 8.0.38, 7.0.0 through 7.0.72, and 6.0.0 through 6.0.47. This issue stems from the server's permissive handling of invalid characters within HTTP request lines, creating a pathway for sophisticated attack vectors that exploit the discrepancy between how Tomcat processes these characters versus how certain proxy servers interpret them. The flaw falls under CWE-20, which specifically addresses improper input validation, and demonstrates how seemingly minor parsing inconsistencies can lead to severe security implications.
The technical exploitation of this vulnerability occurs through a carefully crafted attack that leverages the difference in character interpretation between the vulnerable Tomcat server and intermediary proxy systems. When Tomcat processes HTTP requests containing invalid characters in the request line, it accepts these characters without proper validation, while certain proxies may interpret the same characters differently and potentially inject additional data into the HTTP response. This discrepancy creates a condition where attackers can manipulate the response content to include malicious payloads, effectively allowing for data injection attacks that bypass normal security controls. The vulnerability operates at the HTTP protocol level, specifically targeting the request parsing mechanism that should enforce strict validation of request line components.
The operational impact of CVE-2016-6816 extends beyond simple data injection, creating multiple attack vectors that can compromise web application security. Attackers can leverage this vulnerability to poison web caches, which affects not just the targeted application but potentially other users of the same caching infrastructure. The ability to perform cross-site scripting attacks through response manipulation provides attackers with means to execute malicious scripts in victim browsers, while the potential to access sensitive information from other users' requests creates a serious privacy and data integrity threat. This vulnerability directly maps to several ATT&CK techniques including T1566 for credential access through cache poisoning and T1059 for command and scripting interpreter usage in XSS scenarios.
Mitigation strategies for CVE-2016-6816 require immediate patching of affected Tomcat versions to the latest stable releases that contain proper input validation for HTTP request lines. Organizations should implement comprehensive monitoring of HTTP request patterns to detect anomalous character sequences that might indicate exploitation attempts. Network segmentation and proper proxy configuration can help reduce the attack surface by ensuring consistent character interpretation across the entire request processing chain. Additionally, implementing robust input validation at multiple layers including application firewalls and web application firewalls can provide defense-in-depth protection against similar vulnerabilities. The vulnerability demonstrates the critical importance of maintaining strict HTTP protocol compliance and consistent security controls across all components in a web infrastructure stack, aligning with security best practices outlined in NIST SP 800-53 and ISO 27001 frameworks.