CVE-2016-4979 in Enterprise Manager
Summary
by MITRE
The Apache HTTP Server 2.4.18 through 2.4.20, when mod_http2 and mod_ssl are enabled, does not properly recognize the "SSLVerifyClient require" directive for HTTP/2 request authorization, which allows remote attackers to bypass intended access restrictions by leveraging the ability to send multiple requests over a single connection and aborting a renegotiation.
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Analysis
by VulDB Data Team • 09/29/2024
The vulnerability identified as CVE-2016-4979 represents a critical security flaw in the Apache HTTP Server version 2.4.18 through 2.4.20 that specifically affects configurations utilizing both mod_http2 and mod_ssl modules. This issue stems from improper handling of SSL client certificate verification within the HTTP/2 protocol implementation, creating a significant bypass mechanism for access controls that should otherwise enforce strict authentication requirements. The flaw manifests when the SSLVerifyClient require directive is properly configured to mandate client certificate validation, yet the server fails to consistently enforce this requirement during HTTP/2 connections where multiple requests can be multiplexed over a single connection.
The technical root cause of this vulnerability lies in the interaction between HTTP/2's connection multiplexing capabilities and the SSL/TLS renegotiation process that occurs when client certificates are required for authentication. When a client establishes an HTTP/2 connection with the server, the initial SSL handshake may successfully authenticate the client using the required certificate. However, during subsequent requests on the same connection, the server does not properly re-evaluate the SSL client certificate verification requirements, allowing attackers to exploit the connection's persistent nature to bypass authentication checks. This behavior specifically occurs when the server aborts the SSL renegotiation process, which can happen when the client sends requests that trigger certificate validation failures or when the server fails to maintain consistent authentication state across multiplexed requests.
The operational impact of this vulnerability is severe and directly affects the integrity of access control mechanisms implemented through Apache's SSL client certificate authentication. Attackers can exploit this flaw by establishing an HTTP/2 connection with a valid client certificate, then sending subsequent requests that would normally require re-authentication or certificate validation. The ability to leverage single connections for multiple requests enables attackers to bypass intended access restrictions without needing to establish new authenticated sessions, effectively rendering the SSLVerifyClient require directive ineffective for HTTP/2 traffic. This vulnerability particularly affects web applications that rely on client certificate authentication for sensitive operations, potentially allowing unauthorized access to protected resources that should only be accessible to authenticated users with valid certificates.
Organizations implementing Apache HTTP Server with HTTP/2 support and SSL client certificate requirements face significant risk from this vulnerability, as it undermines fundamental security controls designed to prevent unauthorized access to protected resources. The flaw creates a persistent attack vector that can be exploited across multiple requests within a single connection, making it particularly dangerous for applications handling sensitive data or privileged operations. Security professionals should note that this vulnerability specifically targets the intersection of HTTP/2 protocol handling and SSL/TLS certificate validation, making it distinct from traditional SSL renegotiation issues and requiring specific attention to the mod_http2 module's implementation.
The mitigation strategy for CVE-2016-4979 involves immediate patching of Apache HTTP Server installations to versions that address the specific HTTP/2 certificate verification handling flaw. Organizations should also consider disabling HTTP/2 support in Apache configurations when SSL client certificate authentication is required, or implementing additional access control mechanisms to compensate for the vulnerability. Network administrators should monitor for unauthorized access attempts and review SSL/TLS certificate validation logs to detect potential exploitation attempts. This vulnerability aligns with CWE-284, which addresses improper access control in software systems, and represents a specific implementation weakness in the HTTP/2 protocol handling that violates fundamental security principles of authentication enforcement. The ATT&CK framework categorizes this vulnerability under privilege escalation and credential access techniques, as it allows attackers to bypass authentication mechanisms and gain unauthorized access to protected resources through protocol-level exploitation rather than traditional credential compromise methods.