CVE-2016-2168 in Subversion
Summary
by MITRE
The req_check_access function in the mod_authz_svn module in the httpd server in Apache Subversion before 1.8.16 and 1.9.x before 1.9.4 allows remote authenticated users to cause a denial of service (NULL pointer dereference and crash) via a crafted header in a (1) MOVE or (2) COPY request, involving an authorization check.
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Analysis
by VulDB Data Team • 07/27/2022
The vulnerability identified as CVE-2016-2168 represents a critical denial of service flaw within the Apache Subversion mod_authz_svn module that affects httpd servers running vulnerable versions of the software. This issue specifically targets the req_check_access function which handles authorization checks for version control operations. The vulnerability manifests when authenticated users submit specially crafted HTTP headers as part of MOVE or COPY requests, creating conditions that lead to system instability and potential service disruption.
The technical root cause of this vulnerability lies in inadequate input validation within the authorization checking mechanism of the mod_authz_svn module. When processing MOVE or COPY requests with malformed headers, the req_check_access function fails to properly validate or sanitize the incoming authorization data, resulting in a NULL pointer dereference condition. This occurs because the function attempts to access memory locations that have not been properly initialized or allocated, causing the httpd process to crash and terminate unexpectedly. The flaw is particularly concerning as it only requires authentication to exploit, meaning that any authenticated user with appropriate privileges can trigger the denial of service condition.
The operational impact of CVE-2016-2168 extends beyond simple service interruption as it can severely compromise the availability of Subversion repositories that rely on Apache httpd for access control. Organizations using vulnerable versions of Apache Subversion may experience complete service outages during critical operations when authorized users inadvertently or maliciously trigger the crash condition. The vulnerability affects both the 1.8.x release series prior to 1.8.16 and the 1.9.x series before 1.9.4, representing a significant portion of the Subversion user base that would require immediate attention and patching. This flaw directly relates to CWE-476 which describes NULL pointer dereference conditions, and can be classified under the ATT&CK technique T1499.004 for Network Denial of Service, as it enables attackers to disrupt network services through resource exhaustion or process termination.
Mitigation strategies for this vulnerability involve immediate deployment of patched versions of Apache Subversion where available, specifically versions 1.8.16 and 1.9.4 or later. Organizations should also implement network-level controls such as rate limiting and request filtering to reduce the potential impact of exploitation attempts. Additionally, monitoring systems should be configured to detect unusual patterns of MOVE and COPY requests that may indicate attempted exploitation. The vulnerability highlights the importance of proper input validation and error handling in authorization modules, as well as the necessity of thorough security testing for authentication mechanisms in web-based version control systems. Security administrators should also consider implementing access controls that limit the ability to perform MOVE and COPY operations to only trusted users, reducing the attack surface for this specific vulnerability while patches are being deployed.