CVE-2016-2167 in Subversion
Summary
by MITRE
The canonicalize_username function in svnserve/cyrus_auth.c in Apache Subversion before 1.8.16 and 1.9.x before 1.9.4, when Cyrus SASL authentication is used, allows remote attackers to authenticate and bypass intended access restrictions via a realm string that is a prefix of an expected repository realm string.
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Analysis
by VulDB Data Team • 07/27/2022
The vulnerability described in CVE-2016-2167 represents a critical authentication bypass flaw within Apache Subversion's svnserve component when utilizing Cyrus SASL authentication mechanisms. This issue affects versions prior to 1.8.16 and 1.9.4, creating a significant security risk for organizations relying on Subversion for version control management. The flaw specifically resides in the canonicalize_username function located within the svnserve/cyrus_auth.c file, where improper handling of realm strings enables malicious actors to exploit the authentication system.
The technical implementation of this vulnerability stems from inadequate string comparison logic within the canonicalize_username function. When Cyrus SASL authentication is employed, the system expects a specific realm string that corresponds to the intended repository realm. However, the flawed implementation allows attackers to submit a realm string that serves as a prefix of the expected repository realm string. This prefix matching approach creates an authentication bypass condition where the system incorrectly validates credentials against a partial match rather than requiring an exact realm string match. The vulnerability essentially permits attackers to authenticate using credentials intended for a different but prefixed realm, effectively circumventing access controls.
The operational impact of this vulnerability extends beyond simple unauthorized access to encompass potential data compromise and system integrity violations. Organizations utilizing Subversion with Cyrus SASL authentication face the risk of unauthorized users gaining access to repositories they should not be permitted to access. This authentication bypass could enable attackers to read, modify, or delete sensitive source code and configuration files stored within version control systems. The implications are particularly severe in environments where multiple repositories exist with different security classifications, as the vulnerability allows cross-repository privilege escalation. Additionally, this flaw could facilitate further attacks within the network infrastructure, as compromised repositories might contain sensitive information or serve as entry points for lateral movement.
The vulnerability aligns with CWE-287, which addresses improper authentication issues in software systems, and demonstrates characteristics consistent with ATT&CK technique T1078.002 related to valid accounts and credential access. Organizations should implement immediate mitigations including upgrading to Apache Subversion versions 1.8.16 or 1.9.4, which contain the necessary patches to address the realm string validation logic. Additional defensive measures include implementing network segmentation to limit access to svnserve services, configuring proper access controls, and monitoring authentication logs for suspicious realm string patterns. Security teams should also consider implementing intrusion detection systems capable of identifying anomalous authentication attempts that might indicate exploitation of this vulnerability. The patch addresses the core issue by enforcing strict exact matching of realm strings rather than allowing prefix-based validation, thereby restoring proper authentication controls and preventing unauthorized access to Subversion repositories.