CVE-2016-4745 in macOS
Summary
by MITRE
The Kerberos 5 (aka krb5) PAM module in Apple OS X before 10.12 does not use constant-time operations for determining username validity, which makes it easier for remote attackers to enumerate user accounts via a timing side-channel attack.
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Analysis
by VulDB Data Team • 09/20/2022
The vulnerability described in CVE-2016-4745 represents a critical timing side-channel weakness in the Kerberos 5 PAM module implementation within Apple macOS versions prior to 10.12. This flaw exists within the authentication mechanism that governs how user credentials are validated during the Kerberos authentication process. The vulnerability stems from the module's failure to implement constant-time cryptographic operations when checking username validity, creating an exploitable timing differential that can be measured and analyzed by remote attackers.
The technical implementation of this vulnerability lies in the PAM module's handling of authentication requests where the time taken to process valid versus invalid usernames varies significantly. When a user attempts authentication, the system should take approximately the same amount of time regardless of whether the username exists in the system or not. However, in affected versions of macOS, the Kerberos PAM module exhibits measurable timing differences that correlate directly with username existence. This timing variation occurs because the module performs different computational paths for valid and invalid usernames, with valid usernames typically requiring additional processing steps such as database lookups or credential validation that take measurable time.
This vulnerability directly maps to CWE-203, which specifically addresses "Information Exposure Through Timing Discrepancy," and represents a classic example of how cryptographic implementations can leak sensitive information through timing characteristics. The operational impact of this vulnerability is significant as it enables remote attackers to perform user enumeration attacks against systems using the affected Kerberos PAM module. Through careful measurement of response times to authentication attempts, an attacker can systematically determine which usernames are valid within the target system, effectively creating a user account dictionary that can be used for subsequent attacks including brute force attempts, credential stuffing, or social engineering operations.
The attack surface for this vulnerability extends across any network service that relies on Kerberos authentication through the affected macOS PAM module, including but not limited to remote desktop services, VPN connections, and network file sharing protocols. Attackers can leverage this information to focus their efforts on valid accounts, dramatically reducing the search space required for successful authentication attempts. The vulnerability is particularly concerning in enterprise environments where Kerberos is commonly used for single sign-on solutions and centralized authentication services, as it provides attackers with a method to map the user base of the organization without requiring prior knowledge of valid accounts.
Organizations should prioritize immediate remediation by upgrading to macOS 10.12 or later versions where Apple has implemented proper constant-time operations within the Kerberos PAM module. Additionally, implementing network-level protections such as rate limiting for authentication attempts, account lockout policies, and monitoring for unusual authentication patterns can help mitigate the impact of timing-based enumeration attacks. The implementation of proper constant-time algorithms for all cryptographic operations, as recommended by NIST SP 800-90A and other security standards, should be enforced throughout the authentication stack to prevent similar vulnerabilities from emerging in future implementations. Security monitoring should also include detection of timing variations in authentication responses, which can serve as indicators of potential exploitation attempts against this or similar timing side-channel vulnerabilities.