CVE-2017-11368 in Kerberos 5
Summary
by MITRE
In MIT Kerberos 5 (aka krb5) 1.7 and later, an authenticated attacker can cause a KDC assertion failure by sending invalid S4U2Self or S4U2Proxy requests.
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Analysis
by VulDB Data Team • 12/15/2022
The vulnerability identified as CVE-2017-11368 affects MIT Kerberos 5 versions 1.7 and later, representing a critical security flaw in the Key Distribution Center's handling of authentication requests. This issue specifically targets the S4U2Self and S4U2Proxy protocol mechanisms that enable service-to-service authentication within kerberos environments. The vulnerability stems from insufficient validation of request parameters during the assertion process, allowing malicious actors with valid credentials to exploit the system's trust model through crafted malformed requests.
The technical flaw manifests in the KDC's failure to properly validate the structure and content of S4U2Self and S4U2Proxy requests, which are designed to allow services to obtain tickets on behalf of users without requiring the user to be present at the authentication server. When an authenticated attacker submits invalid or malformed requests, the KDC processes these inputs without adequate checks, leading to assertion failures that can disrupt service availability and potentially compromise the integrity of the authentication system. This vulnerability operates at the protocol level within the Kerberos authentication framework, specifically affecting the ticket granting service and the service ticket issuance process.
The operational impact of CVE-2017-11368 extends beyond simple service disruption, as it represents a potential vector for more sophisticated attacks within enterprise environments that rely heavily on Kerberos authentication. Attackers can leverage this vulnerability to cause denial of service conditions against KDC servers, potentially affecting multiple services that depend on Kerberos for authentication. The flaw also creates opportunities for privilege escalation attempts, as the assertion failure could be exploited to manipulate the authentication flow and potentially gain unauthorized access to resources. Organizations using Kerberos-based authentication systems are particularly vulnerable since the attack requires only authenticated access to the network, making it difficult to detect and prevent.
Mitigation strategies for this vulnerability involve immediate patching of affected Kerberos implementations to version 1.15.2 or later, which includes fixes for the validation issues in S4U2Self and S4U2Proxy request processing. Network administrators should implement monitoring solutions that can detect anomalous authentication patterns and malformed requests to the KDC server. The implementation of additional access controls and authentication logging can help identify potential exploitation attempts. Organizations should also consider implementing network segmentation to limit access to KDC servers and establish strict firewall rules that restrict communication to only necessary services. This vulnerability aligns with CWE-248, which addresses the exposure of an exception or assertion failure, and maps to ATT&CK technique T1550.003 for use of Kerberos authentication. The fix addresses the core issue by implementing robust input validation and ensuring proper error handling during the assertion process, preventing malformed requests from causing system failures or security breaches.