CVE-2010-4021 in Kerberos
Summary
by MITRE
The Key Distribution Center (KDC) in MIT Kerberos 5 (aka krb5) 1.7 does not properly restrict the use of TGT credentials for armoring TGS requests, which might allow remote authenticated users to impersonate a client by rewriting an inner request, aka a "KrbFastReq forgery issue."
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Analysis
by VulDB Data Team • 10/06/2021
The vulnerability described in CVE-2010-4021 represents a critical flaw in the Kerberos 5 authentication system that affects the Key Distribution Center implementation. This issue specifically targets the Fast Repudiation and Forwarding (KrbFast) mechanism designed to protect against replay attacks and ensure secure credential handling. The vulnerability exists within MIT Kerberos 5 version 1.7 where the system fails to properly validate the integrity of Ticket Granting Ticket credentials when they are used to armor Ticket Granting Service requests. This weakness creates a pathway for authenticated attackers to manipulate the inner request components of TGS requests, effectively allowing them to forge authentication tokens that appear legitimate to the target system.
The technical exploitation of this vulnerability stems from insufficient validation of the TGT credentials within the KrbFastReq structure. When a client requests a service ticket through the KDC, the system should verify that the TGT used for armor protection matches the actual client identity making the request. However, in the vulnerable implementation, the KDC accepts TGT credentials without proper verification of their authorization scope, allowing an attacker who has valid credentials to modify the inner request content. This flaw operates at the protocol level where the KDC's validation logic fails to enforce proper credential binding between the outer armor and inner request components. The vulnerability is classified as a weakness in authentication mechanisms and falls under the CWE-305 category for authentication bypass through credential reuse.
The operational impact of this vulnerability is severe as it enables authenticated attackers to impersonate legitimate users within the Kerberos domain. An attacker with access to a valid TGT can leverage this flaw to rewrite inner request parameters, potentially gaining access to services that should be restricted to specific user identities. This capability undermines the fundamental security model of Kerberos where tickets are designed to be unforgeable and bound to specific client identities. The attack can be executed remotely without requiring additional privileges beyond existing authentication access, making it particularly dangerous in environments where Kerberos is used for enterprise authentication. The vulnerability affects systems using MIT Kerberos 5 version 1.7 and potentially earlier versions, creating widespread exposure across organizations that rely on this authentication protocol.
Mitigation strategies for CVE-2010-4021 should focus on immediate patching of affected MIT Kerberos installations to version 1.8 or later where the vulnerability has been addressed. Organizations should implement network segmentation to limit access to KDC services and monitor for unusual authentication patterns that might indicate exploitation attempts. The implementation of additional authentication layers such as multi-factor authentication can provide defense-in-depth against this specific attack vector. Security teams should also consider deploying intrusion detection systems capable of identifying malformed KrbFastReq structures and anomalous credential usage patterns. According to ATT&CK framework, this vulnerability maps to T1550.003 for use of Kerberos, representing a privilege escalation and lateral movement attack vector. Organizations should conduct comprehensive audits of their Kerberos implementations to identify other potential credential handling vulnerabilities and ensure proper configuration of authentication policies to prevent unauthorized credential reuse and manipulation.