CVE-2017-17439 in Heimdal
Summary
by MITRE
In Heimdal 7.1.0, remote unauthenticated attackers are able to crash the KDC by sending a crafted UDP packet containing empty data fields for client name or realm. The parser would unconditionally dereference NULL pointers in that case, leading to a segmentation fault. This is related to the _kdc_as_rep function in kdc/kerberos5.c and the der_length_visible_string function in lib/asn1/der_length.c.
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Analysis
by VulDB Data Team • 01/17/2023
The vulnerability described in CVE-2017-17439 represents a critical denial-of-service weakness within the Heimdal Kerberos implementation that affects version 7.1.0. This flaw exists in the Key Distribution Center component which serves as the central authentication server in Kerberos networks. The vulnerability stems from inadequate input validation within the Kerberos protocol parser, specifically when processing authentication requests. Attackers can exploit this weakness by crafting malicious UDP packets that contain empty data fields for either client name or realm information, effectively triggering a controlled crash of the KDC service.
The technical root cause of this vulnerability lies in the improper handling of NULL pointer dereferences within the Kerberos protocol implementation. When the _kdc_as_rep function in kdc/kerberos5.c encounters a crafted packet with empty client name or realm fields, it proceeds to unconditionally dereference NULL pointers without proper validation. This behavior directly violates the principle of defensive programming and leads to segmentation faults that cause the KDC process to terminate unexpectedly. The der_length_visible_string function in lib/asn1/der_length.c serves as another point of failure where similar NULL pointer dereference conditions can occur during ASN.1 parsing operations. This vulnerability classifies under CWE-476 which specifically addresses NULL pointer dereference conditions that can lead to system crashes and service disruption.
The operational impact of this vulnerability extends beyond simple service disruption as it can be exploited by remote unauthenticated attackers without requiring any prior credentials or access privileges. The attack vector through UDP packets makes it particularly dangerous as it can be executed from anywhere on the network, potentially leading to widespread service availability issues for Kerberos-authenticated environments. Organizations relying on Heimdal Kerberos for authentication services face significant risk of unauthorized service disruption, which could affect critical infrastructure components that depend on Kerberos for secure authentication. The vulnerability essentially allows attackers to perform a form of network-level denial-of-service attack that can be executed with minimal resources and technical expertise.
Mitigation strategies for this vulnerability should focus on immediate patching of the Heimdal implementation to version 7.1.1 or later where the NULL pointer dereference issues have been addressed. Network administrators should also implement monitoring solutions to detect anomalous UDP traffic patterns that might indicate exploitation attempts. The fix typically involves adding proper NULL pointer validation checks before dereferencing pointers in both the _kdc_as_rep function and der_length_visible_string function. Additionally, implementing network segmentation and access controls can help reduce the attack surface by limiting exposure of the KDC service to untrusted networks. This vulnerability aligns with ATT&CK technique T1499.004 which covers network disruption through service availability attacks, emphasizing the importance of robust input validation and proper error handling in network services. Organizations should also consider implementing intrusion detection systems that can identify and alert on malformed Kerberos packets that match the exploit pattern described in this CVE.