CVE-1999-1099 in Kerberos
Summary
by MITRE
kerberos 4 allows remote attackers to obtain sensitive information via a malformed udp packet that generates an error string that inadvertently includes the realm name and the last user.
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Analysis
by VulDB Data Team • 04/16/2026
The vulnerability described in CVE-1999-1099 pertains to the Kerberos version 4 authentication protocol, which was widely deployed in the late 1990s and early 2000s for network authentication. This flaw represents a classic information disclosure vulnerability that occurs during the processing of malformed UDP packets within the Kerberos 4 implementation. The issue arises when the Kerberos server receives a malformed UDP packet and generates an error message that inadvertently includes sensitive information about the authentication process. Specifically, the error string contains both the realm name and the last authenticated user, creating a potential avenue for attackers to gather intelligence about the network's authentication infrastructure and user base.
This vulnerability operates at the application layer and specifically targets the Kerberos 4 protocol's error handling mechanism. When a malformed UDP packet is received, the server's error processing routine includes the realm name and username in the generated error message. The technical flaw stems from improper input validation and error message construction within the Kerberos 4 implementation, where the system fails to sanitize error output that could be exposed to network clients. This represents a CWE-20: Improper Input Validation vulnerability, where the system does not adequately validate or sanitize input data before processing it, leading to information exposure in error messages.
The operational impact of this vulnerability is significant for organizations relying on Kerberos 4 authentication systems. Attackers can exploit this flaw to perform reconnaissance activities without requiring authentication credentials, potentially gaining knowledge about valid realms and user accounts within the network. This information can be leveraged to craft more sophisticated attacks, such as targeted credential harvesting or social engineering campaigns. The vulnerability particularly affects systems where Kerberos 4 is still operational, as many organizations had not yet transitioned to Kerberos 5 by the time this vulnerability was discovered. The exposure of realm names and user information creates a pathway for attackers to map the network's authentication landscape and identify potential targets for further exploitation.
Organizations should implement immediate mitigations to address this vulnerability, including upgrading to Kerberos 5 which resolves this issue through improved error handling mechanisms and proper input validation. Network segmentation and firewall rules can be implemented to limit UDP traffic to Kerberos servers, reducing the attack surface. Additionally, monitoring and logging of Kerberos server error messages should be enhanced to detect potential exploitation attempts. The ATT&CK framework categorizes this vulnerability under T1083: File and Directory Discovery and T1566: Phishing, as it enables reconnaissance activities that can lead to credential theft and further compromise. Regular security audits and vulnerability assessments should be conducted to ensure all Kerberos implementations properly handle malformed packets and do not expose sensitive information through error messages. Organizations should also consider implementing intrusion detection systems that can identify patterns of malformed packet traffic that may indicate exploitation attempts. The vulnerability highlights the importance of proper error handling practices in security-critical applications and demonstrates how seemingly benign error conditions can create significant security risks when sensitive information is inadvertently exposed to unauthorized parties.