Source: cve@mitre.org
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.
Kerberos 4, a legacy authentication protocol, is vulnerable to a remote information disclosure flaw. By sending a crafted UDP packet, attackers can extract the realm name and the last authenticated user, potentially enabling further attacks like password guessing or targeted credential harvesting. This vulnerability, though old, could expose sensitive information on systems still utilizing Kerberos 4.
Step 1: Packet Crafting: An attacker crafts a malformed UDP packet. The packet is designed to be invalid according to the Kerberos 4 protocol, triggering an error response from the server.
Step 2: Packet Delivery: The attacker sends the crafted UDP packet to the Kerberos 4 server, typically on port 88 (or other configured ports).
Step 3: Error Generation: The Kerberos 4 server processes the invalid packet and, due to the vulnerability, generates an error message.
Step 4: Information Leakage: The error message, instead of being generic, includes the Kerberos realm name and the username of the last authenticated user. This information is then sent back to the attacker.
Step 5: Information Harvesting: The attacker receives the error message and extracts the sensitive information (realm and username). This information can be used for reconnaissance and to facilitate further attacks.
The vulnerability stems from a flaw in how Kerberos 4 handles malformed UDP packets. Specifically, when an invalid packet is received, the server generates an error message. This error message, due to a programming error, inadvertently includes the Kerberos realm name and the username of the last user who successfully authenticated. The root cause is likely a lack of proper input validation and sanitization of data used in the error message generation. The software fails to filter sensitive information before including it in the error response, leading to information leakage. This is not a buffer overflow or race condition but rather a logic error in error handling.
Due to the age of the vulnerability and the likely limited use of Kerberos 4, it is unlikely to be directly targeted by sophisticated APTs. However, it could be used as part of a broader reconnaissance phase by any attacker. The vulnerability itself is not listed on the CISA KEV catalog, reflecting its age and the low likelihood of widespread exploitation in modern environments.
Network traffic analysis: Examine UDP traffic on Kerberos ports (typically 88) for malformed packets or unusual patterns.
Log analysis: Review Kerberos server logs for error messages that include realm names or user names. These logs may need to be enabled and configured to capture the relevant information.
Intrusion Detection Systems (IDS): Implement IDS rules to detect malformed Kerberos packets. These rules would need to be specifically tailored to the Kerberos 4 protocol.
Honeypots: Deploying a Kerberos 4 honeypot can help to detect and analyze potential exploitation attempts.
Upgrade: The primary remediation is to upgrade to a more secure version of Kerberos (e.g., Kerberos 5) or a modern authentication system. Kerberos 4 is obsolete and should be replaced.
Network Segmentation: If Kerberos 4 must be used, segment the network to limit access to the Kerberos server. This reduces the attack surface.
Firewall Rules: Implement strict firewall rules to restrict access to Kerberos ports (88, etc.) from untrusted networks.
Monitor Network Traffic: Continuously monitor network traffic for suspicious activity, including malformed packets and unusual access patterns to Kerberos servers.
Disable Kerberos 4: If possible, completely disable Kerberos 4 and migrate to a more secure authentication mechanism.