Source: cve@mitre.org
Vulnerability in BSD Telnet client with encryption and Kerberos 4 authentication allows remote attackers to decrypt the session via sniffing.
BSD Telnet clients utilizing encryption and Kerberos 4 authentication are vulnerable to session decryption. This allows attackers to eavesdrop on sensitive Telnet sessions, potentially gaining access to credentials and confidential data. The vulnerability stems from weaknesses in the encryption implementation, making it susceptible to passive sniffing attacks.
Step 1: Session Initiation: The vulnerable Telnet client establishes a connection to a Telnet server, potentially using Kerberos 4 for authentication and encryption.
Step 2: Encryption Negotiation: The client and server negotiate the encryption method, likely using a weak cipher such as DES or a similar outdated algorithm.
Step 3: Traffic Capture: An attacker passively monitors the network traffic between the client and server, capturing the encrypted Telnet session data.
Step 4: Decryption: The attacker uses readily available tools and knowledge of the weak encryption algorithm to decrypt the captured traffic. This is possible due to the lack of strong key exchange and the use of easily broken ciphers.
Step 5: Information Disclosure: The attacker gains access to the decrypted Telnet session, including usernames, passwords, and any other data transmitted during the session.
The vulnerability lies in the flawed implementation of encryption within the BSD Telnet client, specifically when used with Kerberos 4 authentication. The encryption algorithms used were weak by modern standards and the key exchange mechanisms were vulnerable to passive eavesdropping. Attackers could capture the encrypted Telnet traffic and, due to the weak encryption, decrypt the session using readily available tools. The root cause is the use of outdated and easily broken encryption ciphers and the lack of proper key management and session integrity checks. This allows for a simple ciphertext-only attack, where the attacker only needs to capture the encrypted traffic to recover the plaintext.
This vulnerability is not directly associated with specific APT groups or malware campaigns in modern times due to the age and the decline in Telnet usage. However, the techniques used in this attack (passive sniffing and weak encryption decryption) are fundamental and could be employed by any attacker. CISA KEV status: Not Listed
Network traffic analysis: Examine network traffic for Telnet sessions, especially those utilizing Kerberos 4 authentication. Look for the use of weak ciphers (e.g., DES).
Packet capture analysis: Use tools like Wireshark to capture and analyze Telnet traffic. Look for evidence of successful Kerberos 4 authentication and the presence of encrypted data that can be potentially decrypted.
Log analysis: Review system logs for Telnet connections and authentication attempts. Look for unusual activity or failed login attempts that might indicate an attack.
Intrusion Detection Systems (IDS): Implement IDS rules to detect Telnet traffic and potentially identify the use of weak encryption algorithms.
Security Information and Event Management (SIEM): Correlate logs from various sources to identify suspicious Telnet activity and potential exploitation attempts.
Disable Telnet: The most effective remediation is to disable Telnet entirely. Use SSH (Secure Shell) for secure remote access.
Upgrade to SSH: If Telnet is required for legacy systems, upgrade to a secure SSH implementation with strong encryption and authentication.
Patching: Ensure that all systems are patched with the latest security updates. While specific patches for this vulnerability may not exist, patching helps to mitigate other vulnerabilities that could be exploited in conjunction with this one.
Network Segmentation: Isolate legacy systems using Telnet within a separate network segment to limit the impact of a potential breach.
Strong Authentication: Enforce strong authentication mechanisms, such as multi-factor authentication (MFA), to protect against credential theft.
Regular Security Audits: Conduct regular security audits and penetration testing to identify and address vulnerabilities.