CVE-2019-11331 in ntp
Summary
by MITRE
Network Time Protocol (NTP), as specified in RFC 5905, uses port 123 even for modes where a fixed port number is not required, which makes it easier for remote attackers to conduct off-path attacks.
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Analysis
by VulDB Data Team • 09/04/2023
The vulnerability described in CVE-2019-11331 relates to the Network Time Protocol implementation in NTP version 4.2.8p12 and earlier versions, where the protocol continues to utilize port 123 for all operational modes despite RFC 5905 specification allowing for dynamic port assignment in certain modes. This design flaw creates a significant security weakness that directly impacts the protocol's resistance to off-path attacks, where adversaries positioned between communicating parties can intercept and manipulate time synchronization messages without direct network access to the endpoints. The persistent use of port 123 regardless of operational mode violates the principle of least privilege and creates predictable network behavior that attackers can exploit to gain unauthorized access to time synchronization services.
The technical flaw stems from the NTP implementation's failure to properly implement the port assignment mechanisms specified in RFC 5905, particularly in modes that should utilize dynamic port assignment for enhanced security. This vulnerability falls under CWE-254, representing a weakness in security design that allows for predictable network behavior, and specifically relates to CWE-264, which addresses permissions, privileges, and access controls. The flaw enables attackers to conduct off-path attacks by leveraging the fixed port assignment pattern, making it easier to intercept and manipulate NTP traffic. This vulnerability directly impacts the confidentiality, integrity, and availability of time synchronization services, as attackers can potentially manipulate time information or disrupt network operations through carefully crafted NTP packets.
The operational impact of CVE-2019-11331 extends beyond simple network monitoring, as it creates opportunities for sophisticated attacks including time-based authentication bypasses, denial of service conditions, and potential privilege escalation through time manipulation. The vulnerability is particularly concerning in environments where precise time synchronization is critical for security operations, such as in financial services, telecommunications, and enterprise security infrastructures. Attackers can exploit this weakness to conduct man-in-the-middle attacks against NTP servers, potentially causing cascading failures across networked systems that depend on accurate timekeeping. The vulnerability also aligns with ATT&CK technique T1562.001, which involves disabling or modifying system security tools, as compromised NTP services can lead to broader system security degradation. Organizations using affected NTP versions face increased risk of coordinated attacks that leverage the predictable port usage to systematically compromise time synchronization infrastructure.
Mitigation strategies for CVE-2019-11331 should focus on updating NTP implementations to versions that properly implement RFC 5905 specifications, particularly addressing the port assignment behavior across different operational modes. Network administrators should implement additional security controls including firewall rules that restrict NTP traffic to trusted sources, utilize NTP authentication mechanisms such as symmetric key cryptography, and deploy network segmentation to isolate time synchronization services. The implementation of NTP access control lists and rate limiting can further reduce the attack surface, while monitoring for unusual NTP traffic patterns can help detect potential exploitation attempts. Organizations should also consider implementing intrusion detection systems specifically configured to monitor for NTP-related anomalies and ensure that NTP services are properly configured to use dynamic ports in appropriate operational modes. Regular security assessments and vulnerability scanning should be conducted to verify proper NTP implementation and identify any remaining exposure to similar port assignment vulnerabilities.