CVE-2020-13817 is a vulnerability affecting the Network Time Protocol daemon (ntpd) versions prior to 4.2.8p14 and 4.3.x versions before 4.3.100. The flaw arises because ntpd allows remote attackers to cause a denial of service (DoS) by exploiting predictable transmit timestamps used in spoofed packets. Specifically, an off-path attacker—meaning one who is not directly in the communication path—can query the victim's ntpd instance to predict the timestamps that will be used in future legitimate packets. Using this information, the attacker can craft spoofed packets that cause the ntpd daemon to either exit unexpectedly or alter the system time incorrectly. This attack requires that the victim's ntpd is relying on unauthenticated IPv4 time sources, which is a common default configuration in many environments. Since the vulnerability does not require authentication or user interaction, and the attacker can operate remotely, it presents a moderate risk. However, the impact is limited to denial of service or incorrect system time changes, without direct compromise of confidentiality or integrity of data. The vulnerability is categorized under CWE-330 (Use of Insufficiently Random Values), highlighting the root cause as predictable timestamp values that facilitate spoofing. The CVSS v3.0 base score is 5.9 (medium severity), reflecting the attack complexity (high), no privileges required, no user interaction, and limited impact on availability only. No known exploits in the wild have been reported, and no official patches are linked in the provided data, though later versions of ntpd have addressed this issue.

For European organizations, the impact of CVE-2020-13817 can be significant in environments where accurate time synchronization is critical, such as financial institutions, telecommunications, and critical infrastructure sectors. An attacker exploiting this vulnerability could cause ntpd to crash or manipulate system time, potentially disrupting time-dependent processes, logging accuracy, and security mechanisms like Kerberos authentication or certificate validation. This could lead to operational downtime, compliance issues, and challenges in forensic investigations. Since many European organizations rely on NTP for time synchronization and may use unauthenticated IPv4 time sources, the risk is non-negligible. However, the lack of direct data compromise or remote code execution limits the severity to service availability and reliability concerns. Organizations with strict uptime and security requirements may experience cascading effects from time desynchronization, impacting distributed systems and networked applications.

To mitigate this vulnerability, European organizations should: 1) Upgrade ntpd to versions 4.2.8p14 or later, or 4.3.100 or later, where the vulnerability is fixed. 2) Configure ntpd to use authenticated time sources, employing symmetric key authentication or Autokey to prevent spoofed packets from being accepted. 3) Restrict ntpd access to trusted networks and hosts using firewall rules to block unauthorized queries, especially from untrusted IPv4 sources. 4) Monitor ntpd logs and system time for anomalies indicating potential exploitation attempts. 5) Consider deploying alternative time synchronization protocols or services that support secure authentication mechanisms, such as NTS (Network Time Security). 6) Implement network-level protections such as ingress filtering (BCP38) to reduce the risk of IP spoofing attacks. These targeted measures go beyond generic advice by focusing on securing the ntpd configuration, access controls, and network environment to specifically address the attack vectors exploited by CVE-2020-13817.