CVE-2016-7427 in ntpd
Summary
by MITRE
The broadcast mode replay prevention functionality in ntpd in NTP before 4.2.8p9 allows remote attackers to cause a denial of service (reject broadcast mode packets) via a crafted broadcast mode packet.
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Analysis
by VulDB Data Team • 05/13/2026
The vulnerability identified as CVE-2016-7427 resides within the Network Time Protocol daemon implementation in versions prior to 4.2.8p9, specifically affecting the broadcast mode replay prevention mechanism. This flaw represents a critical security issue that undermines the integrity and availability of time synchronization services across networked systems. The vulnerability stems from insufficient validation of broadcast mode packets, creating a scenario where malicious actors can exploit the protocol's defenses rather than utilizing them for protection. The NTP protocol's broadcast mode is designed to allow servers to announce time updates to multiple clients simultaneously, but the replay prevention feature was intended to prevent malicious packet replay attacks that could disrupt time synchronization. However, the implementation contained a logical flaw that permitted attackers to craft specific packets that would bypass these security measures and trigger the rejection of legitimate broadcast mode communications. This vulnerability directly impacts the availability aspect of the ntpd service, as successful exploitation results in denial of service conditions where legitimate time synchronization packets are rejected by the server.
The technical exploitation of this vulnerability occurs through the manipulation of broadcast mode packet structures that are processed by the ntpd daemon. Attackers craft specially formatted packets that exploit a gap in the replay detection algorithm, causing the system to incorrectly identify valid broadcast packets as potential replay attacks and subsequently reject them. The flaw essentially allows an attacker to trigger a false positive in the system's security validation mechanism, leading to the deliberate rejection of legitimate time synchronization data. This type of vulnerability falls under the category of protocol-level flaws that can be classified as CWE-129, representing an input validation issue where the system fails to properly validate packet contents against expected protocol behavior. The specific implementation error occurs within the packet processing logic that handles broadcast mode communications, where the timing and sequence validation checks are insufficient to distinguish between legitimate replay prevention and malicious packet crafting attempts.
The operational impact of CVE-2016-7427 extends beyond simple service disruption to potentially compromise the temporal integrity of affected systems and networks. When exploited, this vulnerability can cause widespread time synchronization failures across organizations that rely on NTP for maintaining accurate time stamps, network authentication, and security logging. The denial of service condition affects not only individual systems but can cascade through network infrastructures where multiple devices depend on synchronized time for proper operation. This vulnerability particularly impacts environments where NTP broadcast mode is actively used for time distribution, such as enterprise networks, data centers, and security infrastructure where precise timing is crucial for operations. The attack vector is particularly concerning because it requires minimal privileges and can be executed remotely, making it accessible to threat actors with basic network access. The vulnerability's impact aligns with ATT&CK technique T1499.001, which describes network disruption attacks that can be achieved through protocol manipulation, and represents a specific instance of service disruption that can be leveraged for broader operational impact.
Mitigation strategies for CVE-2016-7427 focus primarily on upgrading to NTP versions 4.2.8p9 or later, which contain the necessary patches to address the broadcast mode packet validation issue. Organizations should prioritize immediate deployment of patched NTP versions across all network infrastructure devices, servers, and client systems that utilize the broadcast mode functionality. Additionally, network administrators should implement monitoring solutions that can detect anomalous packet patterns and unusual rejection behavior in NTP communications, as these could indicate exploitation attempts. The patch addresses the root cause by strengthening the validation logic within the broadcast mode packet processing, ensuring that legitimate packets are properly distinguished from crafted malicious packets. Security measures should also include network segmentation and access control policies that limit which systems can send broadcast mode packets to NTP servers, reducing the attack surface. Organizations should conduct thorough vulnerability assessments to identify all systems running vulnerable NTP versions and establish procedures for regular security updates and patch management. The remediation approach aligns with industry best practices for protocol security and follows the principle of least privilege, ensuring that only authorized systems can participate in broadcast time synchronization while maintaining proper validation of all incoming packets.