CVE-2016-4956 in ntpd
Summary
by MITRE
ntpd in NTP 4.x before 4.2.8p8 allows remote attackers to cause a denial of service (interleaved-mode transition and time change) via a spoofed broadcast packet. NOTE: this vulnerability exists because of an incomplete fix for CVE-2016-1548.
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Analysis
by VulDB Data Team • 01/27/2025
The vulnerability described in CVE-2016-4956 represents a critical denial of service weakness in the Network Time Protocol daemon (ntpd) version 4.x prior to 4.2.8p8. This issue specifically targets the broadcast mode functionality of NTP implementations and exploits a flaw in how the system handles interleaved-mode transitions and time changes. The vulnerability stems from an incomplete remediation of a previously identified issue, CVE-2016-1548, which demonstrates the complexity of addressing time synchronization protocol vulnerabilities where fixes may inadvertently introduce new attack vectors. The flaw enables remote attackers to manipulate the time synchronization process through forged network packets that appear to originate from legitimate broadcast sources.
The technical implementation of this vulnerability occurs within the ntpd daemon's handling of broadcast mode packets, where the system fails to properly validate the authenticity and sequence of time synchronization messages. When a spoofed broadcast packet is received, the ntpd process undergoes an interleaved-mode transition that disrupts normal time synchronization operations. This transition mechanism, designed to handle multiple time sources and maintain accurate synchronization, becomes vulnerable to manipulation when the packet sequence or timing information is altered by an attacker. The system's inability to properly distinguish between legitimate and forged broadcast communications leads to cascading failures in the time synchronization process, ultimately resulting in service disruption.
The operational impact of CVE-2016-4956 extends beyond simple denial of service, as it can severely compromise the reliability of time-sensitive network operations that depend on accurate time synchronization. Organizations relying on NTP for critical infrastructure operations may experience widespread service degradation or complete time synchronization failure across their network. This vulnerability particularly affects systems where NTP broadcast mode is enabled and where multiple time sources are managed simultaneously. The attack can be executed remotely without requiring authentication, making it particularly dangerous for network infrastructure that exposes NTP services to untrusted networks. The vulnerability's exploitation can lead to cascading failures in systems that depend on synchronized time for logging, security event correlation, database transactions, and other time-dependent operations.
Mitigation strategies for this vulnerability require immediate implementation of the patched NTP version 4.2.8p8 or later, which contains the complete fix for both CVE-2016-4956 and its precursor CVE-2016-1548. Network administrators should also implement additional defensive measures including disabling broadcast mode where possible, implementing proper NTP authentication mechanisms, and deploying network segmentation to limit exposure of NTP services to untrusted networks. The vulnerability aligns with ATT&CK technique T1499.001 for network denial of service and CWE-284 for improper access control in time synchronization protocols. Organizations should also consider implementing intrusion detection systems capable of identifying anomalous broadcast packet patterns and monitoring for irregular time change events that may indicate exploitation attempts. Regular security assessments and vulnerability scanning should be conducted to ensure proper patch management and prevent similar vulnerabilities from arising due to incomplete fixes in protocol implementations.