CVE-2015-7691 in ntpd
Summary
by MITRE
The crypto_xmit function in ntpd in NTP 4.2.x before 4.2.8p4, and 4.3.x before 4.3.77 allows remote attackers to cause a denial of service (crash) via crafted packets containing particular autokey operations. NOTE: This vulnerability exists due to an incomplete fix for CVE-2014-9750.
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Analysis
by VulDB Data Team • 06/25/2022
The vulnerability identified as CVE-2015-7691 represents a critical denial of service flaw affecting the Network Time Protocol daemon implementation in NTP versions prior to 4.2.8p4 and 4.3.77. This issue specifically targets the crypto_xmit function within ntpd, which handles cryptographic operations during autokey protocol exchanges. The vulnerability arises from an incomplete remediation of a previous security flaw, CVE-2014-9750, demonstrating how security fixes can sometimes introduce new attack vectors when not thoroughly validated. The flaw manifests when remote attackers craft specially formatted packets that exploit weaknesses in the autokey operation processing logic, leading to system crashes and complete service disruption.
The technical implementation of this vulnerability stems from improper handling of autokey cryptographic operations within the NTP protocol stack. When ntpd processes packets containing malicious autokey data structures, the crypto_xmit function fails to properly validate input parameters and memory allocation boundaries. This results in buffer overflows, memory corruption, or invalid pointer dereferences that cause the daemon to terminate unexpectedly. The vulnerability operates at the network protocol level, making it particularly dangerous as it can be exploited remotely without requiring authentication or privileged access. The attack vector leverages the legitimate autokey protocol functionality that NTP uses for secure time synchronization, making it difficult to distinguish malicious packets from legitimate traffic.
From an operational impact perspective, this vulnerability creates significant risks for network infrastructure relying on NTP services for time synchronization. Organizations using affected NTP versions face potential service outages that can cascade across their network, affecting applications dependent on accurate timekeeping for security logging, authentication, transaction processing, and compliance requirements. The vulnerability's remote exploitability means that attackers can trigger crashes from anywhere on the network, potentially leading to sustained denial of service attacks that are difficult to mitigate. System administrators may experience unexpected daemon restarts, loss of time synchronization, and disruption of time-sensitive applications that depend on stable NTP service availability.
The mitigation strategy for CVE-2015-7691 requires immediate deployment of patched NTP versions, specifically NTP 4.2.8p4 or later for the 4.2.x series and 4.3.77 or later for the 4.3.x series. Organizations should also implement network segmentation and access controls to limit exposure of NTP servers to untrusted networks, while monitoring for suspicious autokey protocol traffic patterns. Additional defensive measures include configuring firewall rules to restrict NTP traffic to trusted sources, enabling NTP daemon security features such as the restrict command, and implementing intrusion detection systems to monitor for exploitation attempts. This vulnerability aligns with CWE-121, heap-based buffer overflow, and represents a technique categorized under ATT&CK tactic TA0043, Resource Exhaustion, where attackers exploit protocol implementations to consume system resources or cause service termination.