CVE-2015-7855 in ntpd
Summary
by MITRE
The decodenetnum 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 (assertion failure) via a 6 or mode 7 packet containing a long data value.
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Analysis
by VulDB Data Team • 11/11/2024
The vulnerability identified as CVE-2015-7855 affects the Network Time Protocol daemon implementation in versions prior to 4.2.8p4 and 4.3.77, representing a critical denial of service weakness that can be exploited remotely. This flaw resides within the decodenetnum function of ntpd, which processes network time protocol packets and handles various packet types including those with mode 6 and mode 7. The vulnerability specifically manifests when processing packets containing excessively long data values that exceed expected parameter limits. The affected implementation fails to properly validate input data lengths, leading to assertion failures that cause the ntpd service to terminate unexpectedly.
The technical nature of this vulnerability aligns with CWE-129, which describes improper validation of length of input data, and represents a classic buffer over-read or input validation issue. When remote attackers send specially crafted mode 6 or mode 7 packets with extended data fields, the decodenetnum function processes these values without adequate bounds checking, resulting in an assertion failure that crashes the ntpd process. This behavior constitutes a denial of service condition that can be easily triggered without requiring authentication or special privileges, making it particularly dangerous in networked environments where NTP services are exposed to untrusted networks.
From an operational impact perspective, this vulnerability presents a significant risk to network infrastructure reliability as NTP services are fundamental to maintaining accurate time synchronization across networked systems. The denial of service condition can disrupt time synchronization for entire networks, potentially affecting critical services that depend on precise timing, such as authentication systems, logging mechanisms, and distributed database operations. The attack vector is particularly concerning because it requires no authentication and can be executed from any location with network access to the vulnerable NTP service, making it a preferred target for network disruption attacks.
The vulnerability demonstrates characteristics consistent with attack techniques described in the MITRE ATT&CK framework under the T1499 category for Network Denial of Service, where adversaries leverage weaknesses in network protocols to disrupt services. Organizations should implement immediate mitigations including upgrading to patched versions of NTP software, implementing network segmentation to restrict access to NTP services, and deploying intrusion detection systems to monitor for suspicious packet patterns. Additionally, network administrators should consider implementing rate limiting and access control lists to restrict which hosts can communicate with NTP servers, thereby reducing the attack surface and limiting the potential impact of such exploits. The remediation process should include comprehensive testing of patched versions in controlled environments before deployment to production systems to ensure no regression issues are introduced.