CVE-2016-5117 in OpenNTPD
Summary
by MITRE
OpenNTPD before 6.0p1 does not validate the CN for HTTPS constraint requests, which allows remote attackers to bypass the man-in-the-middle mitigations via a crafted timestamp constraint with a valid certificate.
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Analysis
by VulDB Data Team • 08/08/2020
The vulnerability identified as CVE-2016-5117 affects OpenNTPD versions prior to 6.0p1 and represents a critical flaw in the certificate validation process for HTTPS constraint requests. This issue stems from the software's failure to properly validate the Common Name (CN) field within X.509 certificates, creating a significant security gap that undermines the intended man-in-the-middle protection mechanisms. The flaw specifically impacts the validation of timestamp constraints that rely on HTTPS connections, where attackers can exploit this weakness to bypass established security controls.
The technical implementation of this vulnerability lies in the certificate validation logic within OpenNTPD's HTTPS constraint handling mechanism. When processing timestamp constraints, the software performs certificate validation but fails to adequately verify that the certificate's Common Name matches the expected hostname or domain. This incomplete validation allows attackers to craft malicious certificates that contain valid certificate chains but have misleading Common Name fields. The vulnerability essentially permits certificate validation to succeed even when the certificate is not legitimately associated with the target server, thereby undermining the fundamental trust model that HTTPS and certificate-based authentication rely upon.
The operational impact of CVE-2016-5117 is substantial, as it enables remote attackers to execute successful man-in-the-middle attacks against systems using vulnerable OpenNTPD versions. Attackers can intercept and manipulate timestamp constraint requests without detection, potentially compromising the integrity of time synchronization services that are critical for security operations. This vulnerability particularly affects environments where NTP servers are configured to use HTTPS constraints for authentication, making it possible for malicious actors to inject false timestamps or redirect traffic to compromised servers. The implications extend beyond simple time synchronization issues, as incorrect timestamps can affect log analysis, security certificate validation, and the overall integrity of time-sensitive security operations.
This vulnerability aligns with CWE-295, which addresses improper certificate validation, and corresponds to techniques described in the MITRE ATT&CK framework under T1566 for credential access through man-in-the-middle attacks. The flaw demonstrates a classic example of insufficient validation that allows attackers to bypass security controls designed to prevent unauthorized access. Organizations using vulnerable versions of OpenNTPD face risks including potential data manipulation, unauthorized system access, and compromised security auditing capabilities. The vulnerability essentially undermines the trust relationship between NTP clients and servers, making it possible for attackers to establish false trust relationships without proper authentication.
The recommended mitigation strategy involves upgrading to OpenNTPD version 6.0p1 or later, which includes proper certificate validation that enforces Common Name matching for HTTPS constraint requests. System administrators should also implement additional monitoring for unusual timestamp constraint behavior and consider deploying certificate pinning mechanisms where appropriate. Network segmentation and additional security controls such as firewall rules that restrict NTP traffic to trusted sources can provide additional layers of protection. Organizations should conduct thorough vulnerability assessments to identify all systems running vulnerable versions and ensure proper patch management processes are in place to prevent similar issues from occurring in the future.