CVE-2015-6563 in Mac OS X
Summary
by MITRE
The monitor component in sshd in OpenSSH before 7.0 on non-OpenBSD platforms accepts extraneous username data in MONITOR_REQ_PAM_INIT_CTX requests, which allows local users to conduct impersonation attacks by leveraging any SSH login access in conjunction with control of the sshd uid to send a crafted MONITOR_REQ_PWNAM request, related to monitor.c and monitor_wrap.c.
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Analysis
by VulDB Data Team • 05/28/2026
The vulnerability identified as CVE-2015-6563 represents a critical authentication bypass flaw within the OpenSSH implementation on non-OpenBSD platforms. This issue specifically affects the monitor component of sshd, which serves as a security layer responsible for handling privileged operations while maintaining separation between the privileged and unprivileged processes. The vulnerability stems from insufficient input validation within the MONITOR_REQ_PAM_INIT_CTX request processing mechanism, allowing malicious actors to exploit a design flaw that was not present in the OpenBSD implementation of OpenSSH.
The technical exploitation of this vulnerability occurs through a carefully crafted sequence of operations that leverages the existing SSH login functionality. Attackers with access to any SSH login capability can manipulate the monitor component by sending a specially crafted MONITOR_REQ_PWNAM request that includes extraneous username data. This manipulation exploits a fundamental flaw in the way sshd processes authentication requests, where the system fails to properly validate the integrity of user data passed through the monitor interface. The vulnerability is particularly dangerous because it requires only basic SSH access and control over the sshd process uid, making it accessible to local users who may not have elevated privileges.
The operational impact of this vulnerability extends beyond simple impersonation attacks, as it provides attackers with the ability to escalate their privileges within the system. The flaw allows for the manipulation of authentication contexts, potentially enabling unauthorized users to assume the identities of legitimate users within the system. This type of vulnerability falls under CWE-20, which describes improper input validation, and specifically relates to the improper handling of authentication data within privileged processes. The attack vector is particularly concerning because it can be executed from within the normal SSH authentication flow, making detection more challenging and potentially allowing for persistent access to systems.
Mitigation strategies for CVE-2015-6563 primarily involve upgrading to OpenSSH version 7.0 or later, which contains the necessary patches to address the improper input validation issue. Organizations should also implement additional monitoring of sshd processes and authentication logs to detect anomalous behavior patterns that might indicate exploitation attempts. The vulnerability demonstrates the importance of maintaining proper input validation in privileged code sections and highlights the need for consistent security practices across different platform implementations. Security teams should also consider implementing additional authentication controls and monitoring mechanisms that can detect unauthorized privilege escalation attempts. This vulnerability aligns with ATT&CK technique T1078 which covers valid accounts and privilege escalation through legitimate system access, and specifically addresses the monitoring and control of authentication processes as outlined in the security framework.