CVE-2013-3949 in Mac OS X
Summary
by MITRE
The posix_spawn system call in the XNU kernel in Apple Mac OS X 10.8.x does not prevent use of the _POSIX_SPAWN_DISABLE_ASLR and _POSIX_SPAWN_ALLOW_DATA_EXEC flags for setuid and setgid programs, which allows local users to bypass intended access restrictions via a wrapper program that calls the posix_spawnattr_setflags function.
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Analysis
by VulDB Data Team • 04/20/2018
The vulnerability identified as CVE-2013-3949 resides within the XNU kernel implementation of the posix_spawn system call on Apple Mac OS X 10.8.x operating systems. This flaw represents a critical security oversight that directly impacts the kernel's privilege management mechanisms and memory protection policies. The vulnerability specifically affects the enforcement of security restrictions when executing setuid and setgid programs, creating a pathway for local attackers to circumvent intended access controls that are fundamental to operating system security models.
The technical flaw manifests in the improper handling of posix_spawn attributes when dealing with privileged programs. The XNU kernel's implementation fails to adequately validate or restrict the use of two specific flags: _POSIX_SPAWN_DISABLE_ASLR and _POSIX_SPAWN_ALLOW_DATA_EXEC. These flags, when improperly applied to setuid and setgid programs, can disable crucial security mitigations including Address Space Layout Randomization and data execution prevention mechanisms. This vulnerability directly maps to CWE-276, which addresses incorrect permissions for privileges, and represents a failure in proper privilege separation and security attribute enforcement within the kernel's process creation framework.
The operational impact of this vulnerability is significant for local attackers who can leverage this weakness to bypass security controls that are designed to prevent privilege escalation and code execution in protected environments. By crafting a wrapper program that invokes posix_spawnattr_setflags with these specific flags, an attacker can effectively disable ASLR protections that randomize memory layout and data execution prevention that blocks code injection attacks. This creates opportunities for advanced exploitation techniques including return-oriented programming attacks and heap-based buffer overflows that would otherwise be mitigated by these security features. The vulnerability essentially allows attackers to create a controlled execution environment that removes critical memory protection mechanisms.
The implications extend beyond simple privilege escalation to encompass broader system compromise potential. When ASLR is disabled, attackers gain significantly more predictable memory addresses for exploiting vulnerabilities, while disabling data execution prevents the kernel from enforcing stack canaries and other memory protection mechanisms. This vulnerability aligns with ATT&CK technique T1068, which covers 'Local Privilege Escalation,' and specifically targets the kernel-level execution control mechanisms that are fundamental to maintaining system integrity. The attack vector requires local system access but provides a pathway for elevation of privileges that could enable further compromise of the system.
Mitigation strategies for this vulnerability must address both immediate patching requirements and broader security hardening approaches. The primary remediation involves applying the official security patches released by Apple that correct the validation logic in the XNU kernel's posix_spawn implementation. Additionally, system administrators should implement comprehensive monitoring for unauthorized use of posix_spawn attributes and consider deploying additional security controls such as mandatory access controls, privilege separation mechanisms, and regular security auditing of system call usage patterns. The vulnerability demonstrates the importance of maintaining strict validation of security attributes during privilege transitions and underscores the critical need for kernel-level security enforcement that cannot be bypassed by user-space applications.