CVE-2016-9604 in Linux
Summary
by MITRE
It was discovered in the Linux kernel before 4.11-rc8 that root can gain direct access to an internal keyring, such as '.dns_resolver' in RHEL-7 or '.builtin_trusted_keys' upstream, by joining it as its session keyring. This allows root to bypass module signature verification by adding a new public key of its own devising to the keyring.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 04/06/2023
The vulnerability described in CVE-2016-9604 represents a critical security flaw in the Linux kernel's key management subsystem that allows privilege escalation from root to kernel-level access. This issue affects Linux kernel versions prior to 4.11-rc8 and specifically targets the session keyring functionality that is used to manage cryptographic keys within the kernel. The vulnerability exploits a fundamental design weakness in how the kernel handles internal keyrings, particularly those used for module signature verification and DNS resolution. The affected keyrings include '.dns_resolver' in Red Hat Enterprise Linux 7 and '.builtin_trusted_keys' in upstream kernel implementations, both of which serve critical security functions in the kernel's trust model.
The technical implementation of this vulnerability relies on the ability of a root user to manipulate the session keyring through the keyring subsystem's join operation. When root executes a specific sequence of system calls that join an internal keyring as the session keyring, they gain direct access to the internal keyring's contents and can modify its contents. This operation bypasses the normal security controls that should prevent modification of trusted keyrings. The flaw stems from insufficient access controls and validation checks within the kernel's keyring management code, allowing any process running with root privileges to essentially hijack the keyring's security context. This mechanism violates the principle of least privilege and undermines the kernel's security architecture by allowing arbitrary modification of trusted cryptographic keys.
The operational impact of this vulnerability is severe and far-reaching, as it enables a root user to completely bypass the kernel's module signature verification mechanisms. Once root gains access to the internal keyring, they can inject their own public keys and effectively sign arbitrary kernel modules, allowing them to load unsigned or malicious code into the kernel space. This creates a persistent backdoor that can survive system reboots and provides an attacker with complete control over the kernel's security policies. The vulnerability essentially allows for the complete subversion of the kernel's trust model, as demonstrated by the ability to add custom keys to '.builtin_trusted_keys' which is used for validating kernel module signatures. This capability can be exploited to load rootkits, disable security features, or create persistent access mechanisms that are difficult to detect and remove.
The security implications extend beyond simple privilege escalation, as this vulnerability can be leveraged to circumvent multiple security controls within the Linux kernel's security framework. The ability to modify trusted keyrings undermines the integrity of the kernel's cryptographic operations and can be used to bypass security modules such as module signing, kernel lockdown, and other cryptographic verification mechanisms. This vulnerability aligns with attack patterns described in the MITRE ATT&CK framework under privilege escalation techniques, specifically targeting kernel-level access and credential manipulation. From a CWE perspective, this vulnerability corresponds to CWE-284 Access Control, specifically focusing on insufficient access control over kernel resources and the improper handling of privileged operations within the kernel's key management system. The vulnerability also relates to CWE-377 Insecure Temporary File Creation and CWE-362 Concurrent Execution using Shared Resources, as it involves manipulation of shared kernel resources through improper access controls.
Mitigation strategies for CVE-2016-9604 require immediate kernel updates to versions 4.11-rc8 or later where the vulnerability has been patched. The patch addresses the issue by implementing proper access controls and validation checks within the keyring subsystem to prevent root users from joining internal keyrings that should remain protected. Organizations should also implement monitoring for suspicious keyring operations and system call patterns that might indicate exploitation attempts. Additionally, security hardening measures such as kernel lockdown, module signing enforcement, and restricted root access should be implemented to minimize the attack surface. The vulnerability highlights the importance of proper kernel security architecture design and the need for robust access control mechanisms at all levels of the kernel's security model, particularly in cryptographic subsystems where trust relationships are fundamental to system security.