CVE-2016-2085 in Linux
Summary
by MITRE
The evm_verify_hmac function in security/integrity/evm/evm_main.c in the Linux kernel before 4.5 does not properly copy data, which makes it easier for local users to forge MAC values via a timing side-channel attack.
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Analysis
by VulDB Data Team • 07/27/2022
The vulnerability identified as CVE-2016-2085 resides within the Linux kernel's Extended Verification Module (EVM) subsystem, specifically in the evm_verify_hmac function located at security/integrity/evm/evm_main.c. This flaw represents a critical security weakness that affects Linux kernel versions prior to 4.5, potentially allowing local attackers to manipulate cryptographic integrity checks through sophisticated timing attacks. The EVM subsystem is designed to provide mandatory access controls and integrity verification mechanisms for files and directories, making it a crucial component for maintaining system security and data integrity. When properly functioning, EVM ensures that files cannot be modified without detection by verifying their integrity through HMAC (Hash-based Message Authentication Code) calculations. However, the vulnerability creates a pathway for attackers to bypass these security mechanisms through a timing side-channel attack that exploits the improper copying of data within the verification process.
The technical flaw manifests in the evm_verify_hmac function where the implementation fails to properly copy data during HMAC verification operations, creating a timing discrepancy that attackers can exploit. This improper data handling creates observable timing differences in the cryptographic verification process that correlate with the values being checked. The vulnerability stems from the fact that the function does not perform constant-time comparisons or proper memory handling during the HMAC verification, allowing attackers to measure execution time variations that reveal information about the MAC values being verified. This timing side-channel vulnerability enables attackers to perform statistical analysis and gradually reconstruct valid MAC values through repeated verification attempts. The issue is particularly dangerous because it operates at the kernel level, where local users already possess significant privileges and can execute code with elevated permissions. The flaw directly relates to CWE-347, which addresses improper verification of cryptographic signatures, and represents a classic example of how implementation weaknesses can undermine cryptographic security even when the underlying algorithms remain sound.
The operational impact of CVE-2016-2085 is substantial as it allows local users to forge MAC values, effectively compromising the integrity verification mechanisms that the EVM subsystem is designed to protect. An attacker with local access can leverage this vulnerability to bypass mandatory access controls, modify protected files without detection, and potentially escalate privileges within the system. The timing side-channel attack requires careful measurement and multiple attempts but can be automated to gradually reconstruct valid MAC values, making it a practical threat rather than a theoretical concern. This vulnerability particularly affects systems where EVM is enabled and where integrity verification is critical for security, such as systems with mandatory access controls, secure file systems, or environments where file integrity monitoring is essential. The attack vector is limited to local users who can execute code on the target system, but the potential consequences are severe as it undermines fundamental security assumptions about file integrity and access control. Systems running vulnerable kernel versions are at risk of data tampering, unauthorized modifications, and potential privilege escalation attacks that could compromise the entire system.
The primary mitigation for CVE-2016-2085 involves upgrading to Linux kernel version 4.5 or later, where the vulnerability has been addressed through proper data copying and timing-independent verification mechanisms. System administrators should prioritize kernel updates and ensure that all systems running EVM-enabled configurations are patched promptly. Additionally, organizations should consider implementing monitoring solutions that can detect anomalous timing patterns or repeated verification attempts that might indicate exploitation attempts. The fix implemented in kernel version 4.5 addresses the root cause by ensuring proper data handling during HMAC verification operations and eliminating the timing side-channel that enabled the attack. Security teams should also review their EVM configurations and ensure that integrity verification mechanisms are properly implemented and monitored. This vulnerability highlights the importance of constant-time algorithm implementations and proper side-channel attack prevention measures in cryptographic code. The mitigation strategy aligns with ATT&CK technique T1059.001 for command and scripting interpreter and T1566.001 for spearphishing attachment, as attackers might use this vulnerability to establish persistent access through modified system files. Organizations should also consider implementing additional security controls such as file integrity monitoring, intrusion detection systems, and regular security audits to detect and prevent exploitation of similar timing-based vulnerabilities.