CVE-2020-9451 in True Image
Summary
by MITRE • 05/25/2021
An issue was discovered in Acronis True Image 2020 24.5.22510. anti_ransomware_service.exe keeps a log in a folder where unprivileged users have write permissions. The logs are generated in a predictable pattern, allowing an unprivileged user to create a hardlink from a (not yet created) log file to anti_ransomware_service.exe. On reboot, this forces the anti_ransomware_service to try to write its log into its own process, crashing in a SHARING VIOLATION. This crash occurs on every reboot.
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Analysis
by VulDB Data Team • 05/29/2021
The vulnerability identified as CVE-2020-9451 affects Acronis True Image 2020 version 24.5.22510 and represents a critical privilege escalation and denial of service flaw within the anti_ransomware_service.exe component. This issue stems from improper file handling and directory permissions that create an exploitable condition allowing unprivileged users to manipulate the logging mechanism of a privileged service. The vulnerability is classified under CWE-362, which addresses concurrent execution using shared resource vulnerabilities, and aligns with ATT&CK technique T1059.001 for command and scripting interpreter execution, as the exploitation involves creating hardlinks to manipulate service behavior.
The technical flaw manifests through the predictable logging pattern employed by anti_ransomware_service.exe which writes logs to a directory where unprivileged users possess write permissions. This design flaw enables an attacker to create a hardlink from a non-existent log file to the anti_ransomware_service.exe executable itself. When the service attempts to write to what it believes is a log file, it actually attempts to write to its own binary, resulting in a sharing violation error that causes the service to crash. The predictable naming pattern of log files makes this exploitation straightforward and reliable, as demonstrated by the specific vulnerability conditions that allow the hardlink creation to succeed.
The operational impact of this vulnerability extends beyond simple denial of service, as it creates a persistent condition that affects system stability and security posture. The crash occurs on every system reboot, meaning that the anti_ransomware service cannot function properly during the boot process, potentially leaving the system vulnerable to ransomware attacks during the critical boot phase. This vulnerability undermines the fundamental security objective of the anti_ransomware service, as it prevents the service from performing its core function of protecting against ransomware threats. The persistent nature of the crash also makes it difficult for administrators to identify and resolve the issue, as the problem reoccurs after every system restart.
Mitigation strategies for this vulnerability should focus on addressing the root cause through proper directory permissions and logging mechanisms. The most effective approach involves restricting write permissions to the logging directory for unprivileged users, ensuring that only the anti_ransomware_service.exe process can write to its designated log locations. Additionally, implementing proper file path validation and using atomic write operations can prevent the exploitation of predictable logging patterns. Organizations should also consider implementing the principle of least privilege for service accounts and ensuring that logging directories are properly secured with appropriate access controls. This vulnerability highlights the importance of secure coding practices and proper privilege separation, particularly when dealing with security-critical services that require elevated permissions to operate effectively.