CVE-2021-43041 in Unitrends Backup Appliance
Summary
by MITRE • 12/06/2021
An issue was discovered in Kaseya Unitrends Backup Appliance before 10.5.5. A crafted HTTP request could induce a format string vulnerability in the privileged vaultServer application.
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Analysis
by VulDB Data Team • 12/09/2021
The vulnerability identified as CVE-2021-43041 represents a critical format string vulnerability within the Kaseya Unitrends Backup Appliance software ecosystem. This flaw exists in versions prior to 10.5.5 and specifically targets the privileged vaultServer application component that operates with elevated system privileges. The vulnerability stems from improper input validation within the HTTP request processing pipeline where user-supplied data is directly incorporated into format strings without adequate sanitization or escaping mechanisms. This design oversight creates a pathway for malicious actors to manipulate the application's behavior through carefully crafted HTTP requests that exploit the underlying format string handling functionality.
The technical exploitation of this vulnerability occurs when the privileged vaultServer application processes HTTP requests containing malicious format specifiers within user-controllable input fields. When the application fails to properly escape or validate these inputs before using them in printf-style functions, attackers can inject format specifiers that cause the application to read or write memory locations beyond the intended input boundaries. This fundamental flaw aligns with CWE-134 which specifically addresses the use of untrusted data in format string contexts, making it a classic example of format string vulnerability exploitation. The privileged nature of the vaultServer application means that successful exploitation could potentially allow attackers to execute arbitrary code with the highest system privileges available to the application, effectively providing them with complete control over the backup appliance and potentially the underlying network infrastructure it protects.
The operational impact of this vulnerability extends beyond simple privilege escalation as it represents a critical weakness in enterprise backup and disaster recovery systems that organizations rely upon for business continuity. Organizations using affected versions of the Kaseya Unitrends Backup Appliance face significant risk of unauthorized access to critical backup data, potential data exfiltration, and complete system compromise. The vulnerability's exploitation could result in complete loss of backup integrity, unauthorized modification of backup data, or even use of the compromised appliance as a pivot point for attacking other systems within the network. This scenario aligns with ATT&CK technique T1059.007 for command and script interpreter execution, as successful exploitation would likely involve the execution of malicious commands through the compromised privileged process. The implications are particularly severe for organizations that depend on backup systems for disaster recovery planning and data protection strategies, as a compromised backup appliance could undermine the entire recovery infrastructure.
Mitigation strategies for CVE-2021-43041 focus primarily on immediate remediation through the application of vendor-provided patches and updates to versions 10.5.5 or later. Organizations should prioritize the deployment of these updates across all affected systems while implementing network segmentation to limit access to the backup appliance to authorized personnel only. Additional defensive measures include implementing web application firewalls to detect and block suspicious HTTP requests containing format string patterns, conducting thorough network monitoring for anomalous traffic patterns, and establishing robust access controls for the appliance interfaces. The vulnerability highlights the importance of input validation and secure coding practices, particularly in privileged applications where the consequences of exploitation can be catastrophic. Organizations should also consider implementing regular security assessments and penetration testing to identify similar vulnerabilities in their backup and recovery systems, ensuring that critical infrastructure components maintain adequate security postures against evolving threats.