CVE-2026-2035 in OPNsense
Summary
by MITRE • 02/21/2026
Deciso OPNsense diag_backup.php filename Command Injection Remote Code Execution Vulnerability. This vulnerability allows network-adjacent attackers to execute arbitrary code on affected installations of Deciso OPNsense. Authentication is required to exploit this vulnerability.
The specific flaw exists within the handling of backup configuration files. The issue results from the lack of proper validation of a user-supplied string before using it to execute a system call. An attacker can leverage this vulnerability to execute code in the context of root. Was ZDI-CAN-28131.
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Analysis
by VulDB Data Team • 02/22/2026
The CVE-2026-2035 vulnerability represents a critical command injection flaw in Deciso OPNsense's diag_backup.php script that enables remote code execution with root privileges. This vulnerability specifically targets the backup configuration file handling functionality within the OPNsense firewall management interface. The flaw arises from insufficient input validation when processing user-supplied filenames, creating an avenue for attackers to inject malicious commands that get executed with elevated privileges. The vulnerability is particularly concerning because it requires only network adjacency for exploitation, meaning attackers within the same network segment can leverage this weakness without requiring direct physical access or additional authentication mechanisms beyond what is already present in the system. The attack surface is further expanded by the fact that authentication is required but can potentially be bypassed through various means including credential theft or session hijacking attacks that are common in networked environments.
The technical implementation of this vulnerability stems from improper sanitization of user-provided input within the backup file name parameter processing logic. When administrators or authorized users interact with the diag_backup.php interface to manage configuration backups, the system fails to properly validate or escape the filename parameter before incorporating it into system commands. This creates a classic command injection scenario where attacker-controlled input gets directly concatenated into shell execution calls without adequate sanitization or encoding. The vulnerability manifests as a direct system call execution where the malicious input flows through the application's input processing pipeline and gets interpreted by the underlying operating system shell, allowing arbitrary command execution. This type of vulnerability maps directly to CWE-77 which defines improper neutralization of special elements used in a command, and represents a clear violation of secure coding principles that mandate input validation and sanitization before system interactions. The root context of this flaw means that any successful exploitation results in complete system compromise, as the commands execute with the highest available privileges within the operating system environment.
The operational impact of CVE-2026-2035 extends far beyond simple unauthorized code execution, as it provides attackers with complete control over the underlying OPNsense system and its network security functions. An attacker who successfully exploits this vulnerability can manipulate firewall rules, modify network configurations, access sensitive data, and potentially establish persistent backdoors within the network infrastructure. The compromised system becomes a potential pivot point for lateral movement throughout the network, allowing attackers to target other systems that may not be directly exposed to the internet. This vulnerability particularly affects enterprise environments where OPNsense firewalls serve as critical network security appliances, potentially allowing attackers to bypass network segmentation controls and gain access to sensitive internal systems. The impact is compounded by the fact that OPNsense installations often serve as the primary network gateway and security control, making successful exploitation equivalent to gaining control over the entire network perimeter. This vulnerability aligns with ATT&CK technique T1059 which describes command and scripting interpreter usage, and T1068 which covers exploit for privilege escalation, demonstrating how this single vulnerability can enable multiple attack phases.
Mitigation strategies for CVE-2026-2035 require immediate action including applying the vendor-provided security patches and updates to address the command injection flaw. Organizations should implement network segmentation and access controls to limit the attack surface, ensuring that only authorized personnel can access the OPNsense management interfaces. Regular security assessments and network monitoring should be deployed to detect anomalous behavior that might indicate exploitation attempts. The implementation of web application firewalls and input validation controls can provide additional layers of protection against similar vulnerabilities. System administrators should conduct thorough review of backup file handling procedures and implement strict input validation for all user-supplied parameters. Network access controls should be enforced to limit the network adjacency requirement, potentially through the use of VLANs or other network segmentation techniques. Security monitoring should include detection of suspicious command execution patterns and unusual backup file operations that might indicate exploitation attempts. The vulnerability serves as a reminder of the critical importance of secure coding practices and input validation in network security applications, emphasizing the need for continuous security testing and vulnerability management programs to prevent similar issues from arising in other components of the system.