CVE-2021-20118 in Nessus Agent
Summary
by MITRE • 09/09/2021
Nessus Agent 8.3.0 and earlier was found to contain a local privilege escalation vulnerability which could allow an authenticated, local administrator to run specific executables on the Nessus Agent host. This is different than CVE-2021-20117.
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Analysis
by VulDB Data Team • 09/12/2021
The vulnerability identified as CVE-2021-20118 represents a critical local privilege escalation flaw within the Nessus Agent software ecosystem. This vulnerability specifically affects Nessus Agent versions 8.3.0 and earlier, where authenticated local administrators can exploit a design flaw to execute arbitrary code with elevated privileges. The issue stems from improper privilege handling within the agent's execution framework, creating a pathway for malicious actors to bypass standard access controls and gain administrative capabilities on the host system. Unlike CVE-2021-20117 which addressed different aspects of the agent's security model, this vulnerability focuses specifically on the privilege escalation mechanisms that govern how the agent processes and executes commands. The flaw exists in the agent's local execution environment where it fails to properly validate or restrict the execution of specific executables, allowing an authenticated user to escalate their privileges through carefully crafted command sequences.
The technical implementation of this vulnerability involves the agent's handling of local command execution processes where it does not adequately enforce privilege boundaries between standard user sessions and administrative functions. When an authenticated local administrator attempts to execute certain system commands through the Nessus Agent interface, the software fails to properly validate the execution context or the privileges associated with the specific executables being invoked. This misconfiguration creates a condition where the agent's execution environment can be manipulated to run commands with elevated permissions, effectively allowing the local user to assume administrative control over the system. The vulnerability is particularly concerning because it operates within the trusted execution environment of the Nessus Agent, making it difficult to detect through standard network-based security measures. The flaw aligns with CWE-276, which addresses improper privileges, and represents a classic example of insufficient privilege checking in security-critical applications. This vulnerability specifically targets the agent's process execution model where it does not properly enforce the principle of least privilege, allowing for unauthorized privilege escalation through legitimate administrative interfaces.
The operational impact of CVE-2021-20118 extends beyond simple privilege escalation, as it provides attackers with a persistent foothold within the network infrastructure that hosts Nessus agents. Once successfully exploited, the vulnerability allows for complete system compromise, enabling attackers to install backdoors, exfiltrate sensitive data, or pivot to other systems within the network. The threat landscape is particularly concerning because Nessus Agents are commonly deployed in enterprise environments where they operate with elevated privileges to perform security scanning functions. This creates a scenario where a single compromised agent could provide attackers with a valuable entry point to escalate privileges across multiple systems. The vulnerability's exploitation requires local access and authentication, but once achieved, it provides a stable and persistent method for maintaining administrative control over the affected system. Organizations running vulnerable Nessus Agent versions face significant risk, as the flaw can be exploited by both insider threats and external attackers who have gained initial access to the system through other means.
Mitigation strategies for CVE-2021-20118 should prioritize immediate patching of all affected Nessus Agent installations to version 8.3.1 or later where the privilege escalation vulnerability has been addressed. Security administrators must implement comprehensive monitoring of local system access and execution patterns to detect potential exploitation attempts. The principle of least privilege should be enforced through proper access control configurations, ensuring that local administrators have only the minimum necessary permissions to perform their duties. Network segmentation and access controls should be implemented to limit lateral movement opportunities for attackers who might gain initial access through other vulnerabilities. Organizations should also conduct thorough security assessments of their Nessus Agent deployments to identify any additional misconfigurations that could compound the risk. The vulnerability demonstrates the importance of regular security updates and patch management processes, as well as the need for continuous monitoring of privileged access within security tools. From an ATT&CK framework perspective, this vulnerability maps to privilege escalation techniques and can be used to establish persistent access within target environments, making it a critical concern for organizations that rely heavily on Nessus for security operations and vulnerability management.