Attack Chain

1. An attacker identifies vulnerable Tenable Nessus Agent instances (versions prior to 11.2.1 or 11.1.4) within a target network, possibly through network scanning or reconnaissance.
2. The attacker crafts and sends a specially malformed request or malicious input to the vulnerable Nessus Agent process.
3. The Nessus Agent, due to the CVE-2026-15265 vulnerability, improperly processes this input, leading to a bypass of internal security mechanisms.
4. This security bypass facilitates the injection and execution of arbitrary code with the privileges of the Nessus Agent.
5. Upon successful code execution, the attacker can leverage the compromised agent to establish persistence on the system.
6. The attacker can then perform actions such as installing additional malware, exfiltrating sensitive data, or pivoting to other systems within the compromised network.

Impact

A successful exploitation of CVE-2026-15265 would result in complete compromise of the system running the vulnerable Tenable Nessus Agent. This could lead to unauthorized remote code execution, giving attackers full control over the affected endpoint. The attacker could bypass existing security controls, disable security software, steal sensitive data, deploy ransomware, or use the compromised system as a pivot point for lateral movement within the network. All organizations using affected versions of Tenable Nessus Agent are potential targets, risking significant data breaches and operational disruption.

Recommendation

• Patch CVE-2026-15265 immediately by upgrading Tenable Nessus Agent to version 11.2.1 or later, or 11.1.4 or later, as recommended in the Tenable security bulletin tns-2026-18.
• Refer to the Tenable security bulletin https://www.tenable.com/security/tns-2026-18 for detailed patching instructions and affected product matrix.
• Monitor process_creation logs for any unusual process activity originating from the Nessus Agent process after patching, as a precautionary measure.