NASL

NASL, short for Nessus Attack Scripting Language, is a domain-specific scripting language used to author checks for the Nessus vulnerability scanner. It enables security professionals to encode tests that determine whether a host or service is vulnerable, misconfigured, or non-compliant with a policy. NASL scripts are packaged as plugins and are loaded by Nessus during a scan, contributing to Nessus’s vulnerability database.

Plugins written in NASL are typically stored as .nasl files and describe metadata such as plugin name, family, description, and risk level. The scripts implement the logic to connect to targets, run service-specific tests, parse responses, and report results in a structured format that Nessus can display and correlate. This plugin architecture allows Nessus to expand its coverage as new vulnerabilities are discovered.

The language features a C-like syntax with functions, variables, conditionals, and loops, and it provides libraries for network I/O, file access, string processing, and common vulnerability checks. NASL supports operations such as sending network probes, parsing protocol responses, handling authentication where appropriate, and producing standardized output that can be interpreted by the Nessus user interface and reporting system.

History and context place NASL at the core of Nessus’s extensibility. It was developed to allow researchers and administrators to implement automated checks without altering the core scanner. Over time, a large ecosystem of NASL plugins has emerged, maintained by Tenable and the wider security community. Responsible use and appropriate authorization are essential in any vulnerability assessment.