MQL Development Skill

Overview

This skill provides guidance for writing MQL (Mondoo Query Language) queries and validating them using either the cnspec CLI or Mondoo's MCP tools.

Two-tier knowledge system:

• Reference Files (static): MQL syntax docs, platform-specific examples
• Schema Tools (live): Real-time schema lookup and query validation via cnspec CLI or MCP

When to Use

• Writing MQL queries or policies
• Validating MQL syntax before deployment
• Exploring available MQL resources and fields
• Platform-specific query development (AWS, Azure, Linux, Windows, Microsoft 365)

Reference Materials

Located within this skill directory:

File	Purpose
mql-reference.md	Complete MQL syntax and patterns
samples/general.md	General MQL patterns
samples/aws.md	AWS resource patterns
samples/azure.md	Azure resource patterns
samples/linux.md	Linux system patterns
samples/windows.md	Windows system patterns
samples/ms365.md	Microsoft 365 patterns

Schema Discovery & Query Validation

Two equivalent interfaces are available for real-time schema lookup and query validation. Use whichever is available in your environment — they provide the same data.

cnspec CLI (recommended — works everywhere)

The cnspec CLI provides structured JSON output for all schema operations. No MCP server required.

List all providers

cnspec providers list --json

Returns an array of providers with name, version, and connectors:

[
  {"name": "aws", "version": "13.6.2", "connectors": ["aws"]},
  {"name": "os", "version": "13.8.1", "connectors": ["local", "ssh", "docker"]}
]

Get provider details (connectors and flags)

cnspec providers info aws --json
cnspec providers info aws azure --json   # multiple providers

Returns connector details including available flags for each connection type.

List resources in a provider

cnspec providers resources aws --json

Returns all resources with name, title, and field count:

{
  "provider": "aws",
  "total_resources": 111,
  "resources": [
    {"name": "aws.ec2.instance", "title": "Amazon EC2 Instance", "field_count": 52}
  ]
}

Get resource field details

cnspec providers resources aws aws.ec2.instance --json

Returns all fields with types and descriptions:

{
  "name": "aws.ec2.instance",
  "title": "Amazon EC2 Instance",
  "fields": [
    {"name": "arn", "type": "string", "title": "Amazon Resource Name"},
    {"name": "tags", "type": "map[string]string", "title": "Instance tags"}
  ]
}

Validate MQL queries

# Full compilation check — fails with exit 1 on invalid resources/fields
cnspec run local -c "asset.name" --ast

# Lexical parse only — checks syntax, NOT resource/field validity
cnspec run local -c "asset.name" --parse

Important: --parse accepts syntactically valid but semantically wrong queries (e.g., invalid.bogus.thing parses with exit 0). Use --ast to catch invalid resource or field names.

Execute queries

cnspec run local -c "users { name uid }" --json

Policy management

# Lint a policy bundle with structured SARIF output
cnspec policy lint policy.mql.yaml -o sarif

# Format a policy bundle to standard style (modifies file in place)
cnspec policy format policy.mql.yaml

# Sort and format a policy bundle
cnspec policy format policy.mql.yaml --sort

# Generate an example policy bundle scaffold
cnspec policy init example.mql.yaml

Mondoo MCP Server Tools (alternative)

If the Mondoo MCP server is available, you can use these tools instead of the CLI.

MCP Tool	CLI Equivalent
mcp__mondoo-mcp-http__mql-schema-providers	cnspec providers list --json
mcp__mondoo-mcp-http__mql-schema-overview	cnspec providers resources <provider> --json
mcp__mondoo-mcp-http__mql-schema-resource	cnspec providers resources <provider> <resource> --json
mcp__mondoo-mcp-http__mql-schema-suggestion	No CLI equivalent (use LSP)
mcp__mondoo-mcp-http__mql-compiler	cnspec run local -c "query" --ast
mcp__mondoo-mcp-http__mql-bundle-lint	cnspec policy lint file.mql.yaml -o sarif
mcp__mondoo-mcp-http__mql-bundle-format	cnspec policy format file.mql.yaml
mcp__mondoo-mcp-http__mql-policy-bundle	cnspec policy init file.mql.yaml

When to Use What

Need	Best Option
MQL syntax patterns	mql-reference.md
Platform-specific examples	samples/*.md
Resource availability check	cnspec providers resources <provider> --json
Field types and descriptions	cnspec providers resources <provider> <resource> --json
Query compilation validation	cnspec run local -c "query" --ast
Policy structure validation	cnspec policy lint file.mql.yaml -o sarif

Wiring Policies into Content Validation

cnspec policy lint proves a policy compiles. It does not prove a check reaches the verdict you claim, that an IaC variant ever matches an asset, or that a remediation snippet is well-formed and actually fixes the thing it documents. Those live in a separate suite.

When you are authoring or editing policies inside the cnspec repository, every one of those checks lives in content/validation/, and content/validation/README.md is the reference for all of it. Authoring rules are in content/CLAUDE.md.

make test/content              # lint + bundle scans + compliance mappings
make test/content/lint         # run this first, and fix it first
make test/content/iac          # IaC variant fixture suites
make test/content/iac/coverage # every IaC variant has pass+fail fixtures
make test/content/remediation  # remediation code-block linters
make test/content/commands     # remediation CLI and API validators

A new policy has to be registered, or nothing validates it. Every validator except the shell one and the compliance suites is allowlist-driven: it iterates a TARGETS dict or a policy slice. A bundle absent from those lists is not reported as unvalidated, it is never visited, so it merges with its variants untested and its remediation unlinted while CI stays green. Register the policy in the same change that adds it:

Add the policy to	When
content/README.md	always — the user-facing catalog
tfVariantPolicies in content/validation/scans/iac_variants_test.go	it has -terraform-hcl / -terraform-plan / -terraform-state / -cloudformation / -bicep variants
TARGETS in content/validation/remediation/code/<language>.py	it ships that language's remediation. Terraform also needs a PROVIDER_MAP entry per resource prefix
a registry under content/validation/remediation/commands/	it ships id: cli / id: api blocks, or audit: steps that invoke a CLI or REST call

Every IaC variant needs pass and fail fixtures under content/validation/scans/fixtures/iac-variants/<policy>/<variant-uid>/{pass,fail}/<scenario>/, and a coverage gate enforces it at 100%. A variant that asserts exactly what its own filters: require has no possible failing input; record that with a fail/IMPOSSIBLE.md explaining why, which is the only sanctioned way to ship without a fail fixture.

Scope a run to one check rather than running the whole suite — the trailing slash is what makes it work:

go test -tags iac_variants ./content/validation/scans \
  -run 'TestTerraformVariants/mondoo-aws-security/mondoo-aws-security-s3-bucket-encryption-terraform-hcl/'

Three outcomes are distinguished, and the third matters most: passed, failed, and skipped — the check never ran because no asset matched its filters:. A skipped check is a fixture bug, not a pass. It looks identical to a passing one in every report, forever.

A policy with IaC variants leaves its groups unfiltered. A group filter is evaluated before the check's own filter, so a group carrying filters: asset.platform == "<api-platform>" means a Terraform asset never reaches the variant underneath it and every fixture reports as skipped. Let each variant's own filters: select its asset instead, and convert the groups in the same change that adds the first variant.

MQL Quick Reference

Core Syntax

# Basic resource access
resource.property == value

# Filtering
resources.where(condition).all(assertion)

# Data blocks
resource {
  property1
  property2 == expected_value
}

# Variables
v = 23
value = null

# Regular expression matching (NOT =~)
string == /pattern/

# Empty checks
value == empty
value != empty

List Operations

# All entries must match
array.all(condition)

# At least one entry matches
array.contains(condition)

# No entries match
array.none(condition)

# Exactly one entry matches
array.one(condition)

# Filter entries
array.where(condition)

# Current item reference
array.where(_.contains("pattern"))

Common Patterns

# File permissions
file("/etc/passwd").permissions {
  user_readable == true
  user_writeable == true
  group_readable == true
  other_readable == true
}

# Service status
service("ssh").running == true
service("telnet").enabled == false

# Package check
package("nginx").installed == true

# Kernel parameters
kernel.parameters['net.ipv4.ip_forward'] == 0

# Platform detection
asset.platform == "ubuntu"
asset.family.contains("linux")

Anti-Patterns to Avoid

# Don't use =~ for regex
string =~ /pattern/      # Bad
string == /pattern/      # Good

# Don't use deprecated platform
platform == "ubuntu"          # Bad
asset.platform == "ubuntu"    # Good

# Don't nest .where() clauses
events.where(parameters.where(_['name'] == "NEW_VALUE"))  # Bad
events.where(parameters.any(_['name'] == "NEW_VALUE"))    # Good

# Always handle null values
users.all(shell == "/bin/bash")                     # Bad
users.where(shell != null).all(shell == "/bin/bash") # Good

Workflow

1. Understand requirements - What resources need to be checked?
2. Explore schema - Use cnspec providers resources <provider> --json
3. Check samples - Look for similar patterns in samples/*.md
4. Write query - Follow patterns from mql-reference.md
5. Validate - Use cnspec run local -c "query" --ast to verify syntax
6. Test - Run with cnspec run against target systems
7. Wire up validation - For policies in the cnspec repository, add fixtures for any IaC variant and register the policy with the validators that cover its remediation, in the same change. See Wiring Policies into Content Validation

Platform-Specific Guidance

AWS

• Use aws.* resources
• Check samples/aws.md for IAM, EC2, S3 patterns
• Explore: cnspec providers resources aws --json

Azure

• Use azure.subscription.* resources
• Check samples/azure.md for VM, storage, security patterns
• Both full subscription and single resource scan patterns

Linux

• Use file, service, package, users, kernel resources
• Check samples/linux.md for common patterns
• Handle platform variants (debian, redhat, etc.)

Windows

• Use registrykey, secpol, auditpol, windows resources
• Check samples/windows.md for registry and policy patterns
• Handle server vs workstation differences

Microsoft 365

• Use microsoft.* resources
• Check samples/ms365.md for domain patterns