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CloudFormation Template Linter Tool - Client-Side cfn-lint Subset for YAML and JSON Templates

First Published: 2026-04-19
Last Updated: 2026-04-19

Features

• Client-Side Processing: All linting runs locally in your browser; templates never touch a server.
• YAML Short-Form Tags: Full support for !Ref, !Sub, !GetAtt, !Join, !Select, !Split, !FindInMap, !If, !Equals, !And, !Or, !Not, !Base64, !Cidr, !ImportValue, !Transform, and more.
• Auto Format Detection: YAML and JSON are detected automatically; JSON falls back to YAML parsing if needed.
• Reference Integrity Checks: Validates that every !Ref, !GetAtt, !FindInMap, and !Sub variable points to a declared Parameter, Resource, Mapping, or pseudo parameter.
• IAM Wildcard Warnings: Flags Resource: "*" and Action: "*" with Effect: Allow in IAM policies.
• Unused Parameter / Condition Warnings: Detects declared-but-never-used entries.
• YAML ↔ JSON Conversion: Bundled converter expands short-form tags to Fn::* form when converting to JSON.
• Sample Presets: Good / Warnings / Errors samples load instantly for exploration.
• Debounced Auto-Lint: Linting runs automatically 400 ms after typing stops.
• Copyable Report: Copy all findings to the clipboard as plain text for pasting into PRs, tickets, or chat.
• Mobile Friendly: Responsive layout works on phones and tablets.
• Offline Capable: Once loaded, the tool continues to work without an internet connection.

How to Use

1. Paste a CloudFormation template into the textarea, drop a file onto the drop zone, or click one of the three sample preset buttons.
2. The format indicator shows whether YAML or JSON was detected.
3. Lint runs automatically after a short pause, or you can click Run Lint to trigger it immediately.
4. Review the findings table: each row lists Level, Rule ID, Path, and Message.
5. Use Copy Report to copy all findings as text, or YAML ↔ JSON to convert between formats.
6. Click Clear All to reset the tool.

Rules Implemented

• E1001: Template top-level structure. Requires a Resources section and allows only the standard top-level keys (AWSTemplateFormatVersion, Description, Metadata, Parameters, Mappings, Conditions, Resources, Outputs, Transform, Rules).
• E2001: !Ref to an undeclared Parameter.
• E3001: !Ref or !GetAtt to an undeclared Resource.
• E3002: Logical IDs (resource / parameter / mapping / condition / output names) must be alphanumeric (A-Z, a-z, 0-9).
• E6001: !FindInMap references a Mapping that is not defined.
• E1017: !If references a Condition that is not defined.
• E1019: !Sub uses ${Variable} that is not a Parameter, Resource, pseudo parameter, or local substitution variable.
• W1020: IAM policy uses Resource: "*" or Action: "*" with Effect: Allow (least-privilege warning).
• W2001: A Parameter is declared but never referenced.
• W3001: A Condition is declared but never referenced.

Important Notes

• This is not a full cfn-lint replacement. Resource property type validation against the CloudFormation Resource Specification is out of scope for this MVP.
• Parameter Default values are not used to evaluate intrinsic functions. Checks are purely structural.
• SAM (AWS::Serverless::*) resources are not expanded. Nested stacks and Custom Resources are linted as written.
• The YAML ↔ JSON conversion normalizes short-form tags to their Fn::* form.
• Templates larger than 5 MB are rejected for file drops. Very large templates may take longer to lint depending on your browser.
• Always run the official cfn-lint CLI and aws cloudformation validate-template before deploying to production.

References:
Tech Blog with curated related content
Web Tools Collection