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A more friendly CloudFormation JSON and YAML Validator

cfn-lint is a tool used to improve your CloudFormation development cycle. If you are here, you are probably fed up of waiting for 10 minutes or more before CloudFormation gives you an error due to a typo. This tool aims to remove that waste of time. The tool takes the CloudFormation template, and resolves all the Intrinsic functions defined before checking the Properties of Resources defined.

The tool can be used over the commandline using cfn-lint, or can be used as a module within another JavaScript application.

Note: This tool is currently case-sensitive in relation to AWS CloudFormation types, for example aws::lambda::function != AWS::Lambda::Function.

Depreciation/Retirement Notice

This tool was developed before AWS released cfn-python-lint, available at The AWS tool is better supported, provides additional checks and has best practice recommendations - if you are looking at implementing a linting tool into your workflow, use

Due to the superiority of the alternative tool, this project, cfn-lint has entered retirement. To support the existing community using this tool (both the CLI and API), the cfn-lint npm package will remain and will be supported in the following ways:

  • Minor bug fixes
  • Updates to SAM and CloudFormation Specification
  • Security updates

Additional features may be added at the discretion of the project maintainers, however, we strongly advise you use cfn-python-lint at instead of this project.


You can install with npm:

$ npm install -g cfn-lint

If you get /usr/bin/env: 'node': No such file or directory ensure your system has NodeJs installed. On Debian systems you may need to symlink node to nodejs due to namespacing issues (ln -s /usr/bin/nodejs /usr/bin/node).

How to use?

cfn-lint validate my_template.yaml

cfn-lint validate my_template.json

cfn-lint validate my_template.yaml --parameters key="my value",key2=value2,key3=3

cfn-lint validate my_template.yaml --parameters key="my value",key2=value2 --no-guess-parameters

cfn-lint validate my_template.yaml --pseudo AWS::StackName="My-Stack"

cfn-lint validate my_template.yaml --parameters key="my value" --pseudo AWS::Region=ap-northeast-1,AWS::AccountId=000000000000

cfn-lint docs AWS::Lambda::Function

cfn-lint docs AWS::Lambda::Function.Code

cfn-lint docs AWS::Lambda::Function.Code.S3Bucket

Note: The order of --parameters and --pseudo currently matters. This should be addressed in a later release.

Example Output

0 infos
0 warn
2 crit
Resource: Resources > MyInstance > Properties
Message: Required property ImageId missing for type AWS::EC2::Instance

Resource: Resources > MyInstance > Properties
Message: Required property Value missing for type Tag

Template invalid!


Input Parameters

--parameters <param values>: Provide a list of comma-separated key=value pairs of parameters to use when validating your template. If a parameter is not specified here, cfn-lint will guess a mock value based on the Parameter's Type and AllowedValues. e.g.--parameters InstanceType=t2.micro,Memory=512

AWS Pseudo Parameter Override

--pseudo <psuedo param values>: Provide a list of comma-separated key=value pairs of CloudFormation pseudo-parameters to use when validating your template. e.g.--pseudo AWS::Region=ap-southeast-2

Import Values

--import-values <param values>: Provide a list of comma-separated key=value pairs of CloudFormation import values to use when validating your template. e.g.--import-values MyDeployedVPC=vpc-1a2b3c4d

Parameter Guessing

--guess-parameters: Guess any parameters if they don't have any Default value in the template. Parameters will be guessed/mocked based on their AllowedValues or Type. This is the default behaviour; it's only included as an option for explicitness.

--no-guess-parameters: Disable the guessing of parameters if they don't have a Default. If you don't provide them on the CLI in this situation, a critical error will be raised instead of the parameter value being mocked.

--only-guess-parameters <param names>: Only guess the provided parameters, and disable the guessing of all others without Defaults. A critical error will be raised for missing parameters, as above. e.g. --only-guess-parameters InstanceType,Memory

User-defined Resource Attributes

--custom-resource-attributes <attribute values>: Provide a list of comma-separated key=value pairs of resource attributes using either resource-type notation (e.g. AWS::CloudFormation::CustomResource.SomeAttribute) or logical-name notation (e.g. SomethingPretty.SomeAttribute) and their expected values, to use when validating your template. If a custom resource attribute is not specified here, cfn-lint will guess a mock value. e.g.--custom-resource-attributes AWS::CloudFormation::CustomResource.SomeAttribute=[1\\,2],Custom::Dooby.SomeAttribute=[1\\,2],SomethingPretty.SomeAttribute=123

Verbose Output

--verbose: Provide verbose output and stack traces when template parsing fails.

** Note ** : Parameter values that contain commas must be escaped using a backslash (e.g. Param1=[1\,2\,3]). Depending on your command-line interpreter you may have to use double backslashes or enclose the entire argument in quotes, such is the case with Bash and other Bourne-based shells (e.g. --parameters 'Param1=[1\,2\,3]').

What can cfn-lint do?

  • Read JSON + YAML (Including YAML short form)
  • Detect invalid property names
  • Detect invalid Ref
  • Detect invalid mappings
  • Detect invalid format
  • Missing or invalid AWSTemplateFormatVersion
  • Fn::FindInMap
  • Fn::GetAtt
  • Fn::GetAZs
  • Fn::Join
  • Fn::Base64
  • Fn::Sub
  • Fn::If
  • Fn::Equals
  • Fn::Not
  • Fn::Or
  • Fn::And
  • Fn::ImportValue
  • Fn::Select
  • Fn::Split
  • Fn::ImportValue
  • Condition support
  • Ref
  • Go to the documentation from Command Line (see above examples)
  • Detecting invalid property types
  • Detect missing required properties
  • SAM template validation (experimental support)

Feature backlog

  • Verbose parsing errors when reading invalid JSON
  • Test coverage for Conditions and Property checking
  • Refactor Property checking to be clearer
  • Circular dependency checking
  • Extended validation flag (calls AWS CloudFormation API)
  • Watch file flag to revalidate on file save
  • Download latest resources during build

Features that would be nice, but aren't currently possible

  • Detect conditional required properties (Information doesn't exist in AWS Resource Specification)


cfn-lint can also be used as a Node library:

const cfnLint = require('cfn-lint')

The following methods are considered public:

cfnLint.validateFile(fileNamestring, options?: ValidationOptions)ValidationResult

Validates a file, and returns an ValidationResult with the results.

cfnLint.validateJsonObject(objectany, options?: ValidationOptions)ValidationResult

Validates an object, and returns an ValidationResult with the results. The object is what you might get from JSON.parseing a Cloudformation template.

interface ValidationOptions {
  parameters?: {
    Param1: Param1value,
    // ...
  pseudoParameters?: {
    'AWS::Region': 'ap-southeast-2',
    // ...
  guessParameters?: string[] | undefined // default undefined

parameters get passed into the template's Parameters before validation, and pseudoParameters are used to override AWS' pseudo-parameters, like AWS::Region, AWS::AccountId, etc.

If guessParameters is set to a list of parameter names, a critical error will be raised if any Parameter with no Default is not specified in the parameters or guessParameters options. An empty list can be used to enforce that all parameters must be specified in parameters. Leaving as undefined preserves the default loose behaviour, where parameters are guessed as needed without causing an error.

interface ErrorRecord {
  message: string,
  resource: string,
  documentation: string
interface ValidationResult {
  templateValid: boolean,
  errors: {
    crit: ErrorRecord[],
    warn: ErrorRecord[],
    info: ErrorRecord[]
  outputs: {
      [outputName: string]: string;
  exports: {
      [outputName: string]: string;

Represents the result of a validation.

Deploying your template

CloudFormation tends to involve a bit of trail and error. To enable quick development, the following method can be used to prevent the annoying 'ROLLBACK' scenarios where the whole stack must be deleted and recreated.

Deploy a template with the following content, name it what you want your final stack to be called.

    Type: AWS::S3::Bucket

After each change to your template, simply update the stack you just created. If the stack failed to deploy for some reason you can perform an 'Update Stack' operation, without needing to delete and recreate the stack from scratch. You can also use this method to keep parameters populated during the development phase. This method will work using the AWS Console or the CLI tools.


No errors were thrown for my template but it still failed when deploying! Is this tool pointless?!

Hopefully cfn-lint caught some errors before you deployed your template, however, catching every single error before deployment is extremely tricky. Please help out the community by explaining how you managed to get an error from CloudFormation of which the validator did not detect, the following information will help in adding a check to the utility:

  • The Resource/Statement which caused the error
  • The error thrown from CloudFormation
  • The working Resource/Statement which resolves the error

You can add an issue on the Github Issue Page. Thanks for helping out!

Doesn't the AWS API provide this functionality?

Partially. The AWS API cloudformation validate-template only checks the validity of the template, this includes the expected parameters for intrinsic functions, resource types and general structure of the JSON or YAML template it has been given. The AWS API does not detect incorrect resource property names or incorrect resource property types (e.g. string, bool, list) which are typically only detected when CloudFormation attempts to deploy the resources with the incorrect configurations.

How does cfn-lint know what is valid?

AWS provide a CloudFormation specification file for each region. cfn-lint uses the US East (N. Virginia) CloudFormationResourceSpecification.json file available on the AWS CloudFormation Resource Specification page. Currently, updates of this JSON file are manual and baked into each major, minor or bug fix release as needed.

Got any other questions?

Ask a question on the Github Issue Page!



This project is written in TypeScript, a superset of JavaScript which allows for type checking at compile time. The project currently aims to support Node 4 and above.

Unit Tests

Tests can be run using npm test. There should be a test case for every template error, for example an invalid parameter for Fn::Join or an incorrect reference for Ref. The tests depend on Mocha and chai.

IDE Setup

If you use a JetBrains editor, the following Run Configuration can be setup for the Mocha test suite:

Node Options: None Extra Mocha Options: None Test Directory: /lib/test

Local testing of CLI

Environmental Variables

  • DEBUG - Set to receive runtime debug messages
$ npm link
$ cfn-lint <some command>

Resource Attribute Mocking

In order to simulate the intrinsic functions to catch errors before deployment, attributes for Properties and Resources are mocked. This can be seen within the assignResourcesOutputs() and assignParameterOutputs() of validator.es6. A resource will always have a Ref of mock-ref-RESOURCENAME and will be attached other attributes which can be used with the Fn::GetAtt function. This allows for checking if attributes exist for a specific resource before deploying the template for real.

Intrinsic Functions

Each intrinsic function has a signature of doIntrinsicXYZ(ref, key) and is called by the resolveIntrinsicFunction(ref, key) function. The doIntrinsic.. function should return the resolved value of the function. For example, for Fn::Sub, an input of "My ${MyInstance}" would return a string similar to "My i-0a0a0a0a0a.

Parameterized types

Type parameterization is a technique primarily used in cfn-lint for supporting SAM validation as well as user-defined resource attributes, by allowing a certain resource type or property type to have it's specification enhanced/overridden by another registered type, dynamically.

Prime examples for a parameterized resource type, property type and even a mixture of both would respectively be: AWS::S3::Bucket<somethingCool>, AWS::S3::Bucket.BucketEncryption<somethingAwesome> and AWS::S3::Bucket<somethingCool>.BucketEncryption<somethingAwesome>; where somethingCool and somethingAwesome can be any registered resource (e.g. AWS::CodeBuild::Project) or property type (e.g. AWS::CodeBuild::Project.Artifacts).

In regards to user-defined resource attributes that are applicable to a given resource based on its' logical ID, the parameter's value would be the logical ID (e.g. AWS::S3::Bucket<somethingCool>, where somethingCool is the logical ID that's present in the specific template), which has been previously registered as a type with it's respective specification.

Apart from the above mentioned, parameterization is also used for specifying the item type of an aggregate type (i.e. List or Map) and usually occurs within the Type attribute of property specifications when a property may take multiple forms such as in SAM templates.

SAM validation process

Due to the fact that SAM templates support various types or even a mixture of types for a given template property value and that cfn-lint's validation algorithm is designed for the CFN specification that does not, therefore, in order to attain compatibility between the different formats, a property type inference procedure was implemented, namely applySAMPropertyOverrides, that scans the template, attempts to determine the actual type of a given SAM property based on it's value type or shape in template, if applicable and if successful it then registers a logical name override for the resource in question with the disambiguated specification.

In addition, SAM templates also require a transformation process, entailing new resources to be generated according to the node-tree matching patterns defined in the data/sam_20161031_output_resources.json file and placed in a distinct section, namely SAMOutput, by the doSAMTransform procedure so that they can later be utilized within intrinsics.

cfn-lint supports SAM template validation by extending the current CFN specification with additional SAM specific resources that are defined in the data/sam_20161031_cfn.json file (i.e. applySpecificationOverrides), optionally enhancing the definitions of specific entries that reside under the Resources section of a to-be-validated template with definitions extracted from the Globals section of the aforementioned template (i.e. processSAMGlobals) and finally, before the actual template resource validation takes place, the template is recursively scanned and specific resources are generated under the SAMOutput section (i.e. doSAMTransform), in a two phase approach, one that occurs before values are assigned to parameters and resource properties/attributes as well as intrinsic resolution and another one after the aforementioned so that the inference process can disambiguate between an arbitrary number of SAM types that define a specific template resource property.

Known limitations:

  • Primitive types Integer, Long and Double are indistinguishable;
  • Primitive types String and Timestamp are indistinguishable;
  • SAM policy templates are not implemented;

Release Instructions

  1. For each PR, edit the file, adding a fixed, changed or added message to the Unreleased block.
  2. Create a milestone in Github for all issues and PRs which will be included in the release, for example, v1.7.0.
  3. Create a PR for the release (branch release-VERSION), moving the contents of the to a new version, update the links at the bottom of the file.
  4. Ensure the latest CFN Specification is present (AWS CloudFormation Resource Specification) in the data/aws_resources_specification.json file, update if required.
  5. Ensure the latest SAM Specification is present (SAM Schema) in the data/sam_20161031_schema.json file, update if required.
  6. Update the CFN compatible SAM specification file, namely data/sam_20161031_cfn.json, by executing:
cd lib && node sam2CFN.js && cd ..
  1. Verify changes and merge into master with the title Update CHANGELOG for VERSION (#PRNUM).
  2. Go to Releases and click "Draft a new release".
  3. Enter the tag of the version and the same for the release title, for example, v1.7.0.
  4. Copy the section of the file for the release into the describe section.
  5. Click publish release. This will build a publish a package to npm.
  6. The unit tests can generally be trusted, however, a quick test will do no harm and might catch something missed. Next, test the release in a clean environment, ensuring the latest version is pulled.
> docker run -it node /bin/bash
$ npm install -g cfn-lint
$ cat <<EOF >working.yaml
     Type: AWS::S3::Bucket
$ cfn-lint validate working.yaml
$ cat <<EOF >failure.yaml
     Type: AWS::S3::Buckets
$ cfn-lint validate failure.yaml


Please check out the Contributor's Guide for more information on how to get started.


This document is available in:

If you wish to translate this document to another language, we welcome your contribution!

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