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cfsec

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What is it?

cfsec scans your yaml or json CloudFormation configuration files for common security misconfigurations.

Installation

Home Brew - Mac and Linux

brew tap cfsec/cfsec

Chocolatey - Windows

choco install cfsec

Scoop - Windows

scoop install cfsec

Installing latest from source

go install github.com/aquasecurity/cmd/cfsec@latest

An Example

Given the CloudFormation configuration file below;

---
AWSTemplateFormatVersion: "2010-09-09"
Description: An example Stack for a bucket
Parameters:
  BucketName:
    Type: String
    Default: naughty-bucket
  EncryptBucket:
    Type: Boolean
    Default: false
Resources:
  S3Bucket:
    Type: 'AWS::S3::Bucket'
    Properties:
      BucketName:
        Ref: BucketName
      PublicAccessBlockConfiguration:
        BlockPublicAcls: false
        BlockPublicPolicy: false
        IgnorePublicAcls: true
        RestrictPublicBuckets: false
      BucketEncryption:
        ServerSideEncryptionConfiguration:
          - BucketKeyEnabled: !Ref EncryptBucket

Running the command cfsec example.yaml

The output would be

  Result 1

  [aws-s3-block-public-acls][HIGH] Public access block does not block public ACLs
  /home/owen/code/aquasecurity/cfsec/example/bucket.yaml:12-24
   11 |   S3Bucket:
   12 |     Type: 'AWS::S3::Bucket'
   13 |     Properties:
   14 |       BucketName:
   15 |         Ref: BucketName
   16 |       PublicAccessBlockConfiguration:
   17 |         BlockPublicAcls: false    [false]
   18 |         BlockPublicPolicy: false
   19 |         IgnorePublicAcls: true
   20 |         RestrictPublicBuckets: false
   21 |       BucketEncryption:
   22 |         ServerSideEncryptionConfiguration:
   23 |         - BucketKeyEnabled: !Ref EncryptBucket
   24 | 


  Impact:     PUT calls with public ACLs specified can make objects public
  Resolution: Enable blocking any PUT calls with a public ACL specified

  More Info:
  - https://cfsec.dev/docs/s3/block-public-acls/#s3 
  - https://docs.aws.amazon.com/AmazonS3/latest/userguide/access-control-block-public-access.html 

  Result 2

  [aws-s3-block-public-policy][HIGH] Public access block does not block public policies
  /home/owen/code/aquasecurity/cfsec/example/bucket.yaml:12-24
   11 |   S3Bucket:
   12 |     Type: 'AWS::S3::Bucket'
   13 |     Properties:
   14 |       BucketName:
   15 |         Ref: BucketName
   16 |       PublicAccessBlockConfiguration:
   17 |         BlockPublicAcls: false
   18 |         BlockPublicPolicy: false    [false]
   19 |         IgnorePublicAcls: true
   20 |         RestrictPublicBuckets: false
   21 |       BucketEncryption:
   22 |         ServerSideEncryptionConfiguration:
   23 |         - BucketKeyEnabled: !Ref EncryptBucket
   24 | 


  Impact:     Users could put a policy that allows public access
  Resolution: Prevent policies that allow public access being PUT

  More Info:
  - https://cfsec.dev/docs/s3/block-public-policy/#s3 
  - https://docs.aws.amazon.com/AmazonS3/latest/dev-retired/access-control-block-public-access.html 

  Result 3

  [aws-s3-enable-bucket-encryption][HIGH] Bucket does not have encryption enabled
  /home/owen/code/aquasecurity/cfsec/example/bucket.yaml:12-24
   11 |   S3Bucket:
   12 |     Type: 'AWS::S3::Bucket'
   13 |     Properties:
   14 |       BucketName:
   15 |         Ref: BucketName
   16 |       PublicAccessBlockConfiguration:
   17 |         BlockPublicAcls: false
   18 |         BlockPublicPolicy: false
   19 |         IgnorePublicAcls: true
   20 |         RestrictPublicBuckets: false
   21 |       BucketEncryption:
   22 |         ServerSideEncryptionConfiguration:
   23 |         - BucketKeyEnabled: !Ref EncryptBucket    [false]
   24 | 


  Impact:     The bucket objects could be read if compromised
  Resolution: Configure bucket encryption

  More Info:
  - https://cfsec.dev/docs/s3/enable-bucket-encryption/#s3 
  - https://docs.aws.amazon.com/AmazonS3/latest/userguide/bucket-encryption.html 

  Result 4

  [aws-s3-enable-bucket-logging][MEDIUM] Bucket does not have logging enabled
  /home/owen/code/aquasecurity/cfsec/example/bucket.yaml:12-24
   11 |   S3Bucket:
   12 |     Type: 'AWS::S3::Bucket'
   13 |     Properties:
   14 |       BucketName:
   15 |         Ref: BucketName
   16 |       PublicAccessBlockConfiguration:
   17 |         BlockPublicAcls: false
   18 |         BlockPublicPolicy: false
   19 |         IgnorePublicAcls: true
   20 |         RestrictPublicBuckets: false
   21 |       BucketEncryption:
   22 |         ServerSideEncryptionConfiguration:
   23 |         - BucketKeyEnabled: !Ref EncryptBucket
   24 | 


  Impact:     There is no way to determine the access to this bucket
  Resolution: Add a logging block to the resource to enable access logging

  More Info:
  - https://cfsec.dev/docs/s3/enable-bucket-logging/#s3 
  - https://docs.aws.amazon.com/AmazonS3/latest/dev/ServerLogs.html 

  Result 5

  [aws-s3-enable-versioning][MEDIUM] Bucket does not have versioning enabled
  /home/owen/code/aquasecurity/cfsec/example/bucket.yaml:12-24
   11 |   S3Bucket:
   12 |     Type: 'AWS::S3::Bucket'
   13 |     Properties:
   14 |       BucketName:
   15 |         Ref: BucketName
   16 |       PublicAccessBlockConfiguration:
   17 |         BlockPublicAcls: false
   18 |         BlockPublicPolicy: false
   19 |         IgnorePublicAcls: true
   20 |         RestrictPublicBuckets: false
   21 |       BucketEncryption:
   22 |         ServerSideEncryptionConfiguration:
   23 |         - BucketKeyEnabled: !Ref EncryptBucket
   24 | 


  Impact:     Deleted or modified data would not be recoverable
  Resolution: Enable versioning to protect against accidental/malicious removal or modification

  More Info:
  - https://cfsec.dev/docs/s3/enable-versioning/#s3 
  - https://docs.aws.amazon.com/AmazonS3/latest/userguide/Versioning.html 

  Result 6

  [aws-s3-no-public-buckets][HIGH] Public access block does not restrict public buckets
  /home/owen/code/aquasecurity/cfsec/example/bucket.yaml:12-24
   11 |   S3Bucket:
   12 |     Type: 'AWS::S3::Bucket'
   13 |     Properties:
   14 |       BucketName:
   15 |         Ref: BucketName
   16 |       PublicAccessBlockConfiguration:
   17 |         BlockPublicAcls: false
   18 |         BlockPublicPolicy: false
   19 |         IgnorePublicAcls: true
   20 |         RestrictPublicBuckets: false    [false]
   21 |       BucketEncryption:
   22 |         ServerSideEncryptionConfiguration:
   23 |         - BucketKeyEnabled: !Ref EncryptBucket
   24 | 


  Impact:     Public buckets can be accessed by anyone
  Resolution: Limit the access to public buckets to only the owner or AWS Services (eg; CloudFront)

  More Info:
  - https://cfsec.dev/docs/s3/no-public-buckets/#s3 
  - https://docs.aws.amazon.com/AmazonS3/latest/dev-retired/access-control-block-public-access.html 


  6 potential problems detected.

More Information

cfsec scans single file Stack configurations with support for Parameters, Mappings and Resources.

Ignoring Findings

Ignores are available in yaml configurations only.

To add an ignore to a resource - on the line of the check add the ignore.

For example, to ignore S3 bucket encryption errors, you might use

---
Resources:
  UnencrypedBucketWithIgnore:
    Type: AWS::S3::Bucket
    Properties:
      AccessControl: Private
      BucketName: unencryptedbits
      BucketEncryption:
        ServerSideEncryptionConfiguration:
          - BucketKeyEnabled: false # cfsec:ignore:aws-s3-enable-bucket-encryption
    

Supported Intrinsic functions

Not all CloudFormation intrinsic functions are supported, we cover the list below

Ref
Fn::Base64
Fn::Equals
Fn::FindInMap
Fn::GetAtt
Fn::Join
Fn::Select
Fn::Split
Fn::Sub

In yaml configurations, cfsec supports both standard an short notation i.e; !Base64 or Fn::Base64

Limitations

  • Not all intrinsic functions are supported
    • ImportValue requires access to the AWS account which is not currently supported
    • GetAtt is extremely naive. We don't have visibility of attribute values so it is best effort
  • No support for nested stacks. cfsec takes the individual files in isolation with no visibility of what exists in the AWS account

Comments, Suggestions, Issues

cfsec is very early stages, and we are committed to making it the best it can be. Please raise issues or suggestions through GitHub issues or discussion as appropriate.

cfsec is an Aqua Security open source project. Learn about our open source work and portfolio here. Join the community, and talk to us about any matter in GitHub Discussion or Slack.