Custodian policies for Infrastructure Code

This package allows cloud custodian to evaluate policies directly against infrastructure as code source assets.

It also provides a separate cli for better command line ux for source asset evaluation.

Install

We currently only support python > 3.10 on mac and linux, to run on windows we recommend using our docker images.

pip install c7n_left

We also provide signed docker images. These images are built on top of chainguard's wolfi linux distribution which is designed to be minimal, auditable, and secure.

docker pull cloudcustodian/c7n_left:dev

Images signatures can be verified using cosign

export IMAGE=$(docker image inspect cloudcustodian/c7n-left:dev -f '{{index .RepoDigests 0}}')
cosign verify $IMAGE \
   --certificate-identity 'https://github.com/cloud-custodian/cloud-custodian/.github/workflows/docker.yml@refs/heads/main' \
   --certificate-oidc-issuer 'https://token.actions.githubusercontent.com'

Usage

❯ c7n-left run --help

Usage: c7n-left run [OPTIONS]

  evaluate policies against IaC sources.

  c7n-left -p policy_dir -d terraform_root --filters "severity=HIGH"

  WARNING - CLI interface subject to change.

Options:
  --format TEXT
  --filters TEXT                  filter policies or resources as k=v pairs
                                  with globbing
  -p, --policy-dir PATH
  -d, --directory PATH
  -o, --output [cli|github|json]
  --output-file FILENAME
  --output-query TEXT
  --summary [policy|resource]
  --help                          Show this message and exit.

We'll create an empty directory with a policy in it

policies:
  - name: test
    resource: terraform.aws_s3_bucket
    metadata:
      severity: medium
    filters:
      - server_side_encryption_configuration: absent

And now we can use it to evaluate a terraform root module

❯ c7n-left run -p policies -d module
Running 1 policies on 1 resources
test - terraform.aws_s3_bucket
  Failed
  File: s3.tf:1-8
  1 resource "aws_s3_bucket" "example" {                                                                                
  2   bucket = "my-custodian-test-bucket"                                                                               
  3   acl    = "private"                                                                                                
  4                                                                                                                     
  5   tags = {                                                                                                          
  6     original-tag = "original-value"                                                                                 
  7   }                                                                                                                 
  8 }                                                                                                                   

Evaluation complete 0.00 seconds -> 1 Failures
           Summary - By Policy           
┏━━━━━━━━━━┳━━━━━━━━┳━━━━━━━━━━━━━━━━━━━┓
┃ Severity ┃ Policy ┃ Result            ┃
┡━━━━━━━━━━╇━━━━━━━━╇━━━━━━━━━━━━━━━━━━━┩
│ medium   │ test   │ 1 failed 0 passed │
└──────────┴────────┴───────────────────┘
0 compliant of 1 total, 1 resource has 1 policy violation

For running in docker, you'll need to use volume mounts to provide access to the policy directory and terraform root module.

docker run -ti --rm -v $(pwd)/policies:/policies -v $(pwd)/root-module:/module \
       cloudcustodian/c7n-left:dev run -p /policies -d /module

If the terraform root module has other remote module dependencies, you'll need to fetch those first using terraform before running c7n-left.

terraform get -update

CLI Filters

Which policies and which resources are evaluated can be controlled via command line via --filters option.

Available filters

• name - policy name
• category - policy category
• severity - minimum policy severity (unknown, low, medium, high, critical)
• type - resource type, ie. aws_security_group
• id - resource id ie. aws_vpc.example

Multiple values for a given filter can be specified as comma separate values, and all filters except severity support globbing.

Examples

# run all encryption policies on ebs volumes and sqs queues
c7n-left run -p policy_dir -d terraform --filters="category=encryption type=aws_ebs_volume,aws_sqs_queue"

# run all medium and higher level policies cost policies
c7n-left run -p policy_dir -d terraform --filters="severity=medium category=cost"

policy values for severity and category are specified in its metadata section. ie

policies:
  - name: check-encryption
    resource: [aws_ebs_volume, aws_sqs_queue]
    metadata:
      category: [encryption, security]
      severity: high
    filters:
       - kms_master_key_id: absent

Outputs

if your using this in github actions, we have special output mode for reporting annotations directly into pull requests with --output github

We also display a summary output after displaying resource matches, there are two summary displays available, the default policy summary, and a resource summary which can be enabled via --summary resource.

Policy Language

Policies for c7n-left support a few additional capabilities beyond what's common for custodian policies.

Policies can be specified against multiple resource types either as an array or glob.

policies:
  - name: check-encryption
    resource: [aws_ebs_volume, aws_sqs_queue]

A traverse filter is available that allows for multi-hop graph traversal from a resource to any related resource.

ie, here's a policy against an aws ec2 instance, that checks if any of the security groups attached to the instance, have a permission defined that allows access from 0.0.0.0/0

policies:
 - name: check-security-group-open-cidr
   resource: terraform.aws_instance
   description: "EC2 should not be open to world on ssh"
   filters:
     - type: traverse
       resources:
         - aws_security_group
         - aws_security_ingress_permission
       attrs:
         - Ipv4: 0.0.0.0/0

Policy Testing

c7n-left supports writing and running tests for policies.

To create a test for a policy, create a tests directory next to your policy files.

Within that tests directory, create a sub directory with the policy name.

Next add terraform files to this sub directory. Typically you would add both terraform files that would match the policy and those that should not.

Finally you add assertions in a left.plan[.yaml|.json] file. The format of the file is an array of dictionaries. The dictionaries are used to match against the policy findings. The data its matching against is what is found by using c7n-left run --output json. Each key/value pair in the dictionary is matched against the finding.

So putting it all together, we've setup our tests as follows

❯ tree policy-dir-a/
policy-dir-a/
├── alb.yaml
└── tests
    └── alb-deletion-protection-disabled
        ├── left.plan.yaml
        ├── negative1.tf
        └── positive1.tf

3 directories, 4 files

❯ cat policy-dir-a/alb.yaml 
policies:
  - name: alb-deletion-protection-disabled
    resource: [terraform.aws_lb, terraform.aws_alb]
    description: |
      Application Load Balancer should have deletion protection enabled
    metadata:
      severity: low
      category: "Insecure Configurations"
    filters:
      - enable_deletion_protection: empty

❯ cat policy-dir-a/tests/alb-deletion-protection-disabled/left.plan.yaml 
- "resource.__tfmeta.filename": "positive1.tf"

and now we can run a test

❯ c7n-left test -p policy-dir-a/
Discovered 1 Tests
Failure alb-deletion-protection-disabled [{'resource.__tfmeta.filename': 
'positive1.tf'}] checks not used

1 Test Complete (0.05s) 1 Failure

A test fails if either an assertion in the plan file does not match one policy finding, or if a policy finding is not matched by an assertion.