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Cloudsplaining

Cloudsplaining is an AWS IAM Security Assessment tool that identifies violations of least privilege and generates a risk-prioritized HTML report.

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Documentation

For full documentation, please visit the project on ReadTheDocs.

Overview

Cloudsplaining identifies violations of least privilege in AWS IAM policies and generates a pretty HTML report with a triage worksheet. It can scan all the policies in your AWS account or it can scan a single policy file.

It helps to identify IAM actions that do not leverage resource constraints. It also helps prioritize the remediation process by flagging IAM policies that present the following risks to the AWS account in question without restriction:

Cloudsplaining also identifies IAM Roles that can be assumed by AWS Compute Services (such as EC2, ECS, EKS, or Lambda), as they can present greater risk than user-defined roles - especially if the AWS Compute service is on an instance that is directly or indirectly exposed to the internet. Flagging these roles is particularly useful to penetration testers (or attackers) under certain scenarios. For example, if an attacker obtains privileges to execute ssm:SendCommand and there are privileged EC2 instances with the SSM agent installed, they can effectively have the privileges of those EC2 instances. Remote Code Execution via AWS Systems Manager Agent was already a known escalation/exploitation path, but Cloudsplaining can make the process of identifying theses cases easier. See the sample report for some examples.

You can also specify a custom exclusions file to filter out results that are False Positives for various reasons. For example, User Policies are permissive by design, whereas System roles are generally more restrictive. You might also have exclusions that are specific to your organization's multi-account strategy or AWS application architecture.

Motivation

Policy Sentry revealed to us that it is possible to finally write IAM policies according to least privilege in a scalable manner. Before Policy Sentry was released, it was too easy to find IAM policy documents that lacked resource constraints. Consider the policy below, which allows the IAM principal (a role or user) to run s3:PutObject on any S3 bucket in the AWS account:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:PutObject"
      ],
      "Resource": "*"
    }
  ]
}

This is bad. Ideally, access should be restricted according to resource ARNs, like so:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": [
        "s3:GetObject"
      ],
      "Resource": "arn:aws:s3:::my-bucket/*"
    }
  ]
}

Policy Sentry makes it really easy to do this. Once Infrastructure as Code developers or AWS Administrators gain familiarity with the tool (which is quite easy to use), we've found that adoption starts very quickly. However, if you've been using AWS, there is probably a very large backlog of IAM policies that could use an uplift. If you have hundreds of AWS accounts with dozens of policies in each, how can we lock down those AWS accounts by programmatically identifying the policies that should be fixed?

That's why we wrote Cloudsplaining.

Cloudsplaining identifies violations of least privilege in AWS IAM policies and generates a pretty HTML report with a triage worksheet. It can scan all the policies in your AWS account or it can scan a single policy file.

Installation

Homebrew

brew tap salesforce/cloudsplaining https://github.com/salesforce/cloudsplaining
brew install cloudsplaining

Pip3

pip3 install --user cloudsplaining

Shell completion

To enable Bash completion, put this in your .bashrc:

eval "$(_CLOUDSPLAINING_COMPLETE=bash_source cloudsplaining)"

To enable ZSH completion, put this in your .zshrc:

eval "$(_CLOUDSPLAINING_COMPLETE=zsh_source cloudsplaining)"

Scanning a single IAM policy

You can also scan a single policy file to identify risks instead of an entire account.

cloudsplaining scan-policy-file --input-file examples/policies/explicit-actions.json

The output will include a finding description and a list of the IAM actions that do not leverage resource constraints.

The output will resemble the following:

Issue found: Data Exfiltration
Actions: s3:GetObject

Issue found: Resource Exposure
Actions: ecr:DeleteRepositoryPolicy, ecr:SetRepositoryPolicy, s3:BypassGovernanceRetention, s3:DeleteAccessPointPolicy, s3:DeleteBucketPolicy, s3:ObjectOwnerOverrideToBucketOwner, s3:PutAccessPointPolicy, s3:PutAccountPublicAccessBlock, s3:PutBucketAcl, s3:PutBucketPolicy, s3:PutBucketPublicAccessBlock, s3:PutObjectAcl, s3:PutObjectVersionAcl

Issue found: Unrestricted Infrastructure Modification
Actions: ecr:BatchDeleteImage, ecr:CompleteLayerUpload, ecr:CreateRepository, ecr:DeleteLifecyclePolicy, ecr:DeleteRepository, ecr:DeleteRepositoryPolicy, ecr:InitiateLayerUpload, ecr:PutImage, ecr:PutImageScanningConfiguration, ecr:PutImageTagMutability, ecr:PutLifecyclePolicy, ecr:SetRepositoryPolicy, ecr:StartImageScan, ecr:StartLifecyclePolicyPreview, ecr:TagResource, ecr:UntagResource, ecr:UploadLayerPart, s3:AbortMultipartUpload, s3:BypassGovernanceRetention, s3:CreateAccessPoint, s3:CreateBucket, s3:DeleteAccessPoint, s3:DeleteAccessPointPolicy, s3:DeleteBucket, s3:DeleteBucketPolicy, s3:DeleteBucketWebsite, s3:DeleteObject, s3:DeleteObjectTagging, s3:DeleteObjectVersion, s3:DeleteObjectVersionTagging, s3:GetObject, s3:ObjectOwnerOverrideToBucketOwner, s3:PutAccelerateConfiguration, s3:PutAccessPointPolicy, s3:PutAnalyticsConfiguration, s3:PutBucketAcl, s3:PutBucketCORS, s3:PutBucketLogging, s3:PutBucketNotification, s3:PutBucketObjectLockConfiguration, s3:PutBucketPolicy, s3:PutBucketPublicAccessBlock, s3:PutBucketRequestPayment, s3:PutBucketTagging, s3:PutBucketVersioning, s3:PutBucketWebsite, s3:PutEncryptionConfiguration, s3:PutInventoryConfiguration, s3:PutLifecycleConfiguration, s3:PutMetricsConfiguration, s3:PutObject, s3:PutObjectAcl, s3:PutObjectLegalHold, s3:PutObjectRetention, s3:PutObjectTagging, s3:PutObjectVersionAcl, s3:PutObjectVersionTagging, s3:PutReplicationConfiguration, s3:ReplicateDelete, s3:ReplicateObject, s3:ReplicateTags, s3:RestoreObject, s3:UpdateJobPriority, s3:UpdateJobStatus

Scanning an entire AWS Account

Downloading Account Authorization Details

We can scan an entire AWS account and generate reports. To do this, we leverage the AWS IAM get-account-authorization-details API call, which downloads a large JSON file (around 100KB per account) that contains all of the IAM details for the account. This includes data on users, groups, roles, customer-managed policies, and AWS-managed policies.

cloudsplaining download

cloudsplaining download --profile myprofile

It will download a JSON file in your current directory that contains your account authorization detail information.

Create Exclusions file

Cloudsplaining tool does not attempt to understand the context behind everything in your AWS account. It's possible to understand the context behind some of these things programmatically - whether the policy is applied to an instance profile, whether the policy is attached, whether inline IAM policies are in use, and whether or not AWS Managed Policies are in use. Only you know the context behind the design of your AWS infrastructure and the IAM strategy.

As such, it's important to eliminate False Positives that are context-dependent. You can do this with an exclusions file. We've included a command that will generate an exclusions file for you so you don't have to remember the required format.

You can create an exclusions template via the following command:

cloudsplaining create-exclusions-file

This will generate a file in your current directory titled exclusions.yml.

Now when you run the scan command, you can use the exclusions file like this:

cloudsplaining scan --exclusions-file exclusions.yml --input-file examples/files/example.json --output examples/files/

For more information on the structure of the exclusions file, see Filtering False Positives

Scanning the Authorization Details file

Now that we've downloaded the account authorization file, we can scan all of the AWS IAM policies with cloudsplaining.

Run the following command:

cloudsplaining scan --exclusions-file exclusions.yml --input-file examples/files/example.json --output examples/files/

It will create an HTML report like this:

It will also create a raw JSON data file:

{
    "example-authz-details": [
        {
            "AccountID": "012345678901",
            "ManagedBy": "Customer",
            "PolicyName": "InsecureUserPolicy",
            "Arn": "arn:aws:iam::012345678901:user/userwithlotsofpermissions",
            "ActionsCount": 2,
            "ServicesCount": 1,
            "Actions": [
                "s3:PutObject",
                "s3:PutObjectAcl"
            ],
            "Services": [
                "s3"
            ]
        }
    ]
}

See the examples/files folder for sample output.

Filtering False Positives

Resource constraints are best practice - especially for system roles/instance profiles - but sometimes, these are by design. For example, consider a situation where a custom IAM policy is used on an instance profile for an EC2 instance that provisions Terraform. In this case, broad permissions are design requirements - so we don't want to include these in the results.

You can create an exclusions template via the following command:

cloudsplaining create-exclusions-file

This will generate a file in your current directory titled exclusions.yml.

The default exclusions file looks like this:

# Policy names to exclude from evaluation
# Suggestion: Add policies here that are known to be overly permissive by design, after you run the initial report.
policies:
  - "AWSServiceRoleFor*"
  - "*ServiceRolePolicy"
  - "*ServiceLinkedRolePolicy"
  - "AdministratorAccess" # Otherwise, this will take a long time
  - "service-role*"
  - "aws-service-role*"
# Don't evaluate these roles, users, or groups as part of the evaluation
roles:
  - "service-role*"
  - "aws-service-role*"
users:
  - ""
groups:
  - ""
# Read-only actions to include in the results, such as s3:GetObject
# By default, it includes Actions that could lead to Data Exfiltration
include-actions:
  - "s3:GetObject"
  - "ssm:GetParameter"
  - "ssm:GetParameters"
  - "ssm:GetParametersByPath"
  - "secretsmanager:GetSecretValue"
# Write actions to include from the results, such as kms:Decrypt
exclude-actions:
  - ""

Now when you run the scan command, you can use the exclusions file like this:

cloudsplaining scan --exclusions-file exclusions.yml --input-file examples/files/example.json --output examples/files/

Scanning Multiple AWS Accounts

If your IAM user or IAM role has sts:AssumeRole permissions to a common IAM role across multiple AWS accounts, you can use the scan-multi-account command.

This diagram depicts how the process works:

Diagram for scanning multiple AWS accounts with Cloudsplaining

Note: If you are new to setting up cross-account access, check out the official AWS Tutorial on Delegating access across AWS accounts using IAM roles. That can help you set up the architecture above.

cloudsplaining create-multi-account-config-file \ 
    -o multi-account-config.yml

accounts:
  default_account: 123456789012
  prod: 123456789013
  test: 123456789014

Note: Observe how the format of the file above includes account_name: accountID. Edit the file contents to match your desired account name and account ID. Include as many account IDs as you like.

For the next step, let's say that:

Using the data above, you can run the following command:

cloudsplaining scan-multi-account \
    -c multi-account-config.yml \
    --profile scanning-user \
    --role-name CommonSecurityRole \ 
    --output-bucket my-results-bucket

Note that if you run the above without the --profile flag, it will execute in the standard AWS Credentials order of precedence (i.e., Environment variables, credentials profiles, ECS container credentials, then finally EC2 Instance Profile credentials).

Cheatsheet

# Download authorization details
cloudsplaining download
# Download from a specific AWS profile
cloudsplaining download --profile someprofile

# Scan Authorization details
cloudsplaining scan --input-file default.json
# Scan Authorization details with custom exclusions
cloudsplaining scan --input-file default.json --exclusions-file exclusions.yml

# Scan Policy Files
cloudsplaining scan-policy-file --input-file examples/policies/wildcards.json
cloudsplaining scan-policy-file --input-file examples/policies/wildcards.json  --exclusions-file examples/example-exclusions.yml

# Scan Multiple Accounts
# Generate the multi account config file
cloudsplaining create-multi-account-config-file -o accounts.yml
cloudsplaining scan-multi-account -c accounts.yml -r TargetRole --output-directory ./

FAQ

Will it scan all policies by default?

No, it will only scan policies that are attached to IAM principals.

Will the download command download all policy versions?

Not by default. If you want to do this, specify the --include-non-default-policy-versions flag. Note that the scan tool does not currently operate on non-default versions.

I followed the installation instructions but can't execute the program via command line at all. What do I do?

This is likely an issue with your PATH. Your PATH environment variable is not considering the binary packages installed by pip3. On a Mac, you can likely fix this by entering the command below, depending on the versions you have installed. YMMV.

export PATH=$HOME/Library/Python/3.7/bin/:$PATH

I followed the installation instructions, but I am receiving a ModuleNotFoundError that says No module named policy_sentry.analysis.expand. What should I do?

Try upgrading to the latest version of Cloudsplaining. This error was fixed in version 0.0.10.

References