Ansible and EC2 Auto Scaling Groups: False-positive idempotency errors and a workaround

When using Ansible to deploy and manage EC2 auto scaling groups (ASGs) in AWS, you may encounter, like I have recently, an issue with idempotency errors that can be somewhat befuddling. Basically, when the ec2_asg module is called, one of its properties, vpc_zone_identifier, is used to define the subnets used by the ASG. A typical ASG configuration is to use two subnets, each one in a different availability zone, for a robust HA configuration, like so:

- name: "create auto scaling group"
  local_action:
    module: ec2_asg
    name: "{{ asg_name }}"
    desired_capacity: "{{ desired_capacity }}"
    launch_config_name: "{{ launch_config }}"
    min_size: 2
    max_size: 3
    desired_capacity: 2
    region: "{{ region }}"
    vpc_zone_identifier: "{{ subnet_ids }}"
    state: present

Upon subsequent Ansible plays, when ec2_asg is called, but no changes are made, you can still experience a changed=true result because of how Ansible is ordering the subnet-id’s used in vpc_zone_identifier versus how AWS is ordering them. This makes the play non-idempotent. How does this happen?

It turns out that Ansible’s ec2_asg module sorts the subnet-ids, while AWS does not when it returns those values. Here is the relevant code from the v2.3.0.0 version of ec2_asg.py, notice the sorting that happens in an attempt to match what AWS provides as an order:

 518 for attr in ASG_ATTRIBUTES:
 519     if module.params.get(attr, None) is not None:
 520         module_attr = module.params.get(attr)
 521         if attr == 'vpc_zone_identifier':
 522             module_attr = ','.join(module_attr)
 523         group_attr = getattr(as_group, attr)
 524         # we do this because AWS and the module may return the same list
 525         # sorted differently
 526         if attr != 'termination_policies':
 527             try:
 528                 module_attr.sort()
 529             except:
 530                 pass
 531             try:
 532                 group_attr.sort()
 533             except:
 534                 pass
 535         if group_attr != module_attr:
 536             changed = True
 537             setattr(as_group, attr, module_attr)
 538

While this is all well and good, AWS does not follow any specific ordering algorithm when it returns values for subnet-ids in the ASG context. So, when AWS returns its subnet-id list for the ec2_asg call, Ansible will sometimes have a different order in its ec2_asg configuration and then incorrectly interpret the difference between the two lists as a change and mark it thusly. If you are counting on your Ansible plays to be perfectly idempotent, this is problematic. There is now an open GitHub issue about this specific problem.

The good news is that the latest development version of ec2_asg, which is also written using boto3, does not exhibit this false-positive idempotency error issue. The devel version of ec2_asg (i.e., unreleased 2.4.0.0) is altogether different than what ships in current stable releases. So, these false-positive idempotency errors can occur in releases up to and including version 2.3.1.0 (I have found it in 2.2.1.0, 2.3.0.0, and 2.3.1.0).  Sometime soon, we should have a version of ec2_asg that behaves idempotently. But what to do until then?

One approach is to write a custom library in Python that you use instead of ec2_asg. While feasible, it would involve a lot of time spent verifying integration with both AWS and existing Ansible AWS modules.

Another approach, and one I took recently, is to simply ask AWS what it has for the order of subnet-ids to be in vpc_zone_identifier and then plug that ordering into what I pass to ec2_asg during each run.

Prior to running ec2_asg, I use the command module to run the AWSCLI autoscaling utility and query for the contents of VPCZoneIdentifier. Then I take those results and use them as the ordered list that I pass into ec2_asg afterward:

- name: "check for ASG subnet order due to idempotency failures with ec2_asg"
  command: 'aws autoscaling describe-auto-scaling-groups --region "{{ region }}" --auto-scaling-group-names "{{ asg_name }}" '
  register: describe_asg
  changed_when: false

- name: "parse the json input from aws describe-auto-scaling-groups"
  set_fact: asg="{{ describe_asg.stdout | from_json }}"

- name: "get vpc_zone_identifier and parse for subnet-id ordering"
  set_fact: asg_subnets="{{ asg.AutoScalingGroups[0].VPCZoneIdentifier.split(',') }}"
  when: asg.AutoScalingGroups

- name: "update subnet_ids on subsequent runs"
  set_fact: my_subnet_ids="{{ asg_subnets }}"
  when: asg.AutoScalingGroups

# now use the AWS-sorted list, my_subnet_ids, as the content of vpc_zone_identifier

- name: "create auto scaling group"
  local_action:
    module: ec2_asg
    name: "{{ asg_name }}"
    desired_capacity: "{{ desired_capacity }}"
    launch_config_name: "{{ launch_config }}"
    min_size: 2
    max_size: 3
    desired_capacity: 2
    region: "{{ region }}"
    vpc_zone_identifier: "{{ my_subnet_ids }}"
    state: present

On each run, the following happens:

  1. A command task runs the AWSCLI to describe the autoscaling group in question. If it’s the first run, an empty array is returned. The result is registered as asg_describe.
  2. The JSON data in asg_describe is copied into a new Ansible fact called “asg”
  3. The subnets in use by the ASG and how they are ordered is determined by extracting the VPCZoneIdentifier attribute from the AutoScalingGroup (asg fact). If it’s the first run, this step is skipped because of the when: clause which limits task execution to runs where the ASG already exists (runs 2 and later). It puts this list into the fact called “asg_subnets”
  4. Using the AWS-ordered list from step 3, Ansible sets a new fact called “my_subnet_ids”, which is then specified as the value to vpc_zone_identifier when ec2_asg is called.

I did a test on the idempotency of the play by running Ansible one hundred times after the ASG was created; at no point did I receive a false-positive change. Prior to this workaround, it would happen every run if I happened to be specifying subnet-ids ordered differently than from what AWS returned in terms of their order.

While this is admittedly somewhat kludgy, at least I can be confident that my plays involving AWS EC2 autoscaling groups will actually behave idempotently when they should. In the meantime, while we wait for the next update to Ansible’s ec2_asg module, this workaround can be used successfully to avoid false positive idempotency errors.

Until next time, have fun getting your Ansible on!

Stupid Boto3 Tricks – get_aws_region()

For some use cases, it’s not feasible to rely on an EC2 instance having any boto or AWS configuration information available (e.g., you are using an instance profile/role instead of API keys). This is a problem when it comes to establishing client sessions with services and you need to set the default region as an attribute to the boto3.setup_default_session() module.

Here’s one way to solve this problem via pulling the availability-zone element out of EC2 instance metadata, and then filtering that to drop the AZ portion (e.g., us-east-1b -> us-east-1).

First, import the urllib2 module into your code (Python 2.x):

import urllib2

Then, create a function like so that returns the AWS region name to the calling program:

def get_aws_region():

    # still no equivalent of boto.utils in boto3, so I have to do this janky thing...
    myAz = urllib2.urlopen('http://169.254.169.254/latest/meta-data/placement/availability-zone').read()
    myRegion = myAz[:-1]
    return myRegion

Quick & easy AMI generator

I have been meaning to put together a Lambda function to create an AMI from a custom EC2 instance.  It’s a pretty typical scenario, but I haven’t taken the time to roll my own. Recently, I ran across an article on StackOverflow which provides a CloudFormation template that:

  • constructs an EC2 image,
  • creates a Lambda execution role for AMI building,
  • creates a Lambda function for constructing an AMI, and
  • uses a custom resource to make an AMI from the instance via the Lambda function.

The Lambda function is written in the JavaScript SDK (node.js), is short and sweet, and easy to modify.

So, I modified both the template and Lambda function to make it a little more generic and reusable. Also, I fixed a logic error in a the original Lambda.  Finally, I wanted to customize the name of both the image and AMI, so I created an InstanceName parameter. The only other parameter for the CF template is InstanceType, which I defaulted to t2.micro. Add your desired instance types to the list in that parameter’s AllowedValues attribute. The base AMI for the instance is a region-specific Amazon Linux image. Once the stack is deployed, simply update the template with your userdata changes to create new custom AMI’s. It’s a very helpful tool to have in your CloudFormation toolbox.

The template is available from my aws-mojo repo on GitHub in both JSON and YAML formats.

Enjoy!

cfn-flip – CloudFormation format flipper

In a previous post, I talked about how CloudFormation now supports YAML for templates. The fine folks at AWS Labs have since released a Python package, cfn-flip, that you can install and use from a shell to convert a CF template from one format to the other: if you feed it JSON, it converts to YAML, and vice-versa.  It also works when used as a Python library.

Installing and using cfn-flip is this easy:

[rcrelia@seamus ~]$ pip install cfn-flip
Collecting cfn-flip
 Downloading cfn_flip-0.2.1.tar.gz
Requirement already satisfied: PyYAML in /usr/local/lib/python2.7/site-packages (from cfn-flip)
Requirement already satisfied: six in /usr/local/lib/python2.7/site-packages (from cfn-flip)
Building wheels for collected packages: cfn-flip
 Running setup.py bdist_wheel for cfn-flip ... done
 Stored in directory: /Users/rcrelia/Library/Caches/pip/wheels/1b/dd/d0/184e11860f8712a4a574980e129bd7cce2e6720b1c4386d633
Successfully built cfn-flip
Installing collected packages: cfn-flip
Successfully installed cfn-flip-0.2.1

[rcrelia@seamus ~]$ cat /tmp/foo.json | cfn-flip > /tmp/foo.yaml

 

S3crets

I recently read through Chris Craig’s AWS Security Blog post about limiting S3 bucket access based on specific IAM credentials/roles. There are two parts especially worth mentioning that can be an effective solution for many needs (e.g., distributing secret key values programmatically).

Explicit Deny in an S3 Bucket Policy

First, you construct a specific S3 bucket policy that can be used for controlling access via IAM user id’s (IAM user, IAM instance role & instance profile) as well as the AWS root account. In the policy below, note the explicit Deny statement at the end, which is how you lock down access except for those IAM entities. Make sure you include yourself or root (and you have root access) otherwise you will lock yourself out of the bucket you just created. Best to work with a temp IAM user for testing, fyi.

{
 "Version": "2012-10-17",
 "Statement": [
   {
     "Effect": "Allow",
     "Principal": {
       "AWS": "arn:aws:iam::012345678901:role/my-role"
     },
     "Action": "s3:ListBucket",
     "Resource": "arn:aws:s3:::my-bucket"
   },
   {
     "Effect": "Allow",
     "Principal": {
       "AWS": "arn:aws:iam::012345678901:role/my-role"
      },
     "Action": "s3:GetObject",
     "Resource": "arn:aws:s3:::my-bucket/*"
   },
 {
     "Effect": "Deny",
     "Principal": "*",
     "Action": "s3:*",
     "Resource": [
        "arn:aws:s3:::my-bucket",
        "arn:aws:s3:::my-bucket/*"
      ],
     "Condition": {
        "StringNotLike": {
           "aws:userId": [
              "AIDAIDEADBEEF01234567",
              "AROAJABCD1234EF560123:*",
              "AIPAIBEA2510CD3498765:*",
              "012345678901"
            ]
         }
      }
    }
  ]
}

Note: IAM objects in the Deny statement condition have tell-tale userId patterns as follows:

  • AIDAIDEADBEEF01234567″ – IAM user
  • AROAJABCD1234EF560123″ – IAM role (instance role in this case)
  • AIPAIBEA2510CD3498765″ – IAM instance profile
  • “012345678901” – AWS account number, or root

This policy essentially prohibits all access to “my-bucket” and associated keys, except for those IAM objects listed in the Conditional to the Deny * statement.

Explicit Allow in an IAM Policy

To make sure that the IAM entities you are not Denying access to the S3 bucket in question, you must craft a specific IAM policy. Then attach the policy to the IAM object(s) that require access. This policy is straightforward and is the second piece of this solution:

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

These two policies together form a powerful mechanism for creating a simple distribution point for secrets that you wish to use but not store locally in code or on an instance. A potential variation might include integration with KMS  to provide at-rest encryption as well as programmatic decryption/encryption of your secrets as you move them in and out of S3.

Security as op-ex savings

The journey to the cloud is compelling enough as is, what with its foundation of IaaS components and automation capabilities across all layers of your computing environment. It promotes the use of CI/CD methodologies and best-practices for configuration management. These characteristics impart a considerable savings in terms of operational costs (op-ex) that can run amok in premise deployments of infrastructure.

Still, in my experience, one of the most powerful arguments for using cloud services, and AWS in particular, is the value added by a hosting architecture that is secure by design. How many times have you seen a well-architected application or infrastructure suffer either functional or performance-related problems due to poor security design elements? In addition, the capital outlay (cap-ex) required for premise infrastructure is non-trivial if you want the same breadth and depth of security controls and auditability for compliance that AWS provides customers, again by design. This facet of AWS alone can substantially mitigate op-ex costs associated with running services in the cloud, which can vary dramatically depending on how you solve problems with your infrastructure.

AWS provides substantial documentation on cloud security, and one of the best places to start (or revisit) is the periodic publication “AWS Security Best Practices“, the current version of which can be found in the Developer Documents section of the main AWS cloud security resource collection. If you haven’t read this document yet or lately, below I have compiled some excerpts that touch on some common issues or concerns when deploying infrastructure in AWS. I highly recommend reading the Best Practices document at least a few times a year as the pace of innovation in AWS continues to grow faster each quarter.

In no particular order, here are some noteworthy best practice highlights from the August 2016 publication of the Best Practices guide that help illustrate the value and savings of using AWS viz the costs of traditional datacenter computing environments:

IP Spoofing – Amazon EC2 instances cannot send spoofed network traffic. The AWS-controlled, host-based firewall infrastructure will not permit an instance to send traffic with a source IP or MAC address other than its own.

Distributed Denial Of Service (DDoS) Attacks – AWS API endpoints are hosted on large, Internet-scale, world class infrastructure that benefits from the same engineering expertise that has built Amazon into the world’s largest online retailer. Proprietary DDoS mitigation techniques are used. Additionally, AWS’s networks are multihomed across a number of providers to achieve Internet access diversity

Packet sniffing by other tenants – It is not possible for a virtual instance running in promiscuous mode to receive or “sniff” traffic that is intended for a different virtual instance. While you can place your interfaces into promiscuous mode, the hypervisor will not deliver any traffic to them that is not addressed to them. Even two virtual instances that are owned by the same customer located on the same physical host cannot listen to each other’s traffic. Attacks such as ARP cache poisoning do not work within Amazon EC2 and Amazon VPC.

Secure Access Points – AWS has strategically placed a limited number of access points to the cloud to allow for a more comprehensive monitoring of inbound and outbound communications and network traffic. These customer access points are called API endpoints, and they allow secure HTTP access (HTTPS), which allows you to establish a secure communication session with your storage or compute instances within AWS. To support customers with FIPS cryptographic requirements, the SSL-terminating load balancers in AWS GovCloud (US) are FIPS 140-2-compliant. 

Several services also now offer more advanced cipher suites that use the Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) protocol. ECDHE allows SSL/TLS clients to provide Perfect Forward Secrecy, which uses session keys that are ephemeral and not stored anywhere. This helps prevent the decoding of captured data by unauthorized third parties, even if the secret long-term key itself is compromised.

Instance Isolation – Different instances running on the same physical machine are isolated from each other via the Xen hypervisor. Amazon is active in the Xen community, which provides awareness of the latest developments. In addition, the AWS firewall resides within the hypervisor layer, between the physical network interface and the instance’s virtual interface. All packets must pass through this layer, thus an instance’s neighbors have no more access to that instance than any other host on the Internet and can be treated as if they are on separate physical hosts. The physical RAM is separated using similar mechanisms. Customer instances have no access to raw disk devices, but instead are presented with virtualized disks. The AWS proprietary disk virtualization layer automatically resets every block of storage used by the customer, so that one customer’s data is never unintentionally exposed to another. In addition, memory allocated to guests is scrubbed (set to zero) by the hypervisor when it is unallocated to a guest. The memory is not returned to the pool of free memory available for new allocations until the memory scrubbing is complete.

Firewall (Security Groups) – Amazon EC2 provides a complete firewall solution; this mandatory inbound firewall is configured in a default deny-all mode and Amazon EC2 customers must explicitly open the ports needed to allow inbound traffic. The traffic may be restricted by protocol, by service port, as well as by source IP address (individual IP or Classless Inter-Domain Routing (CIDR) block).

The firewall isn’t controlled through the guest OS; rather it requires your X.509 certificate and key to authorize changes, thus adding an extra layer of security. AWS supports the ability to grant granular access to different administrative functions on the instances and the firewall, therefore enabling you to implement additional security through separation of duties. The level of security afforded by the firewall is a function of which ports you open, and for what duration and purpose. The default state is to deny all incoming traffic, and you should plan carefully what you will open when building and securing your applications. Well-informed traffic management and security design are still required on a per instance basis. AWS further encourages you to apply additional per-instance filters with host-based firewalls such as IPtables or the Windows Firewall and VPNs. This can restrict both inbound and outbound traffic.

Storage Device Decommissioning – When a storage device has reached the end of its useful life, AWS procedures include a decommissioning process that is designed to prevent customer data from being exposed to unauthorized individuals. AWS uses the techniques detailed in DoD 5220.22-M (“National Industrial Security Program Operating Manual “) or NIST 800-88 (“Guidelines for Media Sanitization”) to destroy data as part of the decommissioning process. All decommissioned magnetic storage devices are degaussed and physically destroyed in accordance with industry-standard practices.

Multi-factor Authentication – You can enable MFA devices for your AWS Account as well as for the users you have created under your AWS Account with AWS IAM. In addition, you add MFA protection for access across AWS Accounts, for when you want to allow a user you’ve created under one AWS Account to use an IAM role to access resources under another AWS Account. You can require the user to use MFA before assuming the role as an additional layer of security. 

You can also enforce MFA authentication for AWS service APIs in order to provide an extra layer of protection over powerful or privileged actions such as terminating Amazon EC2 instances or reading sensitive data stored in Amazon S3. You do this by adding an MFA-authentication requirement to an IAM access policy. You can attach these access policies to IAM users, IAM groups, or resources that support Access Control Lists (ACLs) like Amazon S3 buckets, SQS queues, and SNS topics.

AWS Trusted Advisor Security Checks – The AWS Trusted Advisor customer support service not only monitors for cloud performance and resiliency, but also cloud security. Trusted Advisor inspects your AWS environment and makes recommendations when opportunities may exist to save money, improve system performance, or close security gaps. It provides alerts on several of the most common security misconfigurations that can occur, including leaving certain ports open that make you vulnerable to hacking and unauthorized access, neglecting to create IAM accounts for your internal users, allowing public access to Amazon S3 buckets, not turning on user activity logging (AWS CloudTrail), or not using MFA on your root AWS Account.

Amazon Virtual Private Cloud (Amazon VPC) Security – Normally, each Amazon EC2 instance you launch is randomly assigned a public IP address in the Amazon EC2 address space. Amazon VPC enables you to create an isolated portion of the AWS cloud and launch Amazon EC2 instances that have private (RFC 1918) addresses in the range of your choice (e.g., 10.0.0.0/16). You can define subnets within your VPC, grouping similar kinds of instances based on IP address range, and then set up routing and security to control the flow of traffic in and out of the instances and subnets. AWS offers a variety of VPC architecture templates with configurations that provide varying levels of public access:

  • VPC with a single public subnet only. Your instances run in a private, isolated section of the AWS cloud with direct access to the Internet. Network ACLs and security groups can be used to provide strict control over inbound and outbound network traffic to your instances.
  • VPC with public and private subnets. In addition to containing a public subnet, this configuration adds a private subnet whose instances are not addressable from the Internet. Instances in the private subnet can establish outbound connections to the Internet via the public subnet using Network Address Translation (NAT).
  • VPC with public and private subnets and hardware VPN access. This configuration adds an IPsec VPN connection between your Amazon VPC and your data center, effectively extending your data center to the cloud while also providing direct access to the Internet for public subnet instances in your Amazon VPC. In this configuration, customers add a VPN appliance on their corporate datacenter side.
  • VPC with private subnet only and hardware VPN access. Your instances run in a private, isolated section of the AWS cloud with a private subnet whose instances are not addressable from the Internet. You can connect this private subnet to your corporate data center via an IPsec VPN tunnel.

Security features within Amazon VPC include security groups, network ACLs, routing tables, and external gateways. Each of these items is complementary to providing a secure, isolated network that can be extended through selective enabling of direct Internet access or private connectivity to another network.

AWS Identity and Access Management (AWS IAM) – AWS IAM allows you to create multiple users and manage the permissions for each of these users within your AWS Account. A user is an identity (within an AWS Account) with unique security credentials that can be used to access AWS Services. AWS IAM eliminates the need to share passwords or keys, and makes it easy to enable or disable a user’s access as appropriate. AWS IAM enables you to implement security best practices, such as least privilege, by granting unique credentials to every user within your AWS Account and only granting permission to access the AWS services and resources required for the users to perform their jobs. AWS IAM is secure by default; new users have no access to AWS until permissions are explicitly granted.

AWS CloudTrail Security – AWS CloudTrail provides a log of all requests for AWS resources within your account. For each event recorded, you can see what service was accessed, what action was performed, any parameters for the action, and who made the request. Not only can you see which one of your users or services performed an action on an AWS service, but you can see whether it was as the AWS root account user or an IAM user, or whether it was with temporary security credentials for a role or federated user. CloudTrail basically captures information about every API call to an AWS resource, whether that call was made from the AWS Management Console, CLI, or an SDK. If the API request returned an error, CloudTrail provides the description of the error, including messages for authorization failures. It even captures AWS Management Console sign-in events, creating a log record every time an AWS account owner, a federated user, or an IAM user simply signs into the console.

The Security Best Practices document contains many more descriptions and illustrations of AWS’s secure-by-design environment and services. Take some time over the holidays and review the document, with an eye towards op-ex savings in the coming new year. Security is not a luxury, and it definitely shouldn’t cost like one.

Safe travels on your journey to/in the cloud!

 

 

cfn_nag – a security linter for CloudFormation

It’s a little too easy to make non-secure configurations of resources in CloudFormation when you are focused on getting the entire stack to render correctly. By the time you are done building and testing a template, you must take extra time to revisit all your resources to make sure you are following good security and IaaS practices.

Enter cfn_nag, a handy little Ruby gem created by Stelligent that can help identify problems in your CloudFormation templates before you publish them. According to the README for the repo on Github, Stelligent says this about cfn_nag:

The cfn-nag tool looks for patterns in CloudFormation templates that may indicate insecure infrastructure. Roughly speaking it will look for:

  • IAM rules that are too permissive (wildcards)
  • Security group rules that are too permissive (wildcards)
  • Access logs that aren’t enabled
  • Encryption that isn’t enabled

Under the covers, cfn_nag is using jq to parse the JSON input files you provide to it for inspection. In my case, I simply installed jq first using homebrew:

[rcrelia@fuji vpc-scenario-2-reference (master=)]$ brew install jq
==> Installing dependencies for jq: oniguruma
==> Installing jq dependency: oniguruma
==> Downloading https://homebrew.bintray.com/bottles/oniguruma-6.0.0.yosemite.bottle.tar.gz
######################################################################## 100.0%
==> Pouring oniguruma-6.0.0.yosemite.bottle.tar.gz
🍺 /usr/local/Cellar/oniguruma/6.0.0: 16 files, 1.3M
==> Installing jq
==> Downloading https://homebrew.bintray.com/bottles/jq-1.5_1.yosemite.bottle.tar.gz
######################################################################## 100.0%
==> Pouring jq-1.5_1.yosemite.bottle.tar.gz
🍺 /usr/local/Cellar/jq/1.5_1: 18 files, 958.5K

Once I had jq, I installed cfn_nag:

[rcrelia@fuji vpc-scenario-2-reference (master=)]$ gem install cfn-nag
Fetching: trollop-2.1.2.gem (100%)
Successfully installed trollop-2.1.2
Fetching: multi_json-1.12.1.gem (100%)
Successfully installed multi_json-1.12.1
Fetching: little-plugger-1.1.4.gem (100%)
Successfully installed little-plugger-1.1.4
Fetching: logging-2.0.0.gem (100%)
Successfully installed logging-2.0.0
Fetching: cfn-nag-0.0.19.gem (100%)
Successfully installed cfn-nag-0.0.19
Parsing documentation for trollop-2.1.2
Installing ri documentation for trollop-2.1.2
Parsing documentation for multi_json-1.12.1
Installing ri documentation for multi_json-1.12.1
Parsing documentation for little-plugger-1.1.4
Installing ri documentation for little-plugger-1.1.4
Parsing documentation for logging-2.0.0
Installing ri documentation for logging-2.0.0
Parsing documentation for cfn-nag-0.0.19
Installing ri documentation for cfn-nag-0.0.19
Done installing documentation for trollop, multi_json, little-plugger, logging, cfn-nag after 1 seconds
5 gems installed

At this point, I had a working version of cfn_nag and immediately checked some recent templates. Here is output from running against one of my aws-mojo “Scenario 2” templates I recently posted about:

[rcrelia@fuji vpc-scenario-2-reference (master=)]$ cfn_nag --input-json-path ./aws-vpc-instance-securitygroups.json
------------------------------------------------------------
./aws-vpc-instance-securitygroups.json
------------------------------------------------------------------------------------------------------------------------
| WARN
|
| Resources: ["PubInstSGIngressHttp", "PubInstSGIngressHttps"]
|
| Security Group Standalone Ingress found with cidr open to world. This should never be true on instance. Permissible on ELB
------------------------------------------------------------
| WARN
|
| Resources: ["PrivInstSGEgressGlobalHttp", "PrivInstSGEgressGlobalHttps", "PubInstSGEgressGlobalHttp", "PubInstSGEgressGlobalHttps"]
|
| Security Group Standalone Egress found with cidr open to world.

Failures count: 0
Warnings count: 6

Pretty neat! In this case, these warnings are anticipated due to how I designed the VPC security groups to make use of network routing through NAT instances as well as the public NAT instances themselves being able to receive traffic globally in the public zones.

Obviously, you may want to consider adding your own cfn_nag rules to the stock set it ships with, to reflect your own specific security and configuration concerns.

To see a list of all the rules that come pre-configured in cfn_nag, simply run cfn_nag_rules:

[rcrelia@fuji vpc-scenario-2-reference (master=)]$ cfn_nag_rules

WARNING VIOLATIONS:
CloudFront Distribution should enable access logging
Elastic Load Balancer should have access logging configured
Elastic Load Balancer should have access logging enabled
IAM managed policy should not allow * resource
IAM managed policy should not allow Allow+NotAction
IAM managed policy should not allow Allow+NotResource
IAM policy should not allow * resource
IAM policy should not allow Allow+NotAction
IAM policy should not allow Allow+NotResource
IAM role should not allow * resource on its permissions policy
IAM role should not allow Allow+NotAction
IAM role should not allow Allow+NotAction on trust permissinos
IAM role should not allow Allow+NotResource
Lambda permission beside InvokeFunction might not be what you want? Not sure!?
S3 Bucket likely should not have a public read acl
S3 Bucket policy should not allow Allow+NotAction
SNS Topic policy should not allow Allow+NotAction
SQS Queue policy should not allow Allow+NotAction
Security Group Standalone Egress found with cidr open to world.
Security Group Standalone Ingress cidr found that is not /32
Security Group Standalone Ingress found with cidr open to world. This should never be true on instance. Permissible on ELB
Security Group egress with port range instead of just a single port
Security Group ingress with port range instead of just a single port
Security Groups found egress with port range instead of just a single port
Security Groups found ingress with port range instead of just a single port
Security Groups found with cidr open to world on egress
Security Groups found with cidr open to world on egress array
Security Groups found with cidr open to world on ingress array. This should never be true on instance. Permissible on ELB
Security Groups found with cidr open to world on ingress. This should never be true on instance. Permissible on ELB
Security Groups found with cidr that is not /32
Specifying credentials in the template itself is probably not the safest thing

FAILING VIOLATIONS:
A Cloudformation template must have at least 1 resource
AWS::EC2::SecurityGroup must have Properties
AWS::EC2::SecurityGroupEgress must have Properties
AWS::EC2::SecurityGroupEgress must not have GroupName - EC2 classic is a no-go!
AWS::EC2::SecurityGroupIngress must have Properties
AWS::EC2::SecurityGroupIngress must not have GroupName - EC2 classic is a no-go!
AWS::IAM::ManagedPolicy must have Properties
...snip...

There are two classes of notifications, warning violations and failing violations. There is good guidance in each set, but again, you may find that you want to edit/add your own rules to increase the value of cfn_nag for your infrastructure.