Edit an existing IAM Role trust policy using boto3

Question

I need to update/append IAM role trust policy with Deny statement using boto3. If i use update_assume_role_policy , it is overwriting the previous policy instead of appending the new changes. So i tried to read existing policy using get_role and then append my statement to it, but running into below challenges

1. If i do string replace - My code sample

    policy = '"Statement" : [ {"Sid": "Test","Effect": "Deny","Principal":{"AWS": "123456"},"Action": "*","Resource": "*"},{'        
            response = iam.get_role(RoleName= ResourceName)
            current_policy=str(response['Role']['AssumeRolePolicyDocument'])
            updated_policy = current_policy.replace('"Statement" : [ {', policy)

This works only if policy string matches '"Statement": [ {'. Its case sensitive and if the previous policy has single quote(') instead of double quote(") around Statement it doesn't work. I can use "re" module and write multiple conditions around it, but it adds too much complexity.

2. If i take policy as dictionary and append value, it adds a " " around my update and it policy looks like

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::123456:root"
      },
      "Action": "sts:AssumeRole",
      "Condition": {}
    },
    "{
      "Effect": "Deny",
      "Principal": {
        "AWS": "arn:aws:iam::890123:root"
      },
      "Action": "sts:AssumeRole",
      "Condition": {}
    }"
  ]
}

Is there a better,easier way to update IAM Role trust policy?

Answer 1

I'm not sure why do you require string operation for that. You can just replace the individual components of your trust policy.

For example:

import boto3

iam = boto3.client('iam')

response = iam.get_role(RoleName='<role-name>') 

trust_policy = response['Role']['AssumeRolePolicyDocument']

print(trust_policy)

# change effect to `Deny`
trust_policy['Statement'][0]['Effect'] = 'Deny'

# change principle to '123456'
trust_policy['Statement'][0]['Principal']['AWS'] = '123456'

print(trust_policy)

You can do same for other components.