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Access Undenied on AWS

Access Undenied parses AWS AccessDenied CloudTrail events, explains the reasons for them, and offers actionable fixes.

Gif demonstrating an example of using AccessUndenied

Access Undenied

Overview

Access Undenied analyzes AWS CloudTrail AccessDenied events, scans the environment to identify and explain the reasons for them, and offers actionable least-privilege remediation suggestions.

Common use cases

Sometimes, the new and more detailed AccessDenied messages provided by AWS will be sufficient. However, that is not always the case.

Some AccessDenied messages do not provide details. Among the services with (many or exclusively) undetailed messages are: S3, SSO, EFS, EKS, GuardDuty, Batch, SQS, and many more.
When the reason for AccessDenied is an explicit deny, it can be difficult to track down and evaluate every relevant policy.
Specifically when the reason is an explicit deny in a service control policy (SCP), one has to find and every single policy in the organization that applies to the account.
When the problem is a missing Allow statement, AccessUndenied automatically offers a least-privilege policy based on the CloudTrail event.

Simple Startup

Install AccessUndenied:

pip install access-undenied-aws

Analyze a CloudTrail event file:

access-undenied-aws --file event_history.json

Installation

Installation from pip

python -m pip install access-undenied-aws

Installation from source code (development)

To install from source code, you can set up a venv (optionally), and within that venv.

python -m pip install --editable .

Usage

Getting events

Access Undenied works by analyzing a CloudTrail event where access was denied and the error code is either AccessDenied or Client.UnauthorizedOperation, it works on an input of one or more CloudTrail events. You can get them from wherever you get events, they can be found in the event history in the console, or by the LookupEvents API, or through whatever system you use in order to filter and detect events: Athena, Splunk, others. You can either download the records file (the default format for multiple events) or just copy and paste a single event. For examples of how to do this:

Getting CloudTrail events via the LookupEvents API with the CLI.
Getting Cloudtrail events from the AWS Console's event history.

Permissions

Access Undenied runs with the default permissions of the environment running the cli command, and accepts the --profile flag for using a different profile from .aws/credentials.

access-undenied-aws --profile my-profile analyze --events-file cloudtrail_events.json

(note that the location of the profile flag must be before the sub-command (which in this case is analyze).

The role running access-undenied-aws should be granted the appropriate permissions, to do so:

Attach the SecurityAudit managed policy.
If you would like to scan cross-account assets and analyze service control policies, attach the following inline policy. This policy allows AccessUndenied to assume roles in your other accounts:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AccessUndeniedAssumeRole",
      "Effect": "Allow",
      "Action": "sts:AssumeRole",
      "Resource": [
        "arn:aws:iam::<management_account_id>:role/AccessUndeniedRole",
        "arn:aws:iam::<account_1_id>:role/AccessUndeniedRole",
        "arn:aws:iam::<account_2_id>:role/AccessUndeniedRole",
        "..."
      ]
    }
  ]
}

If you do not wish to attach SecurityAudit, you may instead attach the updating least-privilege AccessUndenied policy.

Same account assets only, no SCPs

When both the resource and the principal are in the same account as the credentials used to run AccessUndenied and Service Control Policies (SCPs) do not need to be considered, it is sufficient to just run AccessUndenied with default credentials or a profile, and you do not need to set up any additional profiles.

Cross-account assets and SCPs

To consider assets in multiple accounts and/or SCPs in the management account, we need to set up AWS cross-account roles with the same policy and the same name as each other (the default is AccessUndeniedRole)

when setting up these roles, remember to set up the appropriate trust policy (trusting the credentials in the source account, the one you're running AccessUndenied in):

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::<source_account>:role/AccessUndeniedRole"
      },
      "Action": "sts:AssumeRole",
      "Condition": {}
    }
  ]
}

Attach SecurityAudit managed policy to the identity , or the updating least-privilege AccessUndenied policy

CLI Commands

Simplest command

access-undenied-aws analyze --events-file cloudtrail_events.json

All options:

Options:
  -v, --verbosity LVL  Either CRITICAL, ERROR, WARNING, INFO or DEBUG
  --profile TEXT       the AWS profile to use (default is default profile)
  --help               Show this message and exit.

Commands:
  analyze   Analyzes AWS CloudTrail events and explains the reasons for...
  get-scps  Writes the organization's SCPs and organizational tree to a file

Analyze

This command is used to analyze AccessDenied events. It can be used either with the management-account-role-arn parameter to retrieve SCPs, or with the scp-file parameter to use a policy data file created by the get_scps command.

Options:
  --events-file FILENAME          input file of CloudTrail events  [required]
  --scp-file TEXT                 Service control policy data file generated
                                  by the get_scps command.
  --management-account-role-arn TEXT
                                  a cross-account role in the management
                                  account of the organization, which must be
                                  assumable by your credentials.
  --cross-account-role-name TEXT  The name of the cross-account role for
                                  AccessUndenied to assume. default:
                                  AccessUndeniedRole
  --output-file TEXT              output file for results (default: no output
                                  to file)
  --suppress-output / --no-suppress-output
                                  should output to stdout be suppressed
                                  (default: not suppressed)
  --help                          Show this message and exit.

Example:

access-undenied-aws analyze --events-file events_file.json

Get SCPs

This command is used to writes the organization's SCPs and organizational tree to an organizational policy data file. This command should be run from the management account.

Options:
  --output-file TEXT  output file for scp data (default: scp_data.json)
  --help              Show this message and exit.

Example:

access-undenied-aws get-scps

Then when running analyzing (from the same account or a different account)

access-undenied-aws analyze --events-file events_file.json --scp-file scp_data.json

Output Format

{
  "EventId": "55555555-12ad-4f70-9140-d44428038119",
  "AssessmentResult": "Missing allow in an identity-based policy",
  "ResultDetails": {
    "PoliciesToAdd": [
      {
        "AttachmentTargetArn": "arn:aws:iam::123456789012:role/MyRole",
        "Policy": {
          "Version": "2012-10-17",
          "Statement": [
            {
              "Effect": "Allow",
              "Action": "rds:DescribeDBInstances",
              "Resource": "arn:aws:rds:ap-northeast-3:123456789012:db:*"
            }
          ]
        }
      }
    ]
  }
}

This output for example, tells us that access was denied because of there is no Allow statement in an identity-based policy. To remediate, we should attach to the IAM role arn:aws:iam::123456789012:role/MyRole the policy:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "rds:DescribeDBInstances",
      "Resource": "arn:aws:rds:ap-northeast-3:123456789012:db:*"
    }
  ]
}

Output Fields

AccessDeniedReason:

The reason why access was denied. Possible Values

Missing allow in:

Identity policy
Resource policy (in cross-account access)
Both (in cases of cross-account access)
Permissions boundary
Service control policy (with allow-list SCP strategy)

Explicit deny from:

Identity policy
Resource policy
Permissions boundary
Service control policy

Invalid action:

a principal or action that cannot be simulated by access undenied.

"Allowed" An "Allowed" result means that access undenied couldn't find the reason for AccessDenied, this could be for a variety of reasons:

Policies, resources and/or identities have changed since the CloudTrail event and access now actually allowed
Unsupported resource policy type
Unsupported policy type (VPC endpoint policy, session policy, etc.)
Unsupported condition key

ResultDetails

These are the details of the result, explaining the remediation steps, this section may contain either PoliciesToAdd or ExplicitDenyPolicies.

PoliciesToAdd

These are the policies which need to be added to enable least-privilege access. Each policy contains:

AttachmentTargetArn: the entity to which the new policy should be attached
Policy: The content of the policy to be added

ExplicitDenyPolicies

These are the policies cause explicit deny, which need to be removed or modified to facilitate access. AccessUndenied also gives the specific statement causing the Deny outcome.

AttachmentTargetArn: the entity to which the policy causing explicit deny is currently attached
PolicyArn: The arn (if applicable) of the policy causing explicit deny. For the sake of convenience, resource policies are represented by generic placeholder arns such as: arn:aws:s3:::my-bucket/S3BucketPolicy
PolicyName: The policy name, if applicable. Resource policies are represented by generic placeholder names such as S3BucketPolicy
PolicyStatement: The specific statement in the aforementioned policy causing explicit deny

Acknowledgements

This project makes use of Ian Mckay's iam-dataset Ben Kehoe's aws-error-utils.

Appendices

Running AccessUndenied from a Lambda function

Full README here

Setting up a venv

python -m venv .venv

Platform	Shell	Command to activate virtual environment
POSIX	bash/zsh	$ source .venv/bin/activate
	fish	$ source .venv/bin/activate.fish
	csh/tcsh	$ source .venv/bin/activate.csh
	PowerShell Core	$ .venv/bin/Activate.ps1
Windows	cmd.exe	C:> .venv\Scripts\activate.bat
	PowerShell	PS C:> .venv\Scripts\Activate.ps1

Getting CloudTrail events via the LookupEvents API with the CLI

This section is directly based on this AWS support page. It has been adapted so that the command outputs raw events rather than an ascii table.

Run the following AWS CLI command:

aws cloudtrail lookup-events --start-time "yyyy-mm-ddThh:mm:ss+0000" --end-time "yyyy-mm-ddThh:mm:ss+0000" \
  --query "Events[*].CloudTrailEvent" --output text | jq -r ". | \
  select(.userIdentity.arn == \"arn:aws:sts::123456789012:assumed-role/role-name/role-session-name\" \
  and .eventType == \"AwsApiCall\" and .errorCode != null \
  and (.errorCode | ascii_downcase | (contains(\"accessdenied\") or contains(\"unauthorized\"))))" | \
  jq -s '{Records:.}' > lookup_events_output.json

Note: The rate of lookup requests to CloudTrail is limited to one request per second per account. If you exceed this limit, then a throttling error occurs.

You can get errors for all users by removing this line:

.userIdentity.arn == \"arn:aws:sts::123456789012:assumed-role/role-name/role-session-name\" and

The command outputs errors to lookup_events_output.json, which can be analyzed by Access Undenied (using additional parameters as needed).

access-undenied-aws analyze --events-file lookup_events_output.json

Getting CloudTrail events from the AWS Console's event history

Open the AWS console
Go to "CloudTrail"
In the sidebar on the left, click Event History
Find the event you're interested in checking. Unfortunately, the console doesn't let you filter by ErrorCode, so you'll have to filter some other way, e.g. by username or event name.
Download the event:
1. By clicking the event, copying the event record, and pasting it to a json file locally. or,
2. By clicking download events -> download as JSON in the top-right corner. (Access Undenied will handle all events where the ErrorCode is AccessDenied or Client.UnauthorizedOperation)

With the event saved locally, you may use the cli command

Example Cloudtrail event

One event in file:

{
  "awsRegion": "us-east-2",
  "eventID": "5ac7912b-fd5d-436a-b60c-8a4ec1f61cdc",
  "eventName": "ListFunctions20150331",
  "eventSource": "lambda.amazonaws.com",
  "eventTime": "2021-09-09T14:01:22Z",
  "eventType": "AwsApiCall",
  "userIdentity": {
    "accessKeyId": "ASIARXXXXXXXXXXXXXXXX",
    "accountId": "123456789012",
    "arn": "arn:aws:sts::123456789012:assumed-role/RscScpDisallow/1631196079303620000",
    "principalId": "AROARXXXXXXXXXXXXXXXX:1631196079303620000",
    "sessionContext": {
      "attributes": {
        "creationDate": "2021-09-09T14:01:20Z",
        "mfaAuthenticated": "false"
      },
      "sessionIssuer": {
        "accountId": "123456789012",
        "arn": "arn:aws:iam::123456789012:role/RscScpDisallow",
        "principalId": "AROARXXXXXXXXXXXXXXXX",
        "type": "Role",
        "userName": "RscScpDisallow"
      },
      "webIdFederationData": {}
    },
    "type": "AssumedRole"
  },
  "errorCode": "AccessDenied",
  "errorMessage": "User: arn:aws:sts::123456789012:assumed-role/RscScpDisallow/1631196079303620000 is not authorized to perform: lambda:ListFunctions on resource: * with an explicit deny",
  "sourceIPAddress": "xxx.xxx.xxx.xxx",
  "readOnly": true,
  "eventVersion": "1.08",
  "userAgent": "aws-cli/2.2.16 Python/3.8.8 Linux/4.19.128-microsoft-standard exe/x86_64.ubuntu.20 prompt/off command/lambda.list-functions",
  "requestID": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxxx",
  "managementEvent": true,
  "recipientAccountId": "123456789012",
  "eventCategory": "Management"
}

Multiple events in file:

{
  "Records": [
    {
      "awsRegion": "us-east-1",
      "eventID": "xxxxxxxx-xxxx-xxxx-xxxx-8234c1555c12"
      //... rest of cloudtrail_event ...
    },
    {
      //... another cloudtrail_event ...
    }
    // more events...
  ]
}

Least privilege AccessUndenied policy

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AccessUndeniedLeastPrivilegePolicy",
      "Effect": "Allow",
      "Action": [
        "ecr:GetRepositoryPolicy",
        "iam:Get*",
        "iam:List*",
        "iam:SimulateCustomPolicy",
        "kms:GetKeyPolicy",
        "lambda:GetPolicy",
        "organizations:List*",
        "organizations:Describe*",
        "s3:GetBucketPolicy",
        "secretsmanager:GetResourcePolicy",
        "sts:DecodeAuthorizationMessage"
      ],
      "Resource": "*"
    }
  ]
}