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https://github.com/tenable/access-undenied-aws

Access Undenied parses AWS AccessDenied CloudTrail events, explains the reasons for them, and offers actionable remediation steps. Open-sourced by Ermetic.
https://github.com/tenable/access-undenied-aws

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Access Undenied parses AWS AccessDenied CloudTrail events, explains the reasons for them, and offers actionable remediation steps. Open-sourced by Ermetic.

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# Access Undenied on AWS



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



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![Gif demonstrating an example of using AccessUndenied](examples/example.gif)



- [Access Undenied](#access-undenied)

  - [Overview](#overview)

    - [Common use cases](#common-use-cases)

  - [Simple Startup](#simple-startup)

  - [Installation](#installation)

    - [Installation from pip](#installation-from-pip)

    - [Installation from source code (development)](#installation-from-source-code-development)

  - [Usage](#usage)

    - [Getting events](#getting-events)

    - [Permissions](#permissions)

      - [Same account assets only, no SCPs](#same-account-assets-only-no-scps)

      - [Cross-account assets and SCPs](#cross-account-assets-and-scps)

    - [CLI Commands](#cli-commands)

      - [Analyze](#analyze)

      - [Get SCPs](#get-scps)

  - [Output Format](#output-format)

    - [Output Fields](#output-fields)

      - [AccessDeniedReason:](#accessdeniedreason)

      - [ResultDetails](#resultdetails)

        - [PoliciesToAdd](#policiestoadd)

        - [ExplicitDenyPolicies](#explicitdenypolicies)

  - [Acknowledgements](#acknowledgements)

  - [Appendices](#appendices)

    - [Running AccessUndenied from a Lambda function](#running-accessundenied-from-a-lambda-function)

    - [Setting up a venv](#setting-up-a-venv)

    - [Getting CloudTrail events via the LookupEvents API with the CLI](#getting-cloudtrail-events-via-the-lookupevents-api-with-the-cli)

    - [Getting Cloudtrail events from the AWS Console's event history](#getting-cloudtrail-events-from-the-aws-consoles-event-history)

    - [Example Cloudtrail event](#example-cloudtrail-event)

    - [Least privilege AccessUndenied policy](#least-privilege-accessundenied-policy)



## 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](https://aws.amazon.com/blogs/security/aws-introduces-changes-to-access-denied-errors-for-easier-permissions-troubleshooting/)

will be sufficient. However, that is not always the case.

1. 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.

2. When the reason for AccessDenied is an explicit deny, it can be difficult to track down 

and evaluate every relevant policy.

3. 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.

4. 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](#setting-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-via-the-lookupevents-api-with-the-cli).

- [Getting Cloudtrail events from the AWS Console's event history](#getting-cloudtrail-events-from-the-aws-consoles-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:

1. Attach the `SecurityAudit` managed policy.

2. 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: 

```json

{

  "Version": "2012-10-17",

  "Statement": [

    {

      "Sid": "AccessUndeniedAssumeRole",

      "Effect": "Allow",

      "Action": "sts:AssumeRole",

      "Resource": [

        "arn:aws:iam:::role/AccessUndeniedRole",

        "arn:aws:iam:::role/AccessUndeniedRole",

        "arn:aws:iam:::role/AccessUndeniedRole",

        "..."

      ]

    }

  ]

}

```

If you do not wish to attach `SecurityAudit`, you may instead attach the updating [least-privilege

 AccessUndenied policy](#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](#permissions) 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):



```json

{

  "Version": "2012-10-17",

  "Statement": [

    {

      "Effect": "Allow",

      "Principal": {

        "AWS": "arn:aws:iam:::role/AccessUndeniedRole"

      },

      "Action": "sts:AssumeRole",

      "Condition": {}

    }

  ]

}

```

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

 AccessUndenied policy](#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](#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

```json

{

  "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:

```json

{

  "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](https://github.com/iann0036/iam-dataset) Ben

Kehoe's [aws-error-utils](https://github.com/benkehoe/aws-error-utils).

## Appendices

### Running AccessUndenied from a Lambda function

[Full README here](examples/lambda/README.md)

### 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](https://aws.amazon.com/premiumsupport/knowledge-center/troubleshoot-iam-permission-errors/).

It has been adapted so that the command outputs raw events rather than an ascii table.

1.    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



1. Open the AWS console

2. Go to "CloudTrail"

3. In the sidebar on the left, click Event History

4. 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.

5. 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](#cli-arguments)



### Example Cloudtrail event



One event in file:



```json

{

  "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:



```json

{

  "Records": [

    {

      "awsRegion": "us-east-1",

      "eventID": "xxxxxxxx-xxxx-xxxx-xxxx-8234c1555c12"

      //... rest of cloudtrail_event ...

    },

    {

      //... another cloudtrail_event ...

    }

    // more events...

  ]

}

```



### Least privilege AccessUndenied policy

```json

{

  "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": "*"

    }

  ]

}

```