The serverless revolution has promised to free us from the shackles of infrastructure management. But as we’ve flocked to services like AWS Lambda and Azure Functions, we’ve inadvertently created a new and insidious type of security risk: the “LambdaChain” exploit.
This isn’t a vulnerability in the traditional sense. It’s not a bug in the code, but a flaw in the architecture. It’s a chain of seemingly innocuous IAM misconfigurations that, when linked together, can lead to a full AWS account takeover.
In this tutorial, we’ll walk you through a LambdaChain exploit from an attacker’s perspective, showing you how a single, overly-permissive Lambda function can be the first domino to fall in a catastrophic security breach.
### The Stage: A Single, Overly-Permissive Lambda
Our story begins with a common scenario: a Lambda function that’s designed to process user uploads to S3. The developer, in a hurry to get the function working, has given it an overly-permissive IAM role. The role has read and write access to all S3 buckets in the account, as well as permission to call other AWS services.
The developer has also made a crucial mistake: they’ve included the AWS SDK in the function’s deployment package. This is a common practice, but it’s also a security risk. The SDK includes a wealth of information about the AWS account, including the account ID, the region, and a list of all the services that are available.
### The Attack Chain: From RCE to Admin
Now, let’s put on our black hats and see how we can exploit this situation.
**Step 1: Gaining a Foothold**
Our first step is to gain a foothold in the Lambda function. We can do this by exploiting a file upload vulnerability in the web application that’s using the function. We upload a malicious file that, when processed by the Lambda function, gives us a reverse shell.
**Step 2: Enumerating the Environment**
Now that we’re inside the Lambda function, we can start to enumerate the environment. We use the AWS SDK to get a list of all the S3 buckets, DynamoDB tables, and other resources in the account. We also use the `sts:GetCallerIdentity` API call to get the ARN of the Lambda function’s IAM role.
**Step 3: Pivoting Through the Account**
This is where the LambdaChain exploit gets its name. We start to pivot through the account, using the permissions of the Lambda function’s IAM role to access other services.
* First, we use the S3 permissions to read the contents of a sensitive bucket. We find a file that contains the credentials for a DynamoDB table.
* Next, we use the DynamoDB permissions to read the contents of the table. We find a record that contains the credentials for a user in Secrets Manager.
* Finally, we use the Secrets Manager permissions to retrieve the user’s credentials. The user, it turns out, is an administrator.
**Step 4: The Takeover**
We now have administrator access to the AWS account. We can create new users, delete existing resources, and exfiltrate all of the data in the account. The LambdaChain exploit is complete.
### How to Prevent This
The LambdaChain exploit is a scary scenario, but it’s also a preventable one. Here are some key steps you can take to protect your serverless applications:
* **Apply the principle of least privilege:** This is the most important thing you can do. Give your Lambda functions only the permissions they need to do their jobs.
* **Use automated IAM scanning:** There are a number of tools that can scan your IAM policies and identify potential security risks.
* **Implement service control policies (SCPs):** SCPs are a powerful tool for enforcing security policies across your entire AWS organization. You can use them to prevent your developers from creating overly-permissive IAM roles.
### The Future of Serverless Security
The LambdaChain exploit is a stark reminder that serverless security is not just about securing your code. It’s about securing your entire architecture. As we continue to embrace serverless, we need to be vigilant. We need to understand the new attack vectors that are emerging, and we need to build resilient systems that are designed to withstand them. The security of our cloud environments depends on it.
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