SSTI1

Easy WebExploitation picoCTF2025

1. Start the Challenge

   

2. Some quick reconnaissance tells us that the website is running on a Flask server.

   

3. Putting in <script>alert('Hello world')</script> tells us that the server is vulnerable to XSS injection.

   

4. Here’s where we go a bit off-road. The challenge tells us to use Server-side template injection. That’s a good start as any. I googled server side template injection python flask and came across this page: https://portswigger.net/web-security/server-side-template-injection

5. After trying all the templates, we can infer that the server is running a Jinja2 template engine.

   

6. I searched for server side template injection jinja2 payloads and came across this article https://www.onsecurity.io/blog/server-side-template-injection-with-jinja2/ {{request.application.__globals__.__builtins__.__import__('os').popen('id').read()}} was the demo payload in the article. https://man7.org/linux/man-pages/man3/popen.3.html this tells us that popen() takes in strings and executes them as commands. Jackpot! RCE!

7. However, we need it to execute a different command for us. Here’s the on that gets us the flag.{{request.application.__globals__.__builtins__.__import__('os').popen('grep -r "picoCTF"').read()}}