Problem with readlines() and comparisons

[DeaD_EyE]

The splitlines() method and also the iteration over a file-object, does not strip the line ending.
In real, you compare 123456\n with password and so on. Use the method rstrip() to get rid of line endings and white space on the right side of the str.

You should also use a function instead and a context manager with a for-loop inside:

def test_password():
    gefragt = input("Zu testendes Passwort: ")
    with open("passwoerter.txt") as fd:
        for password in fd:
            if password.rstrip() == gefragt:
                return True
    return False

Using the function:

if test_password():
    print ("Sicher")
else:
    print("Unsicher")