Python 3, 53 bytes

I=input
X=vars(vars()[I()])
X[I()]=X[I()]
class I:1/0

Try it online!

The password is 3 lines of input on STDIN. The flag is the following text, to be printed to STDERR:

Traceback (most recent call last):
  File ".code.tio", line 4, in <module>
    class I:1/0
IndexError: tuple index out of range

(replace .code.tio with whatever the filename of the program is)

I will offer a +50 rep bounty to the first valid Robbers' answer within the two week period.

Python 3, 53 bytes, cracked by EasyasPi

I=input
X=vars(vars()[I()])
X[I()]=X[I()]
class I:1/0

Try it online!

The password is 3 lines of input on STDIN. The flag is the following text, to be printed to STDERR:

Traceback (most recent call last):
  File ".code.tio", line 4, in <module>
    class I:1/0
IndexError: tuple index out of range

(replace .code.tio with whatever the filename of the program is)

Python 3, 53 bytes

I=input
X=vars(vars()[I()])
X[I()]=X[I()]
class I:1/0

Try it online!

Takes 3 lines of input on STDIN. Output is, printed to STDERR, the following text:

Traceback (most recent call last):
  File ".code.tio", line 4, in <module>
    class I:1/0
IndexError: tuple index out of range

(replace .code.tio with whatever the filename of the program is)

I will offer a +50 rep bounty to the first valid Robbers' answer within the two week period.

Python 3, 53 bytes

I=input
X=vars(vars()[I()])
X[I()]=X[I()]
class I:1/0

Try it online!

The password is 3 lines of input on STDIN. The flag is the following text, to be printed to STDERR:

Traceback (most recent call last):
  File ".code.tio", line 4, in <module>
    class I:1/0
IndexError: tuple index out of range

(replace .code.tio with whatever the filename of the program is)

I will offer a +50 rep bounty to the first valid Robbers' answer within the two week period.

Python 3, 53 bytes

I=input
X=vars(vars()[I()])
X[I()]=X[I()]
class I:1/0

Try it online!

Takes 3 lines of input on STDIN. Output is, printed to STDERR, the following text:

Traceback (most recent call last):
  File ".code.tio", line 4, in <module>
    class I:1/0
IndexError: tuple index out of range

(replace .code.tio with whatever the filename of the program is)

Python 3, 53 bytes

I=input
X=vars(vars()[I()])
X[I()]=X[I()]
class I:1/0

Try it online!

Takes 3 lines of input on STDIN. Output is, printed to STDERR, the following text:

Traceback (most recent call last):
  File ".code.tio", line 4, in <module>
    class I:1/0
IndexError: tuple index out of range

(replace .code.tio with whatever the filename of the program is)

I will offer a +50 rep bounty to the first valid Robbers' answer within the two week period.