CWE-204: Response Discrepancy Information Exposure
Weakness ID: 204
The software provides different responses to incoming requests in a way that allows an actor to determine system state information that is outside of that actor's control sphere.
This issue frequently occurs during authentication, where a difference in failed-login messages could allow an attacker to determine if the username is valid or not. These exposures can be inadvertent (bug) or intentional (design).
The following code checks validity of the supplied username and
password and notifies the user of a successful or failed login.
if (IsValidUsername($username) == 1)
if (IsValidPassword($username, $password) == 1)
print "Login Successful";
print "Login Failed - incorrect password";
print "Login Failed - unknown username";
In the above code, there are different messages for when an incorrect
username is supplied, versus when the username is correct but the
password is wrong. This difference enables a potential attacker to
understand the state of the login function, and could allow an attacker
to discover a valid username by trying different values until the
incorrect password message is returned. In essence, this makes it easier
for an attacker to obtain half of the necessary authentication
While this type of information may be helpful to a user, it is also
useful to a potential attacker. In the above example, the message for
both failed cases should be the same, such as:
Version control system allows remote attackers to
determine the existence of arbitrary files and directories via the -X
command for an alternate history file, which causes different error messages
to be returned.
FTP server generates an error message if the user
name does not exist instead of prompting for a password, which allows remote
attackers to determine valid usernames.
Phase: Architecture and Design
Strategy: Separation of Privilege
Compartmentalize the system to have "safe" areas where trust
boundaries can be unambiguously drawn. Do not allow sensitive data to go
outside of the trust boundary and always be careful when interfacing
with a compartment outside of the safe area.
Ensure that appropriate compartmentalization is built into the system
design and that the compartmentalization serves to allow for and further
reinforce privilege separation functionality. Architects and designers
should rely on the principle of least privilege to decide when it is
appropriate to use and to drop system privileges.
Ensure that error messages only contain minimal details that are
useful to the intended audience, and nobody else. The messages need to
strike the balance between being too cryptic and not being cryptic
enough. They should not necessarily reveal the methods that were used to
determine the error. Such detailed information can be used to refine the
original attack to increase the chances of success.
If errors must be tracked in some detail, capture them in log messages
- but consider what could occur if the log messages can be viewed by
attackers. Avoid recording highly sensitive information such as
passwords in any form. Avoid inconsistent messaging that might
accidentally tip off an attacker about internal state, such as whether a
username is valid or not.