The product provides different responses to incoming requests in a way that reveals internal state information to an unauthorized actor outside of the intended control sphere.

Primary

Resultant

Architecture and Design
An observable response discrepancy frequently occurs during authentication, where a difference in failed-login messages could allow an attacker to determine if the username is valid or not. The discrepancy could be inadvertent (bug) or intentional (design).

Implementation
An observable response discrepancy frequently occurs during authentication, where a difference in failed-login messages could allow an attacker to determine if the username is valid or not. The discrepancy could be inadvertent (bug) or intentional (design).

Confidentiality
Access Control
Read Application Data
Bypass Protection Mechanism

Architecture and Design
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 the compartmentalization allows for and reinforces privilege separation functionality. Architects and designers should rely on the principle of least privilege to decide the appropriate time to use privileges and the time to drop privileges.

Implementation

Ensure that error messages only contain minimal details that are useful to the intended audience and no one else. The messages need to strike the balance between being too cryptic (which can confuse users) or being too detailed (which may reveal more than intended). The messages should not reveal the methods that were used to determine the error. Attackers can use detailed information to refine or optimize their original attack, thereby increasing their chances of success.
If errors must be captured in some detail, record them in log messages, but consider what could occur if the log messages can be viewed by attackers. Highly sensitive information such as passwords should never be saved to log files.
Avoid inconsistent messaging that might accidentally tip off an attacker about internal state, such as whether a user account exists or not.

The following code checks validity of the supplied username and password and notifies the user of a successful or failed login.

my $username=param('username');
my $password=param('password');

if (IsValidUsername($username) == 1)
{
if (IsValidPassword($username, $password) == 1)
{
print "Login Successful";
}
else
{
print "Login Failed - incorrect password";
}
}
else
{
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 credentials.
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:

"Login Failed - incorrect username or password"

CVE-2002-2094
This, and others, use ".." attacks and monitor error responses, so there is overlap with directory traversal.
https://www.cve.org/CVERecord?id=CVE-2002-2094

CVE-2001-1483
Enumeration of valid usernames based on inconsistent responses
https://www.cve.org/CVERecord?id=CVE-2001-1483

CVE-2001-1528
Account number enumeration via inconsistent responses.
https://www.cve.org/CVERecord?id=CVE-2001-1528

CVE-2004-2150
User enumeration via discrepancies in error messages.
https://www.cve.org/CVERecord?id=CVE-2004-2150

CVE-2005-1650
User enumeration via discrepancies in error messages.
https://www.cve.org/CVERecord?id=CVE-2005-1650

CVE-2004-0294
Bulletin Board displays different error messages when a user exists or not, which makes it easier for remote attackers to identify valid users and conduct a brute force password guessing attack.
https://www.cve.org/CVERecord?id=CVE-2004-0294

CVE-2004-0243
Operating System, when direct remote login is disabled, displays a different message if the password is correct, which allows remote attackers to guess the password via brute force methods.
https://www.cve.org/CVERecord?id=CVE-2004-0243

CVE-2002-0514
Product allows remote attackers to determine if a port is being filtered because the response packet TTL is different than the default TTL.
https://www.cve.org/CVERecord?id=CVE-2002-0514

CVE-2002-0515
Product sets a different TTL when a port is being filtered than when it is not being filtered, which allows remote attackers to identify filtered ports by comparing TTLs.
https://www.cve.org/CVERecord?id=CVE-2002-0515

CVE-2001-1387
Product may generate different responses than specified by the administrator, possibly leading to an information leak.
https://www.cve.org/CVERecord?id=CVE-2001-1387

CVE-2004-0778
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.
https://www.cve.org/CVERecord?id=CVE-2004-0778

CVE-2004-1428
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.
https://www.cve.org/CVERecord?id=CVE-2004-1428

Response discrepancy infoleak

Allowed
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

can overlap errors related to escalated privileges

PLOVER
2006-07-19
Draft 3
2006-07-19

Eric Dalci
Cigital
2008-07-01
updated Potential_Mitigations, Time_of_Introduction

CWE Content Team
MITRE
2008-09-08
updated Relationships, Relationship_Notes, Taxonomy_Mappings

CWE Content Team
MITRE
2008-10-14
updated Description, Potential_Mitigations

CWE Content Team
MITRE
2009-12-28
updated Demonstrative_Examples

CWE Content Team
MITRE
2010-09-27
updated Description, Name, Observed_Examples

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Demonstrative_Examples, Observed_Examples, References, Relationships

CWE Content Team
MITRE
2012-10-30
updated Potential_Mitigations

CWE Content Team
MITRE
2014-07-30
updated Relationships

CWE Content Team
MITRE
2017-11-08
updated Applicable_Platforms

CWE Content Team
MITRE
2020-02-24
updated Description, Name, Relationships

CWE Content Team
MITRE
2020-12-10
updated Potential_Mitigations

CWE Content Team
MITRE
2023-01-31
updated Related_Attack_Patterns

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2025-04-03
4.17
2025-04-03
updated Description, Diagram, Modes_of_Introduction

CWE Content Team
MITRE
2025-12-11
4.19
2025-12-11
updated Weakness_Ordinalities

Response Discrepancy Information Leak
Response Discrepancy Information Exposure