The product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor, which exposes security-relevant information about the state of the product, such as whether a particular operation was successful or not.
Discrepancies can take many forms, and variations may be detectable in timing, control flow, communications such as replies or requests, or general behavior. These discrepancies can reveal information about the product's operation or internal state to an unauthorized actor. In some cases, discrepancies can be used by attackers to form a side channel.

Primary

Resultant

Side Channel Attack
Observable Discrepancies are at the root of side channel attacks.

Architecture and Design

Implementation

Confidentiality
Access Control
Read Application Data
Bypass Protection Mechanism
An attacker can gain access to sensitive information about the system, including authentication information that may allow an attacker to gain access to the system.

Confidentiality
Read Application Data
When cryptographic primitives are vulnerable to side-channel-attacks, this could be used to reveal unencrypted plaintext in the worst case.

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"

In this example, the attacker observes how long an authentication takes when the user types in the correct password.
When the attacker tries their own values, they can first try strings of various length. When they find a string of the right length, the computation will take a bit longer, because the for loop will run at least once. Additionally, with this code, the attacker can possibly learn one character of the password at a time, because when they guess the first character right, the computation will take longer than a wrong guesses. Such an attack can break even the most sophisticated password with a few hundred guesses.

def validate_password(actual_pw, typed_pw):

if len(actual_pw) <> len(typed_pw):
return 0
for i in len(actual_pw):
if actual_pw[i] <> typed_pw[i]:
return 0

return 1

Note that in this example, the actual password must be handled in constant time as far as the attacker is concerned, even if the actual password is of an unusual length. This is one reason why it is good to use an algorithm that, among other things, stores a seeded cryptographic one-way hash of the password, then compare the hashes, which will always be of the same length.

Non-uniform processing time causes timing channel.
Suppose an algorithm for implementing an encryption routine works fine per se, but the time taken to output the result of the encryption routine depends on a relationship between the input plaintext and the key (e.g., suppose, if the plaintext is similar to the key, it would run very fast).
In the example above, an attacker may vary the inputs, then observe differences between processing times (since different plaintexts take different time). This could be used to infer information about the key.
Artificial delays may be added to ensure that all calculations take equal time to execute.

Suppose memory access patterns for an encryption routine are dependent on the secret key.
An attacker can recover the key by knowing if specific memory locations have been accessed or not. The value stored at those memory locations is irrelevant. The encryption routine's memory accesses will affect the state of the processor cache. If cache resources are shared across contexts, after the encryption routine completes, an attacker in different execution context can discover which memory locations the routine accessed by measuring the time it takes for their own memory accesses to complete.

CVE-2020-8695
Observable discrepancy in the RAPL interface for some Intel processors allows information disclosure.
https://www.cve.org/CVERecord?id=CVE-2020-8695

CVE-2019-14353
Crypto hardware wallet's power consumption relates to total number of pixels illuminated, creating a side channel in the USB connection that allows attackers to determine secrets displayed such as PIN numbers and passwords
https://www.cve.org/CVERecord?id=CVE-2019-14353

CVE-2019-10071
Java-oriented framework compares HMAC signatures using String.equals() instead of a constant-time algorithm, causing timing discrepancies
https://www.cve.org/CVERecord?id=CVE-2019-10071

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-2002-0208
Product modifies TCP/IP stack and ICMP error messages in unusual ways that show the product is in use.
https://www.cve.org/CVERecord?id=CVE-2002-0208

CVE-2004-2252
Behavioral infoleak by responding to SYN-FIN packets.
https://www.cve.org/CVERecord?id=CVE-2004-2252

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

CVE-2003-0078
SSL implementation does not perform a MAC computation if an incorrect block cipher padding is used, which causes an information leak (timing discrepancy) that may make it easier to launch cryptographic attacks that rely on distinguishing between padding and MAC verification errors, possibly leading to extraction of the original plaintext, aka the "Vaudenay timing attack."
https://www.cve.org/CVERecord?id=CVE-2003-0078

CVE-2000-1117
Virtual machine allows malicious web site operators to determine the existence of files on the client by measuring delays in the execution of the getSystemResource method.
https://www.cve.org/CVERecord?id=CVE-2000-1117

CVE-2003-0637
Product uses a shorter timeout for a non-existent user than a valid user, which makes it easier for remote attackers to guess usernames and conduct brute force password guessing.
https://www.cve.org/CVERecord?id=CVE-2003-0637

CVE-2003-0190
Product immediately sends an error message when a user does not exist, which allows remote attackers to determine valid usernames via a timing attack.
https://www.cve.org/CVERecord?id=CVE-2003-0190

CVE-2004-1602
FTP server responds in a different amount of time when a given username exists, which allows remote attackers to identify valid usernames by timing the server response.
https://www.cve.org/CVERecord?id=CVE-2004-1602

CVE-2005-0918
Browser allows remote attackers to determine the existence of arbitrary files by setting the src property to the target filename and using Javascript to determine if the web page immediately stops loading, which indicates whether the file exists or not.
https://www.cve.org/CVERecord?id=CVE-2005-0918

Discrepancy Information Leaks

A6
Information Leakage and Improper Error Handling
CWE More Specific

A7
Improper Error Handling
CWE More Specific

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.

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, Taxonomy_Mappings

CWE Content Team
MITRE
2009-12-28
updated Description, Name

CWE Content Team
MITRE
2011-06-01
updated Common_Consequences

CWE Content Team
MITRE
2012-05-11
updated Common_Consequences, Demonstrative_Examples, Observed_Examples, 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
2019-06-20
updated Relationships, Type

CWE Content Team
MITRE
2020-02-24
updated Alternate_Terms, Applicable_Platforms, Common_Consequences, Demonstrative_Examples, Description, Name, Observed_Examples, Relationships

CWE Content Team
MITRE
2020-08-20
updated Alternate_Terms, Common_Consequences, Demonstrative_Examples, Description, Name, Potential_Mitigations, Related_Attack_Patterns, Relationships, Research_Gaps

CWE Content Team
MITRE
2020-12-10
updated Common_Consequences, Demonstrative_Examples, Description, Name, Potential_Mitigations, Research_Gaps

CWE Content Team
MITRE
2021-07-20
updated Demonstrative_Examples

CWE Content Team
MITRE
2021-10-28
updated Observed_Examples

CWE Content Team
MITRE
2023-04-27
updated Relationships

CWE Content Team
MITRE
2023-06-29
updated Mapping_Notes

CWE Content Team
MITRE
2023-10-26
updated Observed_Examples

CWE Content Team
MITRE
2024-02-29
4.14
2024-02-29
updated Demonstrative_Examples

CWE Content Team
MITRE
2025-04-03
4.17
2025-04-03
updated Demonstrative_Examples

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

Nicole Fern
Cycuity (originally submitted as Tortuga Logic)
2020-06-03
Provided Demonstrative Example for cache timing attack

Discrepancy Information Leaks
Information Exposure Through Discrepancy
Observable Discrepancy
Observable Differences in Behavior to Error Inputs