CWE-203: Observable Discrepancy (Side Channel Attack)

CVE-2025-27667

Code examples

Illustrative examples from MITRE showing how the weakness appears in code.

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

Vulnerable example

perl

my $username=param('username');

Resulting query

plaintext

"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.

Vulnerable example

python

def validate_password(actual_pw, typed_pw):

Non-uniform processing time causes timing channel.

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.

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.

Illustrative examples

Real CVEs that MITRE cites as examples of this weakness.

CVE-2020-8695 — Observable discrepancy in the RAPL interface for some Intel processors allows information disclosure.
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
CVE-2019-10071 — Java-oriented framework compares HMAC signatures using String.equals() instead of a constant-time algorithm, causing timing discrepancies
CVE-2002-2094 — This, and others, use ".." attacks and monitor error responses, so there is overlap with directory traversal.
CVE-2001-1483 — Enumeration of valid usernames based on inconsistent responses
CVE-2001-1528 — Account number enumeration via inconsistent responses.
CVE-2004-2150 — User enumeration via discrepancies in error messages.
CVE-2005-1650 — User enumeration via discrepancies in error messages.
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.
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.
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.
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.
CVE-2002-0208 — Product modifies TCP/IP stack and ICMP error messages in unusual ways that show the product is in use.
CVE-2004-2252 — Behavioral infoleak by responding to SYN-FIN packets.
CVE-2001-1387 — Product may generate different responses than specified by the administrator, possibly leading to an information leak.
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.
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.
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."
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.
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.
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.
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.
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.

CWE-203: Observable Discrepancy

Overview

Background

Real-world CVEs

Common consequences

How it happens

When it is introduced

How to prevent it

Code examples

Illustrative examples

Terminology & mappings

Alternate terms

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-203?

What CVEs are caused by CWE-203?

Is CWE-203 part of the OWASP Top 10?

How do you prevent CWE-203?

What are the consequences of CWE-203?

Is CWE-203 actively exploited?

References

Stay ahead of CWE-203

Overview

Background

Real-world CVEs

Common consequences

How it happens

When it is introduced

How to prevent it

Code examples

Illustrative examples

Related weaknesses

Parent

Child

Terminology & mappings

Alternate terms

Mapped taxonomies

Attack patterns

Frequently asked questions

What is CWE-203?

What CVEs are caused by CWE-203?

Is CWE-203 part of the OWASP Top 10?

How do you prevent CWE-203?

What are the consequences of CWE-203?

Is CWE-203 actively exploited?

References

Stay ahead of CWE-203