CWE-204 Base Incomplete

Observable Response Discrepancy

This vulnerability occurs when an application responds differently to similar requests, unintentionally leaking details about its internal state or logic to unauthorized users.

Definition

What is CWE-204?

This vulnerability occurs when an application responds differently to similar requests, unintentionally leaking details about its internal state or logic to unauthorized users.
Observable Response Discrepancy happens when an application's output—such as error messages, timing, or even subtle differences in page content—changes based on internal conditions. Attackers can probe these differences to infer sensitive information, like whether a username exists, if a file is present on the server, or the structure of a backend database, without triggering standard access controls. To prevent this, developers must ensure their applications provide consistent, generic responses in all scenarios that could reveal system state. This involves standardizing error messages, implementing uniform response times for all outcomes (success or failure), and avoiding any output that changes based on hidden internal data. Treating all failed operations identically from the user's perspective closes this common information leak.
Vulnerability Diagram CWE-204
Observable Response Discrepancy login: alice / wrong → "wrong password" login: zzz / wrong → "user not found" Server if !user → "no such user" else if !pw → "wrong pw" two distinguishable replies + different timing Username enumeration attacker harvests valid accounts Different responses for valid vs invalid identifiers leak existence.
Real-world impact

Real-world CVEs caused by CWE-204

  • 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.

How attackers exploit it

Step-by-step attacker path

  1. 1

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

  2. 2

    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.

  3. 3

    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:

Vulnerable code example

Vulnerable Perl

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

Vulnerable Perl 
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"; 
  }
Secure code example

Secure pseudo

Secure pseudo 
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}
What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist

How to prevent CWE-204

  • Architecture and Design 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.
Detection signals

How to detect CWE-204

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-204 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

Frequently asked questions

What is CWE-204?

This vulnerability occurs when an application responds differently to similar requests, unintentionally leaking details about its internal state or logic to unauthorized users.

How serious is CWE-204?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-204?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-204?

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…

How does Plexicus detect and fix CWE-204?

Plexicus's SAST engine matches the data-flow signature for CWE-204 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-204?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/204.html. You can also reference OWASP and NIST documentation for adjacent guidance.

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Weaknesses related to CWE-204

CWE-203 Parent

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Observable Timing Discrepancy

This vulnerability occurs when an application takes measurably different amounts of time to perform different operations, such as checking…

Resources

Further reading

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