CWE-206 Variant Incomplete

Observable Internal Behavioral Discrepancy

This vulnerability occurs when a system's internal steps or decisions become visible to an attacker because the system behaves differently at each stage. Instead of presenting a single, unified…

Definition

What is CWE-206?

This vulnerability occurs when a system's internal steps or decisions become visible to an attacker because the system behaves differently at each stage. Instead of presenting a single, unified result, the product leaks information about its internal checks, allowing an attacker to map its logic and pinpoint weaknesses.
A secure system should act like a black box, giving attackers minimal feedback. When internal behaviors—like checking a username before verifying a password—produce distinct, observable responses, they create a roadmap for exploitation. Attackers can use these discrepancies to refine their approach, focusing only on the parts of the process that matter, which dramatically reduces the effort needed for a successful attack. For developers, the core principle is to ensure all execution paths for a given operation result in identical external behavior. In the classic login example, both an invalid username and an incorrect password should trigger the same generic error message and timing. By consolidating outputs and eliminating behavioral clues, you remove the signposts attackers rely on to understand and compromise your system's internal state.
Real-world impact

Real-world CVEs caused by CWE-206

  • CVE-2002-2031

    File existence via infoleak monitoring whether "onerror" handler fires or not.

  • CVE-2005-2025

    Valid groupname enumeration via behavioral infoleak (sends response if valid, doesn't respond if not).

  • CVE-2001-1497

    Behavioral infoleak in GUI allows attackers to distinguish between alphanumeric and non-alphanumeric characters in a password, thus reducing the search space.

  • CVE-2003-0190

    Product immediately sends an error message when user does not exist instead of waiting until the password is provided, allowing username enumeration.

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 4

    Iterate until the response leaks data, executes attacker code, or escalates privileges.

Vulnerable code example

Vulnerable pseudo

MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.

Vulnerable pseudo 
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
  // Untrusted input flows directly into the sensitive sink.
  return executeUnsafe(input);
}
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-206

  • Setup generic response pages for error conditions. The error page should not disclose information about the success or failure of a sensitive operation. For instance, the login page should not confirm that the login is correct and the password incorrect. The attacker who tries random account name may be able to guess some of them. Confirming that the account exists would make the login page more susceptible to brute force attack.
Detection signals

How to detect CWE-206

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-206 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-206?

This vulnerability occurs when a system's internal steps or decisions become visible to an attacker because the system behaves differently at each stage. Instead of presenting a single, unified result, the product leaks information about its internal checks, allowing an attacker to map its logic and pinpoint weaknesses.

How serious is CWE-206?

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-206?

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

How can I prevent CWE-206?

Setup generic response pages for error conditions. The error page should not disclose information about the success or failure of a sensitive operation. For instance, the login page should not confirm that the login is correct and the password incorrect. The attacker who tries random account name may be able to guess some of them. Confirming that the account exists would make the login page more susceptible to brute force attack.

How does Plexicus detect and fix CWE-206?

Plexicus's SAST engine matches the data-flow signature for CWE-206 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-206?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/206.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-206

CWE-205 Parent

Observable Behavioral Discrepancy

This vulnerability occurs when an application behaves differently in ways that unauthorized users can detect. These observable differences…

CWE-207 Sibling

Observable Behavioral Discrepancy With Equivalent Products

This vulnerability occurs when a system that should remain anonymous behaves differently than other products with the same purpose,…

Resources

Further reading

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