CWE-670 Class Draft

Always-Incorrect Control Flow Implementation

This weakness occurs when a section of code is structured in a way that always executes incorrectly, regardless of input or conditions. The control flow logic is fundamentally flawed and does not…

Definition

What is CWE-670?

This weakness occurs when a section of code is structured in a way that always executes incorrectly, regardless of input or conditions. The control flow logic is fundamentally flawed and does not match the intended algorithm.
Unlike many security flaws that are triggered by unexpected or malicious data, this issue represents a permanent logic error in the code. The problematic path is always wrong, meaning the software will behave incorrectly every time that specific code segment runs. This is often the result of simple syntactic mistakes, like omitting curly braces in C-style languages, which changes which statements are included in a conditional block. For developers, this means the bug is not situational; it's a constant defect in the program's logic. Identifying these flaws requires carefully reviewing the intended algorithm against the actual code structure, as standard testing with varied inputs might not reveal the root cause—the logic itself is broken from the start.
Real-world impact

Real-world CVEs caused by CWE-670

  • CVE-2021-3011

    virtual interrupt controller in a virtualization product allows crash of host by writing a certain invalid value to a register, which triggers a fatal error instead of returning an error code

How attackers exploit it

Step-by-step attacker path

  1. 1

    This code queries a server and displays its status when a request comes from an authorized IP address.

  2. 2

    This code redirects unauthorized users, but continues to execute code after calling http_redirect(). This means even unauthorized users may be able to access the contents of the page or perform a DoS attack on the server being queried. Also, note that this code is vulnerable to an IP address spoofing attack (CWE-212).

  3. 3

    In this example, the programmer has indented the statements to call Do_X() and Do_Y(), as if the intention is that these functions are only called when the condition is true. However, because there are no braces to signify the block, Do_Y() will always be executed, even if the condition is false.

  4. 4

    This might not be what the programmer intended. When the condition is critical for security, such as in making a security decision or detecting a critical error, this may produce a vulnerability.

  5. 5

    In both of these examples, a message is printed based on the month passed into the function:

Vulnerable code example

Vulnerable PHP

This code queries a server and displays its status when a request comes from an authorized IP address.

Vulnerable PHP 
$requestingIP = $_SERVER['REMOTE_ADDR'];
  if(!in_array($requestingIP,$ipAllowList)){
  	echo "You are not authorized to view this page";
  	http_redirect($errorPageURL);
  }
  $status = getServerStatus();
  echo $status;
```
...*
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-670

  • Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
  • Implementation Validate input at trust boundaries; use allowlists, not denylists.
  • Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
  • Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
  • Operation Monitor logs for the runtime signals listed in the next section.
Detection signals

How to detect CWE-670

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-670 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-670?

This weakness occurs when a section of code is structured in a way that always executes incorrectly, regardless of input or conditions. The control flow logic is fundamentally flawed and does not match the intended algorithm.

How serious is CWE-670?

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-670?

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

How can I prevent CWE-670?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-670?

Plexicus's SAST engine matches the data-flow signature for CWE-670 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-670?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/670.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-670

CWE-691 Parent

Insufficient Control Flow Management

This vulnerability occurs when a program's execution flow isn't properly managed, allowing attackers to bypass critical checks, trigger…

CWE-1265 Sibling

Unintended Reentrant Invocation of Non-reentrant Code Via Nested Calls

This vulnerability occurs when a non-reentrant function is called, and during its execution, another call is triggered that unexpectedly…

CWE-1281 Sibling

Sequence of Processor Instructions Leads to Unexpected Behavior

Certain sequences of valid and invalid processor instructions can cause the CPU to lock up or behave unpredictably, often requiring a hard…

CWE-362 Sibling

Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')

A race condition occurs when multiple processes or threads access a shared resource simultaneously without proper coordination, creating a…

CWE-430 Sibling

Deployment of Wrong Handler

This vulnerability occurs when a system incorrectly assigns or routes an object to the wrong processing component.

CWE-431 Sibling

Missing Handler

This vulnerability occurs when a software component lacks the necessary code to properly handle an error or unexpected event.

CWE-662 Sibling

Improper Synchronization

This vulnerability occurs when a multi-threaded or multi-process application allows shared resources to be accessed by multiple threads or…

CWE-696 Sibling

Incorrect Behavior Order

This weakness occurs when a system executes multiple dependent actions in the wrong sequence, leading to unexpected and potentially…

CWE-705 Sibling

Incorrect Control Flow Scoping

This vulnerability occurs when a program fails to return execution to the correct point in the code after finishing a specific operation…

Resources

Further reading

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