CWE-783 Base Draft Low likelihood

Operator Precedence Logic Error

This vulnerability occurs when a developer writes a conditional expression where the intended logic is broken due to misunderstanding or misapplying the rules of operator precedence.

Definition

What is CWE-783?

This vulnerability occurs when a developer writes a conditional expression where the intended logic is broken due to misunderstanding or misapplying the rules of operator precedence.

At its core, this is a programming mistake where the order in which operations are evaluated (like comparison before logical AND) doesn't match the developer's intent. For example, writing `if (x & 1 == 0)` might be intended to check if the least significant bit is zero, but due to precedence, it's actually evaluated as `if (x & (1 == 0))`, leading to an incorrect result. While this can manifest as a simple bug in non-critical code, it becomes a serious security flaw when it appears in security-critical decision points. A common and dangerous example is within authentication or authorization logic, where a flawed expression could incorrectly grant access or bypass a security check entirely, directly compromising the application's security.

Real-world impact

Real-world CVEs caused by CWE-783

CVE-2008-2516

Authentication module allows authentication bypass because it uses "(x = call(args) == SUCCESS)" instead of "((x = call(args)) == SUCCESS)".
CVE-2008-0599

Chain: Language interpreter calculates wrong buffer size (CWE-131) by using "size = ptr ? X : Y" instead of "size = (ptr ? X : Y)" expression.
CVE-2001-1155

Chain: product does not properly check the result of a reverse DNS lookup because of operator precedence (CWE-783), allowing bypass of DNS-based access restrictions.

How attackers exploit it

Step-by-step attacker path

1
In the following example, the method validateUser makes a call to another method to authenticate a username and password for a user and returns a success or failure code.
2
However, the method that authenticates the username and password is called within an if statement with incorrect operator precedence logic. Because the comparison operator "==" has a higher precedence than the assignment operator "=", the comparison operator will be evaluated first and if the method returns FAIL then the comparison will be true, the return variable will be set to true and SUCCESS will be returned. This operator precedence logic error can be easily resolved by properly using parentheses within the expression of the if statement, as shown below.
3
In this example, the method calculates the return on investment for an accounting/financial application. The return on investment is calculated by subtracting the initial investment costs from the current value and then dividing by the initial investment costs.
4
However, the return on investment calculation will not produce correct results because of the incorrect operator precedence logic in the equation. The divide operator has a higher precedence than the minus operator, therefore the equation will divide the initial investment costs by the initial investment costs which will only subtract one from the current value. Again this operator precedence logic error can be resolved by the correct use of parentheses within the equation, as shown below.
5
Note that the initialInvestment variable in this example should be validated to ensure that it is greater than zero to avoid a potential divide by zero error (CWE-369).

Vulnerable code example

Vulnerable C

In the following example, the method validateUser makes a call to another method to authenticate a username and password for a user and returns a success or failure code.

Vulnerable C

#define FAIL 0
  #define SUCCESS 1
  ...
  int validateUser(char *username, char *password) {
  		int isUser = FAIL;
```
// call method to authenticate username and password* 
  		
  		
  		 *// if authentication fails then return failure otherwise return success* 
  		if (isUser = AuthenticateUser(username, password) == FAIL) {
  		```
  			return isUser;
  		}
  		else {
  			isUser = SUCCESS;
  		}
  		return isUser;
  }

Secure code example

Secure C

However, the method that authenticates the username and password is called within an if statement with incorrect operator precedence logic. Because the comparison operator "==" has a higher precedence than the assignment operator "=", the comparison operator will be evaluated first and if the method returns FAIL then the comparison will be true, the return variable will be set to true and SUCCESS will be returned. This operator precedence logic error can be easily resolved by properly using parentheses within the expression of the if statement, as shown below.

Secure C

...
  if ((isUser = AuthenticateUser(username, password)) == FAIL) {
  ...

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-783

Implementation Regularly wrap sub-expressions in parentheses, especially in security-critical code.

Detection signals

How to detect CWE-783

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.

CWE-783

Don't catalog this weakness. Prove it's reachable.

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Frequently asked questions

What is CWE-783?

This vulnerability occurs when a developer writes a conditional expression where the intended logic is broken due to misunderstanding or misapplying the rules of operator precedence.

How serious is CWE-783?

MITRE rates the likelihood of exploit as Low — exploitation is uncommon, but the weakness should still be fixed when discovered.

What languages or platforms are affected by CWE-783?

MITRE lists the following affected platforms: C, C++.

How can I prevent CWE-783?

Regularly wrap sub-expressions in parentheses, especially in security-critical code.

How does Plexicus detect and fix CWE-783?

Plexicus's SAST engine matches the data-flow signature for CWE-783 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-783?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/783.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-783

CWE-670 Parent

Operator Precedence Logic Error

What is CWE-783?

Real-world CVEs caused by CWE-783

Step-by-step attacker path

Vulnerable C

Secure C

How to prevent CWE-783

How to detect CWE-783

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-783

Always-Incorrect Control Flow Implementation

Use of Incorrect Operator

Incorrect Block Delimitation

Omitted Break Statement in Switch

Reachable Assertion

Execution After Redirect (EAR)

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`