CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
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ID

CWE-783: Operator Precedence Logic Error

Weakness ID: 783
Abstraction: Variant
Structure: Simple
Status: Draft
Presentation Filter:
+ Description
The program uses an expression in which operator precedence causes incorrect logic to be used.
+ Extended Description
While often just a bug, operator precedence logic errors can have serious consequences if they are used in security-critical code, such as making an authentication decision.
+ Relationships

The table(s) below shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

+ Relevant to the view "Research Concepts" (CWE-1000)
NatureTypeIDName
ChildOfClassClass - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More general than a Base weakness.670Always-Incorrect Control Flow Implementation
+ Relevant to the view "Development Concepts" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.569Expression Issues
+ Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the software life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

PhaseNote
ImplementationLogic errors related to operator precedence may cause problems even during normal operation, so they are probably discovered quickly during the testing phase. If testing is incomplete or there is a strong reliance on manual review of the code, then these errors may not be discovered before the software is deployed.
+ Applicable Platforms
The listings below show possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages

C (Rarely Prevalent)

C++ (Rarely Prevalent)

Class: Language-Independent (Rarely Prevalent)

+ Common Consequences

The table below specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

ScopeImpactLikelihood
Confidentiality
Integrity
Availability

Technical Impact: Varies by Context; Unexpected State

The consequences will vary based on the context surrounding the incorrect precedence. In a security decision, integrity or confidentiality are the most likely results. Otherwise, a crash may occur due to the software reaching an unexpected state.
+ Likelihood Of Exploit
Low
+ Demonstrative Examples

Example 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.

(bad code)
Example Language:
#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;
}

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.

(good code)
Example Language:
...

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

...

Example 2

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.

(bad code)
Example Language: Java 
public double calculateReturnOnInvestment(double currentValue, double initialInvestment) {

double returnROI = 0.0;

// calculate return on investment
returnROI = currentValue - initialInvestment / initialInvestment;

return returnROI;
}

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.

(good code)
Example Language: Java 
...

returnROI = (currentValue - initialInvestment) / initialInvestment;

...

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).

+ Observed Examples
ReferenceDescription
Authentication module allows authentication bypass because it uses "(x = call(args) == SUCCESS)" instead of "((x = call(args)) == SUCCESS)".
Chain: Language interpreter calculates wrong buffer size (CWE-131) by using "size = ptr ? X : Y" instead of "size = (ptr ? X : Y)" expression.
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.
+ Potential Mitigations

Phase: Implementation

Regularly wrap sub-expressions in parentheses, especially in security-critical code.
+ Memberships
This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.737CERT C Secure Coding (2008 Version) Section 03 - Expressions (EXP)
MemberOfViewView - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).884CWE Cross-section
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CERT C Secure CodingEXP00-CExactUse parentheses for precedence of operation
CERT Perl Secure CodingEXP04-PLCWE More AbstractDo not mix the early-precedence logical operators with late-precedence logical operators
+ References
[REF-62] Mark Dowd, John McDonald and Justin Schuh. "The Art of Software Security Assessment". Chapter 6, "Precedence", Page 287. 1st Edition. Addison Wesley. 2006.
+ Content History
Submissions
Submission DateSubmitterOrganization
2009-07-16CWE Content TeamMITRE
Modifications
Modification DateModifierOrganization
2009-12-28CWE Content TeamMITRE
updated Observed_Examples
2011-06-27CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples, References, Relationships
2017-11-08CWE Content TeamMITRE
updated Taxonomy_Mappings, Time_of_Introduction

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Page Last Updated: March 29, 2018