CWE

Common Weakness Enumeration

A Community-Developed List of Software Weakness Types

CWE/SANS Top 25 Most Dangerous Software Errors
Home > CWE List > CWE- Individual Dictionary Definition (3.0)  
ID

CWE-768: Incorrect Short Circuit Evaluation

Weakness ID: 768
Abstraction: Variant
Structure: Simple
Status: Incomplete
Presentation Filter:
+ Description
The software contains a conditional statement with multiple logical expressions in which one of the non-leading expressions may produce side effects. This may lead to an unexpected state in the program after the execution of the conditional, because short-circuiting logic may prevent the side effects from occurring.
+ Extended Description

Usage of short circuit evaluation, though well-defined in the C standard, may alter control flow in a way that introduces logic errors that are difficult to detect, possibly causing errors later during the software's execution. If an attacker can discover such an inconsistency, it may be exploitable to gain arbitrary control over a system.

If the first condition of an "or" statement is assumed to be true under normal circumstances, or if the first condition of an "and" statement is assumed to be false, then any subsequent conditional may contain its own logic errors that are not detected during code review or testing.

Finally, the usage of short circuit evaluation may decrease the maintainability of the code.

+ 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
ChildOfClassClass691Insufficient Control Flow Management
+ Relevant to the view "Development Concepts" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory171Cleansing, Canonicalization, and Comparison Errors
+ 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
Implementation
+ 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:

Widely varied consequences are possible if an attacker is aware of an unexpected state in the software after a conditional. It may lead to information exposure, a system crash, or even complete attacker control of the system.
+ Likelihood Of Exploit
Low
+ Demonstrative Examples

Example 1

The following function attempts to take a size value from a user and allocate an array of that size (we ignore bounds checking for simplicity). The function tries to initialize each spot with the value of its index, that is, A[len-1] = len - 1; A[len-2] = len - 2; ... A[1] = 1; A[0] = 0; However, since the programmer uses the prefix decrement operator, when the conditional is evaluated with i == 1, the decrement will result in a 0 value for the first part of the predicate, causing the second portion to be bypassed via short-circuit evaluation. This means we cannot be sure of what value will be in A[0] when we return the array to the user.

(bad code)
Example Language:
#define PRIV_ADMIN 0
#define PRIV_REGULAR 1
typedef struct{
int privileges;
int id;

} user_t;
user_t *Add_Regular_Users(int num_users){
user_t* users = (user_t*)calloc(num_users, sizeof(user_t));
int i = num_users;
while( --i && (users[i].privileges = PRIV_REGULAR) ){
users[i].id = i;

}
return users;

}
int main(){
user_t* test;
int i;
test = Add_Regular_Users(25);
for(i = 0; i < 25; i++) printf("user %d has privilege level %d\n", test[i].id, test[i].privileges);

}

When compiled and run, the above code will output a privilege level of 1, or PRIV_REGULAR for every user but the user with id 0 since the prefix increment operator used in the if statement will reach zero and short circuit before setting the 0th user's privilege level. Since we used calloc, this privilege will be set to 0, or PRIV_ADMIN.

+ Potential Mitigations

Phase: Implementation

Minimizing the number of statements in a conditional that produce side effects will help to prevent the likelihood of short circuit evaluation to alter control flow in an unexpected way.
+ 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.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPFailure to protect stored data from modification
Software Fault PatternsSFP1Glitch in computation
+ Content History
Submissions
Submission DateSubmitterOrganization
2009-03-03CWE Content TeamMITRE
Modifications
Modification DateModifierOrganization
2010-09-27CWE Content TeamMITRE
updated Description
2011-09-13CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITRE
updated Relationships
2014-07-30CWE Content TeamMITRE
updated Relationships, Taxonomy_Mappings
2017-11-08CWE Content TeamMITRE
updated Likelihood_of_Exploit, Taxonomy_Mappings

More information is available — Please select a different filter.
Page Last Updated: January 18, 2018