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-768: Incorrect Short Circuit Evaluation

Weakness ID: 768
Abstraction: Variant
Status: Incomplete
Presentation Filter:
+ Description

Description Summary

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.

+ Time of Introduction
  • Implementation
+ Common Consequences
ScopeEffect
Confidentiality
Integrity
Availability

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

Very 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.

+ Relationships
NatureTypeIDNameView(s) this relationship pertains toView(s)
ChildOfCategoryCategory171Cleansing, Canonicalization, and Comparison Errors
Development Concepts (primary)699
ChildOfWeakness ClassWeakness Class691Insufficient Control Flow Management
Research Concepts (primary)1000
ChildOfCategoryCategory871CERT C++ Secure Coding Section 03 - Expressions (EXP)
Weaknesses Addressed by the CERT C++ Secure Coding Standard (primary)868
ChildOfCategoryCategory998SFP Secondary Cluster: Glitch in Computation
Software Fault Pattern (SFP) Clusters (primary)888
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
CLASPFailure to protect stored data from modification
CERT C++ Secure CodingEXP02-CPPBe aware of the short-circuit behavior of the logical AND and OR operators
Software Fault PatternsSFP1Glitch in computation
+ Content History
Submissions
Submission DateSubmitterOrganizationSource
2009-03-03Internal CWE Team
Modifications
Modification DateModifierOrganizationSource
2010-09-27CWE Content TeamMITREInternal
updated Description
2011-09-13CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings
2012-05-11CWE Content TeamMITREInternal
updated Relationships
2014-07-30CWE Content TeamMITREInternal
updated Relationships, Taxonomy_Mappings

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Page Last Updated: January 18, 2017