CWE-768: Incorrect Short Circuit Evaluation

VariantIncompleteExploit Likelihood: Low

The product 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.

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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 product'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.

Code Examples & CVEs

Demonstrative Examples

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.

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.

BadC

#define PRIV_ADMIN 0#define PRIV_REGULAR 1typedef 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);}

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-768 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-768, Incorrect Short Circuit Evaluation, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Incorrect Short Circuit Evaluation during Implementation

MITRE associates mitigation with Implementation; documented detection approaches include Automated Static Analysis; recorded impacts include Varies by Context. Use these source-defined anchors to turn CWE-768 into implementation, review, and verification checks for the affected component.

CWE-768: Incorrect Short Circuit Evaluation — MITRE CWE

Triage CWE-768 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-768, Incorrect Short Circuit Evaluation, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-768

Apply MITRE's full root-cause mapping guidance when using CWE-768, Incorrect Short Circuit Evaluation. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-768 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-768, Incorrect Short Circuit Evaluation. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-768: Incorrect Short Circuit Evaluation?+

CWE-768: Incorrect Short Circuit Evaluation is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product 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. 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 product'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.

What are the security consequences of Incorrect Short Circuit Evaluation?+

If exploited, CWE-768 (Incorrect Short Circuit Evaluation) it can compromise Confidentiality, Integrity and Availability, leading to outcomes such as Varies by Context.

How do you prevent or mitigate Incorrect Short Circuit Evaluation?+

Recommended mitigations for CWE-768 include: 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.

Which programming languages are affected by Incorrect Short Circuit Evaluation?+

CWE-768 commonly affects C and Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-768 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-768: Incorrect Short Circuit Evaluation

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