type-confusion-anti-pattern

name	type-confusion-anti-pattern
description	Security anti-pattern for type confusion vulnerabilities (CWE-843). Use when generating or reviewing code in dynamic languages that compares values, processes JSON/user input, or uses loose equality. Detects weak typing exploits and type coercion attacks.

Type Confusion Anti-Pattern

Severity: High

Summary

Programs misinterpret data types through loose comparisons, implicit coercion, or improper input handling. Attackers exploit type confusion in weakly-typed languages (JavaScript, PHP) and dynamic data structures (JSON) to bypass security checks, manipulate logic, or achieve code execution.

The Anti-Pattern

The anti-pattern is using loose equality (==) or trusting incoming data types without explicit validation.

BAD Code Example

// VULNERABLE: Loose equality comparison in authentication.

function checkAdminAccess(userId) {
    // Expected: userId is string "123".
    // Attacker input: userId is number 0.
    // JavaScript: "0" == 0 evaluates to true (type coercion).
    if (userId == 0) { // Loose equality
        return true; // Grants admin access if userId is "0" or 0.
    }
    return false;
}

// Scenario 1: userId = "0" (string) gains admin access.
// Scenario 2: userId = 0 (number) bypasses the check.

// PHP example: "0e12345" == "0e56789" (both evaluate to 0).
// If password hash starts with "0e", attacker provides another
// hash starting with "0e" to bypass authentication.

GOOD Code Example

// SECURE: Strict equality and explicit type validation.

// Option 1: Strict equality (===) checks both value AND type.
function checkAdminAccessSecure(userId) {
    // "0" === 0 evaluates to false.
    if (userId === 0) {
        return true;
    }
    return false;
}

// Option 2: Explicitly validate input type.
function processProductId(productId) {
    // Ensure productId is string matching expected format.
    if (typeof productId !== 'string' || !/^\d+$/.test(productId)) {
        throw new Error("Invalid product ID format.");
    }
    // Safe to use productId with known type and format.
    return parseInt(productId, 10);
}

Detection

• Code Review:
  • Loose equality operators: Search for == in JavaScript or PHP code (prefer ===).
  • Implicit type conversions: Look for contexts where a variable of one type might be implicitly converted to another, especially when performing comparisons or operations.
  • Dynamic language features: Be cautious with how user-provided data is used in contexts where the language might automatically infer or coerce types.
• Input Validation: Check if all incoming user input (JSON body, query parameters, form data) is explicitly validated for its expected data type before being processed.
• Dynamic Queries: Review code that constructs queries for NoSQL databases (like MongoDB) or other systems using user input. Attackers can often inject operators ($gt, $ne) by changing the input's type from a string to an object.

Prevention

• Use strict equality: In JavaScript/PHP, always use === instead of ==.
• Validate input types explicitly: Check and enforce expected types before using user data. Safely convert integers and validate ranges.
• Use schema validation: For JSON APIs, use validation libraries (JSON Schema, Joi, Pydantic) that strictly enforce types and formats.
• Protect NoSQL queries: Avoid embedding user-controlled objects in queries. Sanitize or allowlist specific field-value pairs to prevent operator injection.
• Understand language type juggling: Know how your language handles type conversions and stay vigilant where this enables exploits.

Related Security Patterns & Anti-Patterns

• Missing Input Validation Anti-Pattern: Type validation is a fundamental part of comprehensive input validation.
• Integer Overflow Anti-Pattern: A specific type of numeric issue that can arise from unexpected type handling.
• NoSQL Injection: Type confusion is a common vector for exploiting NoSQL databases.

References

• OWASP Top 10 A05:2025 - Injection
• OWASP GenAI LLM05:2025 - Improper Output Handling
• OWASP API Security API8:2023 - Security Misconfiguration
• CWE-843: Access of Resource Using Incompatible Type
• CAPEC-153: Input Data Manipulation
• PHP Type Juggling (OWASP)
• NoSQL Injection Testing (OWASP)
• Source: sec-context