Avoiding mass assignment vulnerabilities in Node.js is crucial to prevent SQL injection attacks and protect sensitive data. Mass assignment occurs when properties are not filtered when binding client-provided data to data models, allowing attackers to create additional objects and modify properties that should be immutable. Node.js applications can be vulnerable to mass assignment due to the use of third-party packages from the npm registry. To defend against mass assignment attacks, developers must remove sensitive fields from user models, use schema validation for user input, and limit acceptable POST request variables. Additionally, using robust solutions like Zod can help prevent prototype pollution and weak validation mechanisms. By implementing these measures, Node.js developers can protect their applications from mass assignment vulnerabilities.