A ReDoS vulnerability can be introduced into a FastAPI application through an insecure regular expression, which can lead to a denial-of-service attack. This can occur when an attacker provides a malicious input to an endpoint that uses a vulnerable regular expression. To mitigate this vulnerability, developers can use tools like Snyk, which scans dependencies for known issues and recommends fixes. By identifying and fixing ReDoS vulnerabilities early, developers can prevent potential security breaches and maintain the integrity of their FastAPI applications. Additionally, integrating Snyk with Git repositories allows developers to automatically scan every commit for vulnerabilities, making it easier to maintain a secure codebase.