How to prevent HTML email injection in Python web apps

HTML injection vulnerabilities can occur in HTML-formatted emails, allowing attackers to insert malicious content into emails sent from legitimate domains, potentially leading to phishing attacks. This issue arises when user-provided data is not properly escaped in email templates, particularly in frameworks like Flask, where only certain file extensions trigger automatic escaping. The article explains how attackers can exploit these vulnerabilities by embedding harmful links in HTML emails, which users might perceive as legitimate. To prevent such attacks, developers should ensure that HTML content is properly escaped, use text emails instead, or employ template engines that automatically escape content. The text emphasizes the importance of understanding the default behaviors of email libraries and suggests using tools like Semgrep to automatically scan code for potential vulnerabilities, integrating these checks into continuous integration processes to maintain a secure codebase.