Proton Exploit May 2026

The Sonar Research team identified the vulnerability during a routine audit of Proton's open-source repositories. The issue stemmed from how the web application handled user-controlled HTML. While senders need the ability to style messages, failing to properly sanitize certain tags can allow malicious tags to execute in a reader's browser. How the Exploit Worked

The vulnerability was strictly limited to the web interface; non-web Proton Mail apps (iOS/Android) were never affected. Protecting Your Data Proton Exploit

If successful, the script would run in the victim's session, allowing the attacker to "see" what the user sees—effectively stealing the decrypted content of their inbox. Proton's Response and Resolution The Sonar Research team identified the vulnerability during