Avoid clicking unexpected links in emails, even from seemingly secure providers.
The attack required a specific sequence of actions to succeed, which limited its real-world viability:
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 Proton Exploit
In most scenarios, the attack only worked if the victim viewed both emails and clicked a specific link in the second one.
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 Avoid clicking unexpected links in emails, even from
In June 2022, security researchers from SonarSource discovered a critical Cross-Site Scripting (XSS) vulnerability in the open-source code of Proton Mail. This flaw could have allowed attackers to bypass end-to-end encryption to steal decrypted emails and impersonate victims. The Discovery
The vulnerability was strictly limited to the web interface; non-web Proton Mail apps (iOS/Android) were never affected. Protecting Your Data While senders need the ability to style messages,
Proton maintained its commitment to security through its Responsible Vulnerability Disclosure Policy .