Mailvelope prior to 3.1.0 is vulnerable to a clickjacking attack against the settings page. As the settings page is intended to be accessible from web applications, the browser's extension isolation mechanisms are disabled (web_accessible_resources). Mailvelope implements additional measures to prevent web applications from directly embedding the settings page, but this mechanism can be bypassed.

Mailvelope prior to 3.3.0 accepts or operates with invalid PGP public keys: Mailvelope allows importing keys that contain users without a valid self-certification. Keys that are obviously invalid are not rejected during import. An attacker that is able to get a victim to import a manipulated key could claim to have signed a message that originates from another person.

Mailvelope prior to 3.3.0 allows private key operations without user interaction via its client-API. By modifying an URL parameter in Mailvelope, an attacker is able to sign (and encrypt) arbitrary messages with Mailvelope, assuming the private key password is cached. A second vulnerability allows an attacker to decrypt an arbitrary message when the GnuPG backend is used in Mailvelope.

Mailvelope prior to 3.3.0 does not require user interaction to import public keys shown on web page. This functionality can be tricked to either hide a key import from the user or obscure which key was imported.