Passbolt sends e-mail to users to warn them about different type of events such as the creation, modification or deletion of a password. Those e-mails may contain user-specified input, such as a password’s title or description.
Passbolt does not escape the user’s input properly, resulting in the user being able to inject HTML code in an e-mail.
An authenticated attacker could share a password containing an img HTML tag in its description with an other user to obtain information about their mail user-agent.
This vulnerability has a very low impact. Most MUA do not embed remote images to protect their users’ privacy.