It is enough to send an email with an attachmentSpecialists from SonarSource, a company specializing in information security, discovered a vulnerability in the free mail service Horde, through which hackers could hack an email account by sending an email with a special attachment.Discuss
The discovered vulnerability is already 9 years old, it could have appeared on November 30, 2012, when the developers updated the code. To start an attack, an attacker just needs to send an email to the victim with a file, the preview of which is the beginning of the attack.
Since Horde is a free service that allows users to read, send emails, and organize mailing list, then we can assume the consequences of hacking the accounts of its users.
Source: Anti-Malware