Mailto links can be used to embed a link to create an email in documents. The recipient and the subject can be specified as parameters. However, some e-mail programs also support undocumented extensions that can be used, for example, to attach any files. This worked on Linux systems.
Mailto links are a common method of making mail addresses clickable on websites. The appropriate mail program is then usually opened with a click. Such a link usually only contains the email address, but variables can also be used to prefill the email subject or the content of the email.
The format of a mailto link is
<a href="mailto:[email protected]">Mail an ju</a>
With further parameters like
subject=Test you can also specify the subject. In some scenarios you can even use the keywords
attach specify further e-mail elements, which then end up automatically in an e-mail that is ready to be sent. So then leads about
Local Files Can Be Attached To Mails With A Parameter
In addition to these rather harmless features, a local file can be attached directly to the mail in some mail programs using the attach parameter. An attacker could take advantage of this to try to steal a key. These are usually located in standard paths. A user might of course notice that there is an unfamiliar attachment to the mail, but it can be assumed that not all users check this before sending the mail. This file attachment feature is not part of the standard specification for mailto links. It is an unofficial extension that is used by some mail programs. According to the release, the feature is supported in the standard mail programs of the Linux desktop environments. Thunderbird itself is not affected, but can be vulnerable if the mailto links are processed using the xdg tool.
These attach parameters are not specified in the mailto standard. As it turned out, the Linux mailers are not necessarily responsible for the data leak. Thunderbird, for example, removed the short term function years ago. But the little helper xdg, which browsers use to open external links, reintroduced the dangerous mailto extensions through the back door.