Is my site at risk? (Boost as vector attack)

Hi there. I've been using Boost for years with wonderful results in several projects. Recently, something I've never seen before caught my attention: in the message log one of our sites I started to see a lot of messages like this:

Warning: mkdir(): Permission denied in boost_mkdir() (line 1347 of /var/www/mysite.com/public_html/sites/all/modules/boost/boost.module).

Knowing that Boost has been working fine for a while, and the cache directory has proper write permissions, I was wondering what was going on. I fired up a new SSH session to the server and navigated to the cache folder to re-check file permissions.

What I found left me scratching my head:

In the cache/normal subdirectory, along with a directory named after my domain, there was a new directory with a strange, absolutely unrelated domain name (I won't publish it here for obvious reasons, but it's from some russian news site with the .az TLD). Inside, it was a file named _.html. Unfortunately (noob mistake) I just deleted the whole folder before saving a copy of their contents, but it looked like the file was created by Boost, as the folder.

File permissions likely are not the issue here: they're properly set, the cache folder is owned by the Apache's user and using ug=rwx,o= as permissions (I actually followed the guide to secure file permissions located here).

What worries me is the possibility that somehow it's possible to trick Boost into creating cache files from bogus domian sources. Could that be possible? What would it take? If so, could this lead to any security issues, or could be exploited in some way?

Anecdotally: as soon as I deleted the bogus folder, the Permission denied in boost_mkdir() messages stopped to appear in the log. And so far, no bogus directories have been created again.

Thank you in advance for any comment or advise.