gui2/viewer/bookmarkmanager.py in Calibre 3.18 calls cPickle.load on imported bookmark data, which allows remote attackers to execute arbitrary code via a crafted .pickle file, as demonstrated by Python code that contains an os.system call.
https://github.com/kovidgoyal/calibre/commit/aeb5b036a0bf657951756688b3c72bd68b6e4a7d