The Bitter (T-APT-17) group is a threat group that usually targets South Asian government organizations, using Microsoft Office programs to distribute malware such as Word or Excel. AhnLab Security Emergency response Center (ASEC) has identified multiple circumstances of the group distributing CHM malware to certain Chinese organizations. CHM files have been used by various threat groups in APT attacks since earlier this year and covered multiple times in ASEC blog posts.
The files used in the recent attack were being distributed as attachments to emails as compressed files. The compressed files contain a CHM file with the following filenames.