Clop, a prolific ransomware gang, has taken credit for stealing data from at least 66 companies by exploiting a bug in widely used corporate file transfer tools made by Cleo Software.

The cybercriminal gang on Tuesday listed on its dark web leak site the partial names of companies it hacked, which TechCrunch has seen, but who had not responded to the gang’s outreach. The gang said it planned to soon reveal the full names of the companies it hacked, likely in an effort to extort the victims into paying the hackers a ransom not to release their stolen files.

This is the Clop gang’s latest mass hack in recent years targeting file transfer tools, used by companies to share often large sensitive datasets over the internet. In years past, Clop previously took credit for hacks targeting hundreds of companies who previously relied on similar file transfer tools, including Accellion, GoAnywhere, and MOVEit.