"As far as we know, most databases do not seem to be affected by the incident. Unfortunately, first detailed analyses suggest that the perpetrators had access to certain customer-relevant data of sister company Credaris, whose systems are partly operated in the same server environment," it said.
Credaris is a financial brokerage.
Comparis said its website - which lets consumers compare prices for goods and services - was working normally again, but access via e-mail and customer hotline may still be limited as it works with cybersecurity specialists on a complete recovery.
It did not comment on whether the incident was linked to a ransomware attack centred on the Florida information technology firm Kaseya that scrambled the data of hundreds of small businesses worldwide. REvil, a prolific, Russia-linked cybercrime syndicate, took credit for the breach.
Comparis did not pay a ransom to regain functionality, a spokesperson said.