More than 10,000 website databases have been taken hostage in recent days by attackers who are demanding hefty ransoms for the data to be restored, a security researcher said Friday.

The affected data is created and stored by the open source MongoDB database application, according to researchers who have been tracking the ongoing attacks all week. On Monday, Victor Gevers, co-founder of the GDI Foundation, reported finding 200 such databases that had been deleted. By Tuesday, John Matherly, founder of the Shodan search engine increased the estimate to 2,000 databases, and by Friday, fellow researcher Niall Merrigan updated the count to 10,500.

Misconfigured MongoDB databases have long exposed user password data and other sensitive information, with the 2015 breach of scareware provider MacKeeper that exposed data for 13 million users being just one example. With the surge in ransomware-style attacks—which threaten to permanently delete or encrypt data unless owners pay a fee—hacks targeting MongoDB are seeing a resurgence. Many poorly secured MongoDB databases can be pinpointed using Shodan, which currently shows 99,000 vulnerable instances

When the ransom-style attacks targeting MongoDB databases first came to light, they were mostly carried out by someone using the online handle Harak1r1. The individual or group was deleting vulnerable databases and promising to restore them if owners paid around $200 in Bitcoin. Over time, other attackers have taken part in similar attacks, in some cases replacing a rival's ransom demand with one of their own. A list of the best-known attackers is here. In all, the attackers have compromised about 10,500 databases. Promises to restore the databases in return for a ransom payment are dubious, since there's no evidence the attackers copied the data before deleting it.

MongoDB maintainers have responded to the reports with a blog post explaining how to detect and respond to attacks. People who administer websites that use MongoDB should ensure they're avoiding common pitfalls by, among other things, blocking access to port 27017 or binding local IP addresses to limit access to servers.