A calendar app in the Mac App Store has been mining cryptocurrency in the background in exchange for giving users additional features — and an option to opt out of mining has been broken. So far, Apple has not taken the scheduling app Calendar 2 down, even after Ars Technica informed the company that Calendar 2 has been mining virtual currency.

The app is supposed to be a buffed-up version of Apple’s Calendar app in macOS, but recently, its developer, Qbix, added extra code to mine monero, a digital coin launched in April 2014 and meant to be a more anonymous version of bitcoin, as you can’t view transactions on a public ledger. That makes Calendar 2 something of a rarity in the App Store — there don’t appear to be other mining apps in the store, let alone apps that use mining as a way to get additional value from non-paying users.

The miner runs in exchange for letting the users access more premium features. Users can opt out by keeping premium features off or paying for them through the App Store.

However, as Ars noted, the app had a bug that kept the miner running, even if users tried to opt out, and a second bug that caused the miner to consume more resources than originally intended. A user noted on Twitter that the app “ate 200% CPU until I found it and killed it. I didn’t expect a miner infection from an App Store vendor. Wow.” The app’s current rating is two out of five in the App Store, with many recent reviews docking stars because of the unwanted mining. Qbix stated that it was in the middle of publishing a fix for the bugs.

Mining programs tend to favor Monero over Bitcoin or Ethereum, as Monero has a more CPU-friendly hashing algorithm. Salon’s website asks readers if they would like to let the media outlet mine monero through readers’ unused computing power, as an alternative to looking at ads. Additionally, Monero has also become an easy target for a spate of malicious mining programs that have emerged in recent months, according to a report from Symantec in December.

While Apple doesn’t have any rules expressly banning mining apps, it wouldn’t be surprising for the company to remove such apps, given this sentence in the guidelines: “Apps should not rapidly drain battery, generate excessive heat, or put unnecessary strain on device resources.” We’ve reached out to Apple for comment.

Update March 13th, 8:35AM ET: Calendar 2 has been pulled from the Mac App Store since the time of publication. It’s not clear if this was done by its developer, Qbix, or by Apple. After publication, Qbix said it would remove the miner from its app.