A hacker has exploited a vulnerability to steal $100 million from Harmony’s Horizon Bridge, which allows users to transfer their crypto assets from one blockchain to another.

Harmony, the U.S. crypto startup behind Horizon, said in a blog post on Friday that it was notified of a “malicious attack” on its proprietary Horizon blockchain bridge on Thursday. Blockchain bridges, also known as cross-chain bridges, facilitate communication between different blockchains and allow users to send assets from one chain to the other. Using Harmony’s Horizon bridge, for example, users can move assets — including tokens, stablecoins, and NFTs — between Ethereum, Binance Smart Chain and Harmony blockchains.

Harmony said the culprit of the attack — which the company singled out in a tweet — stole close to $100 million in cryptocurrency from its blockchain bridge.

According to blockchain analysis company Elliptic, a variety of crypto assets were taken, including Ethereum, Binance Coin, Tether, USD Coin and Dai. Elliptic added that the stolen tokens have now been swapped for Ethereum using decentralized exchanges — a "commonly seen technique with these hacks," it said.

Harmony said in its blog post that immediately following the attack, multiple cybersecurity partners, exchange partners and the FBI were notified and requested to assist with an investigation in identifying the culprit and retrieving stolen assets. “Further, the team has attempted communication with the hacker with an embedded message in a transaction to the culprit’s address,” the blog post read.

Harmony added that it had stopped the Horizon bridge to prevent further transactions. Harmony's bridge for bitcoin was unaffected.