Two top cryptocurrency exchanges have reported distributed denial-of-service (DDoS) attacks on their systems in the last 24 hours.

OKEx, which says it is based in Malta, has apparently suffered two attacks, one Thursday and one Friday, according to its CEO, Jay Hao. On his Weibo account Friday, Hao said the first attack was of a 200 gigabyte severity, but the second had doubled to 400 GB.

Despite the attacks, the exchange’s service is “largely unaffected,” he said. OKEx’s Twitter account has no warnings of interruptions this morning, apart from a brief interlude for a systems upgrade for its futures and options trading facility.

Apparently more severely affected is Bitfinex, which reported a suspected DDoS attack on Twitter Friday morning (UTC time).

The Hong Kong-headquartered platform’s trading services were offline for a period of under an hour, but at press time Bitfinex says services are back to normal.

Bitfinex Chief Technology Officer Paulo Ardoino told CoinDesk later that the “attacker tried to concurrently exploit several platform features to increase load in the infrastructure.”

“We use a variety of different prevention mechanisms to guard against such a Distributed-Denial-of-Service (DDoS) attack. Still, the huge number of different IP addresses used and the sophisticated crafting of the requests towards our API v1 exploited an internal inefficiency in one of our non-core process queues,” Ardoino explained.

With the need to react quickly to avert an escalation in damage, the exchange took the decision to go into maintenance. The CTO said it was “not due to the inability of the platform to resist, rather it was a decision taken in order to quickly bring in the countermeasures and patch for all similar attacks.”

A DDoS attack attempts to overwhelm the hosting servers of an online service, using a flood of fake traffic from multiple computers to disrupt normal activity. A severe attack can take a service offline, or may cause slow responses for website users.

It’s not clear why two major exchanges have been targeted in this way at roughly the same time, or what the attackers hope to gain.