Update: "Our servers are under attack again," Ubisoft says. "Some gamers are experiencing trouble signing in. We're working on it and will keep you posted."

Added the company: "We're happy to say ACII & SH5 are withstanding the efforts to crack them. We see the rumors but still confirm no valid cracked versions exist."

Original:

Further reinforcing outspoken community concerns regarding Ubisoft's new PC DRM scheme, a denial of service attack on the company's authentication servers left a group of gamers unable to play certain titles for roughly seven hours on Sunday.

Ubisoft's new DRM scheme made its debut with Assassin's Creed II PC and Silent Hunter 5 PC, requiring players to have an active internet connection at all times and halting the game if its connection to Ubisoft's online authentication server is cut.

As the company explained this morning through its Twitter:

Apologies to anyone who couldn't play ACII or SH5 yesterday. Servers were attacked which limited service from 2:30pm to 9pm Paris time

95% of players were not affected, but a small group of players attempting to open a game session did receive denial of service errors.

All servers were up and running but the attacks had the effect of blocking new requests from legitimate users

Prior to that explanation, community manager Ubi.Vigil admitted "clearly the extended downtime and lengthy login issues are unacceptable, particularly as I've been told these servers are constantly monitored" and blamed the issues on "exceptional demand."

Shortly after the authentication service premiered, reports of hacker groups circumventing the controversial always-online requirement. Ubisoft quickly dismissed those claims, explaining "a cracked version of [AC2] and [SH5]...is not complete."

However, Ubisoft has said it would consider patching offline support into a game should a functioning crack or workaround become widely available, telling PC Gamer that "it's a valid part of a varied set of options that we would consider."

The company has also pledged to patch offline support into games should the authentication service be permanently discontinued or shut down.