According to PCGamer, this new flaw was spotted by @SkeleMann on Twitter before being then mentioned on Reddit, including by the moderators of the section devoted to Dark Souls 3. The latter would be particularly easy to exploit and could cause serious damage to the players targeted (recovery of confidential information, possibility of launching a remote program, etc).
When contacted, Bandai Namco and FromSoftware ended up taking all of the PC servers of Dark Souls 3, Dark Souls 2, and Dark Souls: Remastered, according to an official tweet, while the Prepare to Die Edition of the first part should soon follow. A precautionary measure that will allow the editor to investigate.
For their part, those responsible for mod Blue Sentinel, which was already intended to correct security concerns, announced that they had updated it to deal with this new flaw.