A group calling itself Bitpico has bragged about attacking the LN (Lightning Network). The group claims it is stress testing the network and the LN developers responded that they are analyzing and trying to close any attack vectors before the currency is used more.
It is almost a traditional DOS attack where nodes were flooded with false transactions to overwhelm them so no real transactions could get through (similar to a web attack that opens frivolous connections to overwhelm the server).
Inevitably almost all networks go through this and it really is a typical cat and mouse game in any public, permissionless blockchains. It is really an IT security nightmare where no one is authenticated or vetted whatsoever. There is little disincentive for organized and well-funded groups not to attack blockchains if they have enough motivation, and clearly many do.