This seems to be the issue, a code injection using custom emojis. Apparently Lemmy is letting some unsanitized data in, which means that Little Bobby Tables strikes again. Somebody was afraid the attackers had seized control of the whole Lemmy network but federation helps hinder the damage here, only individual instances are affected and they'd have to attack each one independently, a single server service would be fully affected in this case.
Beehaw has fully taken down the server as a preventive measure, i imagine they won't be the last ones doing this.