The mod_pubsub module (mod_pubsub.erl) in ejabberd 2.1.8 and 3.0.0-alpha-3 allows remote authenticated users to cause a denial of service (infinite loop) via a stanza with a publish tag that lacks a node attribute.
https://support.process-one.net/browse/EJAB-1498
http://www.process-one.net/en/ejabberd/release_notes/release_note_ejabberd_2.1.9
http://www.openwall.com/lists/oss-security/2011/11/19/2