JRuby computes hash values without properly restricting the ability to trigger hash collisions predictably, which allows context-dependent attackers to cause a denial of service (CPU consumption) via crafted input to an application that maintains a hash table, as demonstrated by a universal multicollision attack against the MurmurHash2 algorithm, a different vulnerability than CVE-2011-4838.
https://www.131002.net/data/talks/appsec12_slides.pdf
https://bugzilla.redhat.com/show_bug.cgi?id=880671
http://www.ocert.org/advisories/ocert-2012-001.html