Rack before 1.1.3, 1.2.x before 1.2.5, and 1.3.x before 1.3.6 computes hash values for form parameters without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted parameters.
www.debian.org/security/2013/dsa-2783
www.kb.cert.org/vuls/id/903934
www.nruns.com/_downloads/advisory28122011.pdf
www.ocert.org/advisories/ocert-2011-003.html
gist.github.com/52bbc6b9cc19ce330829
github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2011-5036.yml
nvd.nist.gov/vuln/detail/CVE-2011-5036
web.archive.org/web/20120201040317/jruby.org/2011/12/27/jruby-1-6-5-1
web.archive.org/web/20130213132312/archives.neohapsis.com/archives/bugtraq/2011-12/0181.html