Carefully crafted sequence of strings can cause a denial of service
attack on the service that parses the sequence to create a Hash
object by using the strings as keys. For instance, this
vulnerability affects web application that parses the JSON data
sent from untrusted entity.
This vulnerability is similar to CVS-2011-4815 for ruby 1.8.7. ruby
1.9 versions were using modified MurmurHash function but it's
reported that there is a way to create sequence of strings that
collide their hash values each other. This fix changes the Hash
function of String object from the MurmurHash to SipHash 2-4.