DoS vulnerability in REXML. Ruby’s REXML library is susceptible to the “billion laughs” denial of service attack where recursively nested entities expand a single entitity reference to a billion characters (kind of like the exploding zip file attack). Rails applications that process user-supplied XML should apply the monkey-patch ASAP; a proper gem update is forthcoming.
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The "billion laughs" trick is one of my most favorite attack vectors EVAR.
Thanks for helping raise the awareness.
Ironically if you click on the example xml with firefox, it crashes. So these billion laugh attacks are surprisingly wide spread.