Cross-Origin Resource Sharing (CORS) is a mechanism for relaxing the Same Origin Policy to enable communication between websites via browsers. It's already widely understood that certain CORS configurations are dangerous. In this presentation, I'll skim over the old knowledge then coax out and share with you an array of under-appreciated but dangerous subtleties and implications buried in the CORS specification. I'll illustrate each of these with recent attacks on real websites, showing how I could have used them to steal bitcoins from two different exchanges, partially bypass Google's use of HTTPS, and requisition API keys from numerous others. I'll also show how CORS blunders can provide an invaluable link in crafting exploit chains to pivot across protocols, exploit the unexploitable via server and client-side cache poisoning, and even escalate certain open redirects into vulnerabilities that are actually notable.
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