![]() What could possibly go wrong? I decided to assess a few bug bounty sites and find out. So, plenty of websites derive allowed origins from user input. Other servers will only send CORS headers if they receive a request containing the Origin header, making associated vulnerabilities extremely easy to miss. If you see a HTTP response with any Access-Control-* headers but no origins declared, this is a strong indication that the server will generate the header based on your input. This is the single most common CORS vulnerability. In other words, using a wildcard effectively disables the Allow-Credentials header.Īs a result of these limitations, many servers programmatically generate the Access-Control-Allow-Origin header based on the user-supplied Origin value. This exception is mentioned in the specification, and also backed up by Mozilla’s documentation: When responding to a credentialed request, server must specify a domain, and cannot use wild carding Then you’ll get the following error in your browser console: Cannot use wildcard in Access-Control-Allow-Origin when credentials flag is true. If you try to disable the SOP entirely and expose your site to everyone by using the following terrifying looking header combination: Access-Control-Allow-Origin: * ![]() There's a hidden safety catch in CORS, too. You might also want to use a wildcard to trust all your subdomains, by specifying something like: Access-Control-Allow-Origin: *.īut that won't work either. However, no browsers actually support this. What if you need to trust multiple origins? The specification suggests that you can simply specify a space-separated list of origins, eg: Access-Control-Allow-Origin: ![]() This creates a trust relationship - an XSS vulnerability on is bad news for this site. The server can enable credential transmission using the following header: Access-Control-Allow-Credentials: true By default this request will be issued without cookies or other credentials, so it can’t be used to steal sensitive user-specific information like CSRF tokens. This permits the listed origin (domain) to make visitors’ web browsers issue cross-domain requests to the server and read the responses - something the Same Origin Policy would normally prevent. Websites enable CORS by sending the following HTTP response header: Access-Control-Allow-Origin: In this post I’ll show how to critically examine CORS configurations from a hacker’s perspective, and steal bitcoins. It’s widely understood that certain CORS configurations are dangerous, but some associated subtleties and implications are easily misunderstood. It's frequently used by web APIs in particular, but in a modern complex website it can turn up anywhere. ![]() What is CORS? (Cross Origin Resource Sharing)Ĭross-Origin Resource Sharing ( CORS) is a technology used by websites to make web browsers relax the Same Origin Policy, enabling cross-domain communication between different websites. I also recommend our free interactive CORS labs. If you have time (or struggle to understand anything) I highly recommend checking out the slides and watching the video. This is a greatly condensed version of my AppSec USA talk. In this post, I'll show how to identify and exploit misconfigured CORS. (or CORS misconfiguration misconceptions)
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