When a CORS request is received, the supplied origin is compared to the whitelist. Some applications that support access from multiple origins do so by using a whitelist of allowed origins. Location='///log?key='+this.responseText ĪPPRENTICE CORS vulnerability with basic origin reflection Errors parsing Origin headers If the response contains any sensitive information such as an API key or CSRF token, you could retrieve this by placing the following script on your website: These headers state that access is allowed from the requesting domain ( ) and that the cross-origin requests can include cookies ( Access-Control-Allow-Credentials: true) and so will be processed in-session.īecause the application reflects arbitrary origins in the Access-Control-Allow-Origin header, this means that absolutely any domain can access resources from the vulnerable domain. For example, consider an application that receives the following request: One way to do this is by reading the Origin header from requests and including a response header stating that the requesting origin is allowed. So some applications take the easy route of effectively allowing access from any other domain. Maintaining a list of allowed domains requires ongoing effort, and any mistakes risk breaking functionality. Some applications need to provide access to a number of other domains. Server-generated ACAO header from client-specified Origin header

Their implementation of CORS may contain mistakes or be overly lenient to ensure that everything works, and this can result in exploitable vulnerabilities. Many modern websites use CORS to allow access from subdomains and trusted third parties. Vulnerabilities arising from CORS configuration issues CORS and the Access-Control-Allow-Origin response header.

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