This CORS misconfiguration could allow an attacker to perform AJAX queries to the vulnerable website from a malicious page loaded by the victim’s user agent.
In order to perform authenticated AJAX queries, the server must specify the header “Access-Control-Allow-Credentials: true” and the “Access-Control-Allow-Origin” header must be set to null or the malicious page’s domain. Even if this misconfiguration doesn’t allow authenticated AJAX requests, unauthenticated sensitive content can still be accessed (e.g intranet websites).
A malicious page can belong to a malicious website but also a trusted website with flaws (e.g XSS, support of HTTP without TLS allowing code injection through MITM, etc).
Solution
If a web resource contains sensitive information, the origin should be properly specified in the Access-Control-Allow-Origin header. Only trusted websites needing this resource should be specified in this header, with the most secured protocol supported.
