The vulnerability in RESTEasy's CorsFilter occurs when it is configured to allow all origins using a wildcard ("*"). In this state, the filter reflects the request's Origin header back in the Access-Control-Allow-Origin response header while also setting Access-Control-Allow-Credentials to true. This combination violates CORS security policies by allowing malicious websites to make credentialed cross-origin requests and access authenticated responses from a victim's session. The vulnerability affects RESTEasy versions >=0 and <6.2.19.Final, and >=7.0.0.Alpha1 and <7.0.5.Final. Red Hat's advisory states that no currently supported Red Hat products are affected by this issue.

The vulnerability allows an attacker controlling a malicious website to perform credentialed cross-origin requests and read sensitive authenticated responses from a victim's browser session, resulting in a loss of confidentiality. There is no indication of impact on integrity or availability. No known exploits in the wild have been reported.

Red Hat has determined that this vulnerability does not affect any currently supported Red Hat products, so no action is required for Red Hat users. For other users of RESTEasy, it is recommended to avoid configuring CorsFilter with a wildcard origin combined with Access-Control-Allow-Credentials set to true. Check the official RESTEasy security advisories and update to versions 6.2.19.Final or later, or 7.0.5.Final or later, where this issue is fixed.