Audience Injection Attacks: A New Class of Attacks on Web-Based Authorization and Authentication Standards

We introduce audience injection attacks, a novel class of vulnerabilities that impact widely used Webbased authentication and authorization protocols, including OAuth 2.0, OpenID Connect, FAPI, CIBA, the Device Authorization Grant, and various well-established extensions, such as Pushed Authorization Requests, Token Revocation, Token Introspection, and their numerous combinations. These protocols underpin services for billions of users across diverse ecosystems worldwide, spanning low-risk applications like social logins to high-risk domains such as open banking, insurance, and healthcare.

Audience injection attacks exploit a critical weakness in a core security mechanism of these protocolsthe handling of so-called audiences in signature-based client authentication mechanisms. This vulnerability allows attackers to compromise fundamental security objectives whenever these mechanisms are utilized across two or more server endpoints. They enable the attacker to impersonate users and gain unauthorized access to their resources, even in high-security protocol families specifically designed for sensitive applications.

We responsibly disclosed these vulnerabilities to the relevant standardization bodies, which recognized their severity. In collaboration with these organizations, we developed fixes and supported a coordinated response, leading to an ongoing effort to update a dozen of standards, numerous major implementations, and far-reaching ecosystems.