The flaw lies in improper encoding or escaping of the WWW‑Authenticate header. The server constructs a Bearer challenge that includes a resource_metadata URL derived from the tenant identifier. Because the tenant value is incorporated directly into a quoted string without sanitization, a tenant name containing a double quote can close the quote and append attacker‑chosen parameters. The attacker can thus inject arbitrary authentication directives, including a resource_metadata pointing to a malicious authorization server. This type of header injection can mislead OAuth‑aware clients into following a forged authorization flow, potentially exposing user credentials or enabling session hijacking. The weakness corresponds to CWE‑116: Improper Encoding or Escaping.

The vulnerability affects the open‑source project ash‑authentication‑oauth2‑server, version range 0.1.3 through 0.3.0 inclusive. Any deployment of this library that has not updated to 0.3.1 or later is susceptible. The issue was identified in the BearerPlug and RequireScopePlug components that build the challenge string.

The attack can be launched by any unauthenticated client that controls the tenant value, such as subdomains, host headers, path segments, or custom headers. Because the vulnerability occurs in a response header, a malicious actor could influence the authentication flow without prior authentication. The exploit requires only a crafted HTTP request, so the risk is medium to high for applications exposed to the internet, particularly in multi‑tenant configurations. The CVSS score of 6.3 indicates significant impact, while the lack of an EPSS score means that no specific exploitation likelihood estimate is available. The vulnerability is not listed in the CISA KEV catalog, indicating no wide‑scale exploitation has been reported yet, but the potential for misuse remains clear.