EEF-CVE-2026-82756

## Summary

Improper Encoding or Escaping of Output vulnerability in ash-project ash\_authentication\_oauth2\_server allows an unauthenticated attacker to inject arbitrary authentication parameters into the WWW-Authenticate challenge header.

BearerPlug and RequireScopePlug built the Bearer resource\_metadata="..." challenge by interpolating a resource\_metadata URL derived from the request tenant directly into the quoted value. In a multi-tenant application that sets the Ash tenant from request-controlled data (a subdomain, the Host, a path segment, or a header), a tenant containing a " closes the quoted value and appends attacker-chosen auth-params, including a second resource\_metadata URL pointing at an attacker-controlled authorization server that spec-following clients follow. Carriage returns and line feeds are rejected by Plug, so this is parameter injection within one header, not response splitting.

This issue affects ash\_authentication\_oauth2\_server: from 0.1.3 before 0.3.1.

## Configuration

Reachable only in a multi-tenant application that derives the Ash tenant from request-controlled input (subdomain, Host, path, or header) and uses BearerPlug or RequireScopePlug; the tenant reaches the challenge through the server's tenant-aware resource\_url.