CVE-2026-82756 in ash_authentication_oauth2_server
Summary
by MITRE • 09/08/2026
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.
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Analysis
by VulDB Data Team • 09/08/2026
The vulnerability identified in the ash_authentication_oauth2_server library represents a critical flaw in how authentication challenges are constructed and transmitted to clients within an OAuth 2.0 framework. Specifically, this is an improper encoding or escaping of output issue that allows unauthenticated attackers to manipulate the WWW-Authenticate header sent by the server. In standard OAuth 2.0 operations, when a client requests access to a protected resource without valid credentials, the server responds with a HTTP 401 Unauthorized status and includes a WWW-Authenticate header containing challenge parameters. This vulnerability exploits the mechanism used by BearerPlug and RequireScopePlug components, which construct these challenges by interpolating dynamic values directly into quoted strings within the header value.
The technical root cause lies in the direct interpolation of request-controlled data into the resource_metadata parameter without adequate sanitization or escaping. In multi-tenant applications utilizing this library, the tenant identifier is often derived from HTTP request attributes such as subdomains, host headers, path segments, or custom headers. When an attacker controls any part of this input and includes a double quotation mark character within it, they can prematurely close the quoted value assigned to resource_metadata. This breaks the syntactic structure of the header field, allowing subsequent characters provided by the attacker to be interpreted as new authentication parameters rather than part of the original string literal.
The operational impact of this vulnerability is significant because it enables parameter injection that redirects authentication flows. By closing the initial quote and appending malicious parameters, an attacker can specify a second resource_metadata URL pointing to an authorization server under their control. Compliant OAuth 2.0 clients will parse these injected parameters and attempt to authenticate against the attacker-controlled endpoint. This facilitates phishing attacks where users are redirected to fake login pages designed to harvest credentials or session tokens. Furthermore, because Plug frameworks typically reject carriage returns and line feeds in header values, this attack is confined to single-header parameter injection rather than HTTP response splitting, but it remains highly effective for credential theft and man-in-the-middle scenarios within the authentication handshake.
From a classification perspective, this vulnerability aligns with CWE-79: Improper Neutralization of Input During Web Page Generation (Common Weakness Enumeration), specifically relating to output encoding failures in header fields. It also maps to MITRE ATT&CK techniques related to credential harvesting and phishing via deceptive redirects. The flaw is particularly dangerous in multi-tenant environments where tenant isolation relies on correct parsing of authentication challenges, as compromising one tenant's challenge mechanism can potentially expose the entire system to unauthorized access if clients blindly follow the injected metadata URLs.
Mitigation strategies must focus on strict input validation and output encoding. Developers should ensure that any data derived from HTTP requests used in header values is strictly validated against a whitelist of allowed characters or properly escaped before being interpolated into strings. Upgrading ash_authentication_oauth2_server to version 0.3.1 or later resolves this issue as the maintainers have implemented proper escaping mechanisms for special characters within the challenge headers. Until an upgrade is performed, application-level workarounds should include validating tenant identifiers to reject any input containing quotation marks and ensuring that all dynamic values inserted into HTTP headers are encoded according to RFC 7230 standards for field content.