Description

In Apache CXF's OAuth2 Dynamic Client Registration endpoint, the authorization server accepts and stores the `scope` value supplied in the client registration request verbatim, without validating it against an AS-defined allowlist. This could lead to a client self-assigning privileged scopes at registration time. Users are recommended to upgrade to versions 4.2.3 or 4.1.8 or 3.6.12, which fix this issue.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.1",
  "pubDate": "2026-08-06T12:16:27.730Z",
  "pubdate": "2026-08-06T12:16:27.730Z",
  "executiveSummary": "Apache CXF contains an input validation vulnerability within its OAuth2 Dynamic Client Registration endpoint, categorized as an insufficient validation flaw. The vulnerability arises because the authorization server accepts and stores the scope parameter supplied in the client registration request verbatim, completely bypassing validation against an authorization server-defined allowlist. The primary impact of this security deficiency is privilege escalation, enabling malicious clients to autonomously self-assign privileged or sensitive access scopes during the initial registration phase. Affected systems include Apache CXF deployments utilizing vulnerable versions of the OAuth2 framework. The risk implication is severe, as unauthorized clients can acquire elevated operational privileges without administrative oversight. Attacker capabilities include arbitrary scope injection during client registration over the network. Exploitation requires the capability to submit standard dynamic client registration requests to the vulnerable endpoint, leveraging the lack of server-side validation on the requested scope parameter.",
  "technicalDetails": "The root cause of the vulnerability resides in the lack of strict input sanitization and enforcement of authorization server-defined scope policies within the OAuth2 Dynamic Client Registration endpoint of Apache CXF. When a client initiates a dynamic registration request, the vulnerable component parses the incoming JSON payload containing the scope parameter. Rather than validating the requested string values against a predefined allowlist of permitted scopes authorized for dynamic registration, the authorization server blindly accepts the supplied values. The system then persists these arbitrary scopes into the underlying storage mechanism associated with the newly registered client entity. The vulnerable component is the OAuth2 Dynamic Client Registration module within Apache CXF. The affected versions include releases prior to 4.2.3, 4.1.8, and 3.6.12. Authentication and privilege requirements for initial exploitation depend on the configuration of the dynamic registration endpoint; however, if open dynamic registration is permitted, unauthenticated or low-privileged actors can submit registration payloads across the network. The payload behavior involves injecting high-privilege or administrative scope strings (such as administrative read/write access identifiers) directly into the scope field of the client registration request. The step-by-step attack flow proceeds as follows: First, the malicious actor formulates an HTTP POST request targeted at the Apache CXF OAuth2 Dynamic Client Registration endpoint. Second, the attacker populates the scope parameter within the registration body with arbitrary, highly privileged scopes that should normally be restricted by the authorization server. Third, the Apache CXF server processes the request, fails to validate the parameter against an allowable set, and successfully registers the client while associating the malicious scopes. Fourth, the attacker utilizes the newly registered client credentials to request OAuth2 access tokens. Finally, because the authorization server honors the persistently stored scopes, the issued access token grants the attacker unauthorized access to protected resources and administrative functions associated with the self-assigned privileged scopes, resulting in complete authorization bypass."
}