CVE-2026-50630: Apache CXF OAuth2 CRLF Injection Vulnerability

Technical summary

CVE-2026-50630 is a CRLF (Carriage Return/Line Feed) injection vulnerability in the OAuth2 AuthorizationUtils class of Apache CXF. The vulnerability stems from unsafe concatenation of the 'realm' parameter into the WWW-Authenticate response header without sanitization of CR (\r, 0x0D) and LF (\n, 0x0A) characters. An attacker controlling the realm value can inject newline sequences to introduce additional HTTP headers, terminate headers prematurely, or split the HTTP response, violating RFC 7230 compliance. The CVSS 3.1 score of 6.5 (Medium) reflects network accessibility, low attack complexity, no privilege requirement, and combined low confidentiality and integrity impact.

Business impact

Successful exploitation can enable cache poisoning attacks where injected headers cause downstream proxies or CDNs to cache poisoned responses. Attackers may inject Set-Cookie headers to establish unauthorized sessions, inject X-Frame-Options bypass headers, or use response splitting to conduct phishing attacks. In federated OAuth2 environments, this could compromise trust relationships between identity providers and service providers, affecting user authentication flows at scale.

Affected systems

Apache CXF versions prior to 4.2.2 (current branch) and 4.1.7 (legacy branch) are vulnerable. Organizations using CXF for OAuth2 authentication, particularly in microservices architectures or API gateway deployments, should prioritize assessment. The vulnerability applies regardless of deployment model (on-premises, cloud) or whether CXF is directly embedded or used as a library.

Exploitability

Exploitation requires the attacker to control or influence the realm parameter passed to the OAuth2 authentication mechanism. In typical deployments, this occurs when realm values are derived from user-supplied input, request headers, or insufficiently validated configuration. Network access is required but no authentication or user interaction is necessary. The attack surface depends on whether realm is externally controllable in your deployment; many standard configurations may have limited exposure, but custom implementations with dynamic realm assignment are at higher risk.

Remediation

Upgrade Apache CXF to version 4.2.2 or 4.1.7 (or later) immediately. These releases include sanitization of CRLF characters in the realm parameter. For environments unable to upgrade immediately, implement input validation and canonicalization of realm values at the application layer, rejecting any realm containing CR or LF bytes. Additionally, deploy HTTP response header validation at reverse proxies or WAF layers to block or sanitize injected headers.

Patch guidance

Apply the following patches: Apache CXF 4.2.2 (for the main branch) or 4.1.7 (for the legacy branch). Verify patch installation by reviewing the vulnerability fix in the AuthorizationUtils class to confirm CRLF character filtering. Test OAuth2 authentication flows post-patching to ensure no functional regression. If you maintain a custom fork or heavily modified CXF deployment, engage your security team to backport the sanitization logic.

Detection guidance

Monitor HTTP response headers for anomalous embedded newlines or unexpected headers appearing after WWW-Authenticate. Implement logging of realm parameter values in OAuth2 authentication requests and alert on realm values containing %0D, %0A, \r, or \n sequences. Review proxy and cache logs for response splitting indicators (e.g., duplicate headers, fragmented responses from the same backend server). Conduct traffic analysis between OAuth2 clients and your CXF endpoints to identify malformed realm injection attempts.

Why prioritize this

Although CVSS 6.5 rates this as Medium severity, context elevates urgency: OAuth2 is often the critical path for authentication and authorization in modern architectures. Successful exploitation can compromise multiple dependent systems through cache poisoning or session hijacking. The attack surface (realm parameter control) should be evaluated in your specific deployment, but the widespread use of OAuth2 and the cascading trust implications in federated environments warrant prioritization above the base CVSS suggests.

Risk score, explained

CVSS 3.1 score 6.5 reflects: Network-accessible vulnerability (AV:N), low attack complexity (AC:L), no privilege required (PR:N), no user interaction (UI:N), confidentiality impact (C:L) from reading injected content or poisoned responses, and integrity impact (I:L) from injecting false headers, with no availability impact (A:N). The score does not account for ecosystem-wide risk in OAuth2 ecosystems or cascading impacts; your organization's criticality and trust dependencies should influence remediation speed beyond the base score.

Frequently asked questions