CVE-2026-59354 in Spring Security
Summary
by MITRE • 08/27/2026
In versions of Spring Security's OAuth2 Authorization Server module 7.0.0 through 7.0.4, when Dynamic Client Registration is explicitly enabled, the registration endpoint performs insufficient validation of certain client metadata fields supplied by the registering client. An attacker who possesses a valid Initial Access Token can register a malicious client with crafted metadata, which, depending on server configuration and how the metadata is later rendered or used, may result in Stored Cross-Site Scripting (XSS), Privilege Escalation, or Server-Side Request Forgery (SSRF).
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Analysis
by VulDB Data Team • 08/27/2026
The vulnerability identified in Spring Security's OAuth2 Authorization Server module versions 7.0.0 through 7.0.4 stems from a critical lack of input validation within the Dynamic Client Registration endpoint when this feature is explicitly enabled. In standard OAuth2 workflows, dynamic client registration allows applications to self-register with an authorization server without manual intervention by administrators. While this enhances developer experience and operational agility, it introduces significant security risks if not properly constrained. The core technical flaw lies in the insufficient sanitization of specific metadata fields provided during the registration process. These fields are typically intended for descriptive or configuration purposes but are stored directly into the system's persistent storage without adequate filtering against malicious payloads. This oversight creates a direct pathway for attackers to inject arbitrary content that can be executed later when the data is retrieved and rendered by the application or associated services.
From an operational perspective, this vulnerability allows any actor possessing a valid Initial Access Token to register a rogue client entity with crafted metadata. The severity of the impact depends heavily on how the authorization server subsequently processes and displays these stored values. If the metadata is reflected in administrative dashboards, user interfaces, or logs without proper encoding, it can lead to Stored Cross-Site Scripting attacks. In such scenarios, an attacker could inject JavaScript payloads that execute in the context of a privileged administrator's browser session when they view the client details. This execution environment grants the malicious script access to sensitive cookies, session tokens, and other confidential data associated with the administrative interface, potentially leading to full account compromise or further lateral movement within the organization's infrastructure.
Beyond cross-site scripting, the improper validation extends to risks involving Server-Side Request Forgery and Privilege Escalation depending on server configuration. If certain metadata fields are used internally by the authorization server to construct URLs for callbacks, token endpoints, or other integration points, an attacker could supply a malicious URL pointing to an internal service or external resource controlled by the adversary. This enables SSRF attacks where the vulnerable server makes requests on behalf of the client, potentially exposing internal network services that are not directly accessible from the internet. Furthermore, if metadata fields influence access control decisions or scope assignments during registration, crafted inputs could manipulate these parameters to escalate privileges beyond what was originally intended for the registered application. This misconfiguration allows a low-privileged entity to gain elevated permissions, undermining the principle of least privilege central to secure OAuth2 implementations.
This vulnerability aligns with CWE-79, which describes Improper Neutralization of Input During Web Page Generation known as Cross-Site Scripting, and CWE-918, Server-Side Request Forgery (SSRF). The attack vector is classified under MITRE ATT&CK technique T1190, Exploit Public-Facing Application, specifically leveraging the dynamic registration feature which acts as a public-facing interface for client onboarding. To mitigate this risk, organizations must ensure that all inputs to the Dynamic Client Registration endpoint are strictly validated against an allowlist of expected characters and formats. Developers should implement robust output encoding mechanisms whenever any part of the registered metadata is rendered in HTML or other executable contexts. Additionally, restricting the fields available during dynamic registration to only those absolutely necessary can reduce the attack surface significantly. Upgrading to a patched version of Spring Security that addresses these validation gaps is essential for maintaining the integrity and security posture of OAuth2-based authentication systems.