CVE-2022-39338 is a vulnerability identified in the user_oidc OpenID Connect user backend component for Nextcloud, specifically affecting versions prior to 1.2.1. The root cause of the vulnerability is improper input validation (CWE-20) of discovery URLs used by the user_oidc backend. This flaw allows an attacker to inject malicious scripts that can be stored and later executed in the context of the Nextcloud web application, constituting a stored cross-site scripting (XSS) vulnerability (CWE-79). However, the impact of this vulnerability is somewhat mitigated by the restrictive Content Security Policy (CSP) applied on the affected endpoint, which limits the execution of injected scripts. Furthermore, exploitation has only been demonstrated in the Safari web browser, indicating a browser-specific attack vector. The vulnerability does not require authentication to exploit but does require the victim to use Safari to trigger the XSS payload. The vulnerability has been addressed in user_oidc version 1.2.1, and users are strongly advised to upgrade to this or later versions to remediate the issue. For users who cannot upgrade immediately, it is recommended to avoid using Safari when interacting with the affected Nextcloud instance to reduce the risk of exploitation. There are no known exploits in the wild at this time, and the vulnerability was publicly disclosed on November 25, 2022.

For European organizations, the primary impact of this vulnerability lies in the potential for stored XSS attacks that could lead to session hijacking, unauthorized actions on behalf of users, or theft of sensitive information accessible through the Nextcloud platform. Given Nextcloud's widespread adoption in Europe, especially among government agencies, educational institutions, and enterprises valuing data sovereignty, exploitation could undermine user trust and data confidentiality. Although the restrictive CSP and browser-specific nature limit the attack surface, organizations with users relying on Safari browsers remain vulnerable. The vulnerability could be leveraged as part of a broader attack chain, potentially facilitating lateral movement or privilege escalation within an organization's infrastructure. The absence of known exploits reduces immediate risk, but the presence of a fix and clear mitigation steps indicates the necessity for prompt action to prevent future exploitation. The impact on integrity and availability is limited, as the vulnerability primarily targets confidentiality and user session integrity through XSS.

1. Immediate upgrade of the user_oidc backend to version 1.2.1 or later is the most effective mitigation and should be prioritized in patch management cycles. 2. For organizations unable to upgrade promptly, implement user awareness campaigns advising users to avoid using the Safari browser when accessing Nextcloud services until the patch is applied. 3. Review and strengthen Content Security Policy configurations to ensure they are as restrictive as possible, potentially adding nonce or hash-based script allowances to further reduce XSS risks. 4. Conduct thorough input validation and sanitization audits on all user-supplied data within Nextcloud customizations or integrations to prevent similar vulnerabilities. 5. Monitor web server and application logs for unusual or suspicious requests targeting the user_oidc discovery URLs, particularly from Safari user agents, to detect attempted exploitation. 6. Employ web application firewalls (WAFs) with rules tuned to detect and block XSS payloads targeting Nextcloud endpoints. 7. Encourage the use of browser security features such as disabling JavaScript on untrusted sites or using browser extensions that mitigate XSS risks, especially for users who must use Safari.