Executive Summary

This vulnerability exists in the Nextcloud User OIDC app where a missing signature verification allowed a malicious ID4me authority to impersonate any user. Essentially, the system did not properly verify the authenticity of login tokens, enabling attackers to bypass authentication by presenting forged tokens.

The issue affected multiple versions of Nextcloud and was fixed by adding signature validation to the ID4ME login tokens, ensuring that only properly signed tokens are accepted.

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Impact Analysis

This vulnerability can lead to an authentication bypass, allowing attackers to impersonate any user without proper credentials.

• Attackers can gain unauthorized access to user accounts.
• Sensitive data and content collaboration resources may be exposed.
• It can result in a loss of trust and potential data breaches.

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Mitigation Strategies

To mitigate this vulnerability, users should upgrade Nextcloud to one of the patched versions: 3.1.0, 4.1.0, 5.1.0, 6.4.0, or 8.3.0.

As a temporary workaround, users can disable the ID4me feature to prevent exploitation until an upgrade is possible.

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Compliance Impact

The vulnerability allows a malicious ID4me authority to impersonate any user due to missing signature verification in the User OIDC app, leading to an authentication bypass.

Such an authentication bypass can result in unauthorized access to sensitive user data, which may violate data protection requirements under regulations like GDPR and HIPAA.

Therefore, if exploited, this vulnerability could compromise the confidentiality and integrity of personal or protected health information, potentially causing non-compliance with these standards.

Upgrading to patched versions or disabling the ID4me feature is advised to mitigate this risk and maintain compliance.

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Detection Guidance

This vulnerability is related to a missing signature verification in the User OIDC app for Nextcloud, allowing a malicious ID4me authority to impersonate any user. Detection would primarily involve verifying the version of the User OIDC app installed on your Nextcloud instance.

To detect if your system is vulnerable, check the installed version of the User OIDC app. Versions before 3.1.0, 5.1.0, 6.4.0, and 8.3.0 are vulnerable.

Suggested commands to check the installed version on your Nextcloud server might include:

• Using the Nextcloud command line tool to list installed apps and their versions: `sudo -u www-data php /var/www/nextcloud/occ app:list`
• Checking the version of the user_oidc app specifically: `sudo -u www-data php /var/www/nextcloud/occ app:info user_oidc`

Additionally, monitoring authentication logs for suspicious ID4me login attempts or unexpected user impersonation events could help detect exploitation attempts, but no specific network detection commands or signatures are provided in the available resources.

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