CVE-2025-59716 is a vulnerability identified in ownCloud Guests prior to version 0.12.5 that permits unauthenticated user enumeration through the /apps/guests/register/{email}/{token} endpoint. The root cause is insufficient validation of the token parameter within the showPasswordForm function. When an attacker submits a request with an email and token, the server's response differs depending on whether the email corresponds to a valid pending guest user or not. This discrepancy allows attackers to confirm the existence of guest user accounts without authentication or user interaction. The vulnerability is classified under CWE-200 (Information Exposure) and CWE-203 (Information Exposure Through Discrepancy). The CVSS v3.1 base score is 5.3, reflecting a medium severity level, with an attack vector of network (AV:N), low attack complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), and limited confidentiality impact (C:L). No integrity or availability impacts are noted. No patches or exploits are currently documented, but the vulnerability poses a risk by enabling attackers to gather valid email addresses of guest users, which can be leveraged for targeted phishing or social engineering attacks. Organizations relying on ownCloud Guests for collaboration should assess their exposure and apply updates or mitigations promptly.

The primary impact of CVE-2025-59716 on European organizations is the exposure of valid guest user email addresses through unauthenticated enumeration. This information disclosure can facilitate targeted phishing campaigns, social engineering, or brute-force attacks against identified users. While the vulnerability does not directly compromise data confidentiality, integrity, or system availability, it lowers the barrier for attackers to identify legitimate users and craft more convincing attacks. Organizations handling sensitive or regulated data, such as those in finance, healthcare, or government sectors, may face increased risk of credential compromise or unauthorized access attempts. Additionally, the reputational damage from successful phishing attacks exploiting this vulnerability could be significant. The lack of known exploits in the wild reduces immediate risk but does not eliminate the potential for future exploitation. European entities using ownCloud Guests in their collaboration infrastructure should consider this vulnerability a moderate threat that requires timely remediation to prevent reconnaissance activities that could lead to more severe attacks.

To mitigate CVE-2025-59716, European organizations should: 1) Upgrade ownCloud Guests to version 0.12.5 or later where the vulnerability is fixed. 2) If immediate patching is not feasible, implement web application firewall (WAF) rules to detect and block suspicious requests targeting the /apps/guests/register/{email}/{token} endpoint, especially those exhibiting enumeration patterns. 3) Monitor web server logs for repeated requests with varying email addresses to identify potential enumeration attempts. 4) Enforce strong email verification and multi-factor authentication (MFA) for guest users to reduce the impact of compromised credentials. 5) Educate users about phishing risks and encourage vigilance against unsolicited emails. 6) Limit exposure of the guest registration endpoint by restricting access via network segmentation or VPN where possible. 7) Regularly review and audit guest user accounts to remove stale or unused entries, minimizing the attack surface. These targeted measures go beyond generic advice by focusing on detection, access control, and user awareness specific to this vulnerability and the ownCloud Guests environment.