CVE-2025-59716 is a vulnerability identified in ownCloud Guests, a component used to manage guest user access in ownCloud environments. The vulnerability affects versions prior to 0.12.5 and involves an unauthenticated user enumeration issue via the endpoint /apps/guests/register/{email}/{token}. The root cause is insufficient validation of the supplied token parameter in 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 in server response enables an attacker to confirm the existence of guest user accounts without needing authentication or user interaction. Although this vulnerability does not allow direct access to user data or account takeover, it leaks information about user presence, which can be leveraged for targeted phishing, social engineering, or further attacks. No CVSS score has been assigned yet, and no known exploits have been reported in the wild. The vulnerability is classified as a security weakness that compromises confidentiality to a limited extent by revealing user enumeration information. The issue is particularly relevant for organizations relying on ownCloud Guests for external collaboration, as it exposes guest user registration details to unauthenticated attackers.

For European organizations, the primary impact of CVE-2025-59716 is the exposure of guest user presence information, which can be exploited to conduct targeted phishing or social engineering campaigns. Organizations handling sensitive or regulated data, such as those in finance, healthcare, or government sectors, may face increased risk if attackers use enumerated email addresses to craft convincing attacks. While the vulnerability does not directly compromise data confidentiality, integrity, or availability, it undermines user privacy and can facilitate subsequent attacks that may lead to more severe breaches. The impact is heightened in environments where guest access is widely used for collaboration with external partners or clients. Additionally, organizations with compliance obligations around data privacy (e.g., GDPR) must consider the implications of unauthorized disclosure of user information. The lack of authentication requirement and ease of exploitation increase the likelihood of reconnaissance activities targeting European entities using ownCloud Guests.

To mitigate CVE-2025-59716, organizations should prioritize updating ownCloud Guests to version 0.12.5 or later once the patch is released by the vendor. Until an official patch is available, administrators can implement monitoring and alerting on access patterns to the /apps/guests/register endpoint to detect unusual or repeated requests indicative of enumeration attempts. Rate limiting or IP blocking mechanisms can be applied to reduce the attack surface. Additionally, reviewing and tightening guest user registration workflows and token validation logic can help minimize information leakage. Organizations should educate users and administrators about the risks of user enumeration and encourage vigilance against phishing attempts that may leverage enumerated email addresses. Finally, conducting regular security assessments and penetration tests on ownCloud deployments will help identify and remediate similar vulnerabilities proactively.