CVE-2021-41166 is a vulnerability identified in the Nextcloud Android application, specifically affecting versions prior to 3.17.1. Nextcloud is a widely used self-hosted productivity platform that allows users to manage files, calendars, contacts, and other data. The Android app serves as a client interface for mobile users to access their Nextcloud instances. The vulnerability arises from incorrect default permissions (CWE-276) related to the handling of image thumbnails within the app. Specifically, an unauthorized application on the same Android device, which lacks the required MANAGE_DOCUMENTS permission, can still access and view image thumbnails of files that the user or the unauthorized app should not have permission to view. This leads to a potential sensitive information disclosure, as thumbnails may reveal image content or metadata that is meant to be protected. The issue is rooted in improper enforcement of permission checks when generating or accessing image thumbnails, allowing unauthorized apps to bypass intended access controls. The vulnerability was addressed in Nextcloud Android app version 3.17.1, which includes a patch to correctly enforce permission requirements and prevent unauthorized access to thumbnails. No known workarounds exist, and there are no reports of active exploitation in the wild. The vulnerability does not require user interaction beyond having the vulnerable app installed, and it affects all users running versions prior to 3.17.1. Since the vulnerability involves information disclosure without modification or deletion of data, it primarily impacts confidentiality. The integrity and availability of data are not directly affected by this issue.

For European organizations using Nextcloud Android clients, this vulnerability could lead to unauthorized disclosure of sensitive image data stored on their Nextcloud instances. Organizations that rely on Nextcloud for storing confidential or regulated information, such as healthcare providers, legal firms, financial institutions, and government agencies, may face privacy breaches if unauthorized apps on employee devices exploit this flaw. Although the vulnerability only exposes thumbnails rather than full images, thumbnails can still contain sensitive visual information or metadata that could aid in reconnaissance or further attacks. The risk is heightened in environments where mobile device security is less controlled, such as BYOD (Bring Your Own Device) scenarios, or where employees install untrusted apps. Since Nextcloud is popular in Europe due to data sovereignty and GDPR compliance preferences, the exposure of personal or regulated data could lead to compliance violations and reputational damage. However, the lack of known exploits and the medium severity rating suggest the immediate risk is moderate. The vulnerability does not allow remote exploitation or direct compromise of the Nextcloud server, limiting the scope to local device security contexts.

The primary mitigation is to update the Nextcloud Android app to version 3.17.1 or later, which contains the patch that corrects the permission enforcement for image thumbnails. Organizations should enforce mobile device management (MDM) policies that restrict installation of unauthorized or untrusted applications on employee devices to reduce the risk of malicious apps exploiting this vulnerability. Additionally, security teams should audit devices for the presence of outdated Nextcloud Android clients and prioritize updates. For environments where immediate app updates are not feasible, consider restricting access to Nextcloud via mobile devices or implementing endpoint security solutions that monitor and control inter-app permissions and data access. Educating users about the risks of installing untrusted apps and encouraging regular app updates can further reduce exposure. Finally, organizations should review their data classification and storage policies to minimize sensitive image data stored on mobile-accessible platforms where possible.