CVE-2021-43863 is a security vulnerability identified in the Nextcloud Android application, specifically affecting versions prior to 3.18.1. Nextcloud is a widely used self-hosted productivity platform that allows users to manage files, calendars, contacts, and other data. The Android client uses content providers to manage and expose data internally within the device. This vulnerability involves an SQL injection flaw (CWE-89) in the FileContentProvider component, which improperly neutralizes special elements used in SQL commands. Additionally, the DiskLruImageCacheFileProvider component suffers from insufficient permission control, allowing unauthorized access to data. These weaknesses enable malicious applications installed on the same Android device to bypass the Android permission system and access or manipulate Nextcloud data without user consent. Exploitation does not require network access but depends on the presence of a malicious app on the device. The vulnerability was patched in version 3.18.1 of the Nextcloud Android app, and no known workarounds exist aside from upgrading. There are no known exploits in the wild as of the published date, but the flaw poses a significant risk to data confidentiality and integrity on affected devices.

For European organizations, this vulnerability presents a risk primarily to employees and users who utilize the Nextcloud Android app on their mobile devices. Since Nextcloud is popular among privacy-conscious users and organizations that prefer self-hosted solutions, especially in Europe where data sovereignty and GDPR compliance are critical, unauthorized access to Nextcloud data could lead to breaches of sensitive corporate or personal information. The SQL injection flaw could allow malicious apps to extract or manipulate stored data, potentially compromising confidentiality and integrity. The insufficient permission control further exacerbates the risk by allowing unauthorized data access. While the vulnerability does not directly affect availability, the breach of confidentiality and integrity could lead to regulatory penalties under GDPR, reputational damage, and loss of trust. The threat is heightened in sectors with strict data protection requirements such as finance, healthcare, and government institutions. Since exploitation requires a malicious app on the same device, the risk is higher in environments where device security hygiene is poor or where users install untrusted applications.

The primary and most effective mitigation is to upgrade the Nextcloud Android app to version 3.18.1 or later, where the vulnerability has been patched. Organizations should enforce mobile device management (MDM) policies that restrict installation of untrusted or unauthorized applications to reduce the risk of malicious apps co-residing on devices. Employing app whitelisting and regular audits of installed applications can further mitigate risk. Users should be educated about the dangers of installing apps from unknown sources. Additionally, organizations should monitor for unusual app behavior or data access patterns on mobile devices. Where possible, sensitive Nextcloud data should be encrypted at rest and in transit, and multi-factor authentication should be enforced for accessing Nextcloud services to reduce the impact of potential data exposure. Finally, organizations should maintain an inventory of Nextcloud app versions in use and ensure timely updates are applied.