CVE-2021-41181 is a vulnerability identified in the Nextcloud Talk Android application versions prior to 12.3.0. Nextcloud Talk is a self-hosted messaging and video calling service widely used for secure communication. The vulnerability arises from improper detection of the device lockscreen state when an incoming call occurs. Specifically, if an attacker gains physical access to a locked Android device running a vulnerable version of the Nextcloud Talk app, and the device receives an incoming call, the attacker can bypass the lockscreen protections and gain unauthorized access to the user's chat messages and files within the app. This exposure is due to the app failing to correctly enforce lockscreen restrictions during call events, leading to an unintended disclosure of sensitive information. The vulnerability is categorized under CWE-200, which pertains to the exposure of sensitive information to unauthorized actors. The issue does not require remote exploitation or network access; instead, it depends on physical access to the device and the occurrence of an incoming call. There are no known workarounds, and the recommended mitigation is to upgrade the Nextcloud Talk Android app to version 12.3.0 or later, where this issue has been resolved. No public exploits have been reported in the wild, and the vulnerability was publicly disclosed on March 8, 2022.

For European organizations, this vulnerability primarily threatens the confidentiality of sensitive communications and files stored within the Nextcloud Talk Android app. Organizations that rely on Nextcloud Talk for internal messaging, especially those handling sensitive or regulated data (e.g., legal, financial, healthcare sectors), could face data leakage if devices are physically compromised. The impact is heightened in environments where mobile devices are frequently used outside secure premises or where device theft or loss is a realistic threat. Although the vulnerability requires physical access and an incoming call event, it could facilitate insider threats or opportunistic attackers who briefly gain possession of a locked device. The integrity and availability of data are not directly affected by this vulnerability. However, the exposure of confidential information could lead to reputational damage, regulatory non-compliance (e.g., GDPR violations), and potential financial losses. Given the widespread adoption of Nextcloud in Europe, especially among privacy-conscious organizations and public institutions, the risk is non-negligible. The lack of known exploits in the wild reduces immediate urgency but does not eliminate the risk, particularly as attackers may develop exploits targeting this vulnerability.

1. Immediate upgrade of the Nextcloud Talk Android application to version 12.3.0 or later on all organizational devices to ensure the vulnerability is patched. 2. Implement strict mobile device management (MDM) policies that enforce device encryption, strong lockscreen authentication, and remote wipe capabilities to mitigate risks from lost or stolen devices. 3. Educate users about the risks of leaving devices unattended and the importance of reporting lost or stolen devices promptly. 4. Restrict physical access to devices in high-risk environments and consider additional hardware security measures such as secure storage or tamper-evident cases. 5. Monitor for unusual access patterns or data exfiltration attempts from mobile devices, integrating mobile endpoint detection and response (EDR) tools where feasible. 6. Review and limit the amount of sensitive data accessible via mobile apps, applying the principle of least privilege and data minimization. 7. Coordinate with Nextcloud administrators to ensure server-side security configurations complement client-side protections, including enforcing app updates and controlling access permissions.