This article provides a detailed analysis of CVE-2025-66557, a medium-severity vulnerability affecting Nextcloud Deck. We’ll explore the technical details, potential impact, and necessary steps to mitigate this risk.
Overview
CVE-2025-66557 affects Nextcloud Deck, a popular kanban-style organization tool integrated within Nextcloud. The vulnerability allows users with “Can share” permissions on a Deck board to modify the permissions of other users sharing the same board. This effectively allows a user to elevate their privileges or downgrade the privileges of others, potentially leading to unauthorized access and data manipulation.
Technical Details
The vulnerability stems from a flaw in the permission logic within Nextcloud Deck. Specifically, the application incorrectly validates whether a user with “Can share” permission has the authority to modify the permissions assigned to other collaborators on a specific board. This missing validation allows malicious or compromised users to elevate their own permissions or restrict access for others without proper authorization.
The specific vulnerable code was addressed in the following commit:
GitHub Commit: f1da8b30a455f02373d44154da04494c949a95ae
CVSS Analysis
- CVSS Score: 5.4 (Medium)
While the vulnerability requires a user to already have “Can share” permissions, the potential for privilege escalation makes this a noteworthy security concern. The CVSS score reflects the limited scope of exploitation but acknowledges the potential impact on data confidentiality and integrity.
Possible Impact
Successful exploitation of CVE-2025-66557 could lead to several negative consequences:
- Unauthorized Data Access: A user could grant themselves higher-level access to sensitive information stored within the Deck board.
- Data Manipulation: Elevated privileges could allow a user to modify or delete critical data.
- Denial of Service: A malicious user could revoke access for legitimate users, disrupting workflows.
- Lateral Movement: In some environments, compromised Nextcloud instances can be used as a stepping stone to attack other systems on the network.
Mitigation and Patch Steps
The vulnerability is patched in the following Nextcloud Deck versions:
- 1.14.6
- 1.15.2
To mitigate the risk, administrators should immediately upgrade their Nextcloud Deck installations to one of these versions or a later release. Here’s how to upgrade:
- Log in to your Nextcloud instance as an administrator.
- Navigate to the Apps section.
- Search for “Deck”.
- If an update is available, click the “Update” button.
- After the update is complete, verify that the Deck app version is 1.14.6 or 1.15.2 or higher.
If immediate patching is not possible, consider temporarily restricting “Can share” permissions to only trusted users until the update can be applied. Regular security audits and user permission reviews are also recommended.
