CVE-2021-39225
📋 TL;DR
CVE-2021-39225 is an authorization bypass vulnerability in Nextcloud Deck that allows authenticated users to access other users' Deck cards without proper permission checks. This affects Nextcloud instances with the Deck app installed before versions 1.2.9, 1.4.5, or 1.5.3. The vulnerability enables unauthorized data access within shared Nextcloud environments.
💻 Affected Systems
- Nextcloud Deck
📦 What is this software?
Deck by Nextcloud
Deck by Nextcloud
Deck by Nextcloud
⚠️ Risk & Real-World Impact
Worst Case
Malicious authenticated users could access sensitive project management data, task assignments, and confidential information from all other users' Deck boards, potentially leading to data theft, corporate espionage, or privacy violations.
Likely Case
Authenticated users accidentally or intentionally accessing other users' project boards and cards, leading to unauthorized information disclosure and potential data leakage within organizations.
If Mitigated
With proper access controls and monitoring, impact is limited to isolated incidents that can be detected and contained through audit logs and user activity monitoring.
🎯 Exploit Status
Exploitation requires authenticated access but is technically simple once authenticated. The vulnerability is well-documented in public advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.2.9, 1.4.5, or 1.5.3
Vendor Advisory: https://github.com/nextcloud/security-advisories/security/advisories/GHSA-2x96-38qg-3m72
Restart Required: No
Instructions:
1. Log into Nextcloud as administrator. 2. Navigate to Apps section. 3. Find Deck app. 4. Update to version 1.2.9, 1.4.5, or 1.5.3. 5. Verify update completed successfully.
🔧 Temporary Workarounds
Disable Deck App
linuxTemporarily disable the Deck app until patching is possible
occ app:disable deck
🧯 If You Can't Patch
- Implement strict access controls and monitor Deck app usage patterns
- Enable detailed logging and audit all Deck-related API calls for unauthorized access attempts
🔍 How to Verify
Check if Vulnerable:
Check Deck app version in Nextcloud admin interface or run: occ app:list | grep deckCheck Version:
occ app:list | grep deckVerify Fix Applied:
Verify Deck app version is 1.2.9, 1.4.5, or 1.5.3 or higher📡 Detection & Monitoring
Log Indicators:
- Unusual Deck API access patterns
- User accessing Deck cards not associated with their account
- Multiple failed permission checks in Deck logs
Network Indicators:
- Unusual volume of Deck API requests from single user
- Requests to Deck endpoints with different user IDs than authenticated user
SIEM Query:
source="nextcloud.log" AND "deck" AND ("permission denied" OR "access denied")🔗 References
- https://github.com/nextcloud/deck/pull/3316
- https://github.com/nextcloud/security-advisories/security/advisories/GHSA-2x96-38qg-3m72
- https://hackerone.com/reports/1331728
- https://github.com/nextcloud/deck/pull/3316
- https://github.com/nextcloud/security-advisories/security/advisories/GHSA-2x96-38qg-3m72
- https://hackerone.com/reports/1331728
