CVE-2021-20795
📋 TL;DR
This CSRF vulnerability in Cybozu Remote Service management screens allows attackers to trick authenticated administrators into performing unintended actions. Administrators of Cybozu Remote Service versions 3.1.8 to 3.1.9 are affected when they visit malicious web pages while logged into the management interface.
💻 Affected Systems
- Cybozu Remote Service
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Administrator credentials could be used to modify system configurations, create new admin accounts, or disrupt service operations through unauthorized administrative actions.
Likely Case
Attackers could modify user permissions, change system settings, or perform other administrative functions without the administrator's knowledge.
If Mitigated
With proper CSRF protections and administrator awareness, the risk is limited to administrators who visit malicious sites while authenticated.
🎯 Exploit Status
Exploitation requires the victim administrator to be authenticated and visit a malicious web page. No authentication bypass is needed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.1.10 or later
Vendor Advisory: https://kb.cybozu.support/article/37422
Restart Required: Yes
Instructions:
1. Download Cybozu Remote Service version 3.1.10 or later from Cybozu support portal. 2. Backup current installation. 3. Install the updated version following Cybozu's upgrade procedures. 4. Restart the Cybozu Remote Service.
🔧 Temporary Workarounds
Implement CSRF Tokens
allAdd anti-CSRF tokens to all management screen forms and validate them server-side.
SameSite Cookie Attribute
allSet SameSite=Strict or SameSite=Lax attributes on session cookies to prevent cross-site requests.
🧯 If You Can't Patch
- Restrict administrator access to trusted networks only
- Implement web application firewall rules to detect and block CSRF attempts
🔍 How to Verify
Check if Vulnerable:
Check Cybozu Remote Service version in administration panel. If version is between 3.1.8 and 3.1.9 inclusive, the system is vulnerable.Check Version:
Check version in Cybozu Remote Service administration interface under System InformationVerify Fix Applied:
Verify version is 3.1.10 or later in administration panel and test management screen forms for CSRF token implementation.📡 Detection & Monitoring
Log Indicators:
- Unusual administrative actions from unexpected IP addresses
- Multiple failed administrative actions in short timeframes
Network Indicators:
- HTTP POST requests to management endpoints without proper referrer headers
- Cross-origin requests to administrative endpoints
SIEM Query:
source="cybozu_logs" AND (action="admin_*" OR user="admin") AND referrer NOT CONTAINS "trusted-domain"