CVE-2021-26473
📋 TL;DR
CVE-2021-26473 is a critical vulnerability in VembuBDR and VembuOffsiteDR backup software that allows unauthenticated attackers to write arbitrary files to the web server via a flawed HTTP API endpoint. This can lead to remote code execution by accessing the uploaded files through the web server, potentially compromising the entire system. Organizations using affected versions of these products are at risk.
💻 Affected Systems
- VembuBDR
- VembuOffsiteDR
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining remote code execution, leading to data theft, ransomware deployment, or lateral movement within the network.
Likely Case
Remote code execution allowing attackers to install malware, exfiltrate sensitive backup data, or disrupt backup operations.
If Mitigated
Limited impact if network segmentation and strict access controls prevent external exploitation, though internal threats may persist.
🎯 Exploit Status
Exploitation is straightforward with publicly available proof-of-concept code, making it highly attractive to attackers.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.2.0.1
Vendor Advisory: https://help.vembu.com/vembu-bdr-suite-release-notes/
Restart Required: Yes
Instructions:
1. Download the latest version (4.2.0.1 or later) from the Vembu website. 2. Backup current configurations. 3. Install the update following vendor instructions. 4. Restart the Vembu services to apply changes.
🔧 Temporary Workarounds
Block Access to Vulnerable Endpoint
allRestrict access to the /sgwebservice_o.php endpoint using web server or firewall rules to prevent exploitation.
For Apache: add 'Deny from all' to .htaccess for /sgwebservice_o.php
For Nginx: add 'location /sgwebservice_o.php { deny all; }' to config
Network Segmentation
allIsolate Vembu servers from the internet and restrict internal access to trusted IPs only.
Use firewall rules to allow only specific IPs to the Vembu web port (default 6060)
🧯 If You Can't Patch
- Implement strict network access controls to limit exposure to the Vembu web interface.
- Monitor logs for suspicious activity targeting /sgwebservice_o.php and set up alerts for file upload attempts.
🔍 How to Verify
Check if Vulnerable:
Check the Vembu software version via the web interface or by examining installed files; versions below 4.2.0.1 are vulnerable.Check Version:
On Windows: Check 'Programs and Features' for Vembu version. On Linux: Run 'vembu --version' or check package manager.Verify Fix Applied:
Confirm the version is 4.2.0.1 or higher in the software settings and test that the /sgwebservice_o.php endpoint no longer accepts arbitrary file writes.📡 Detection & Monitoring
Log Indicators:
- HTTP requests to /sgwebservice_o.php with 'logFilePath' parameter
- Unusual file creation in web-accessible directories
- Errors or warnings in Vembu application logs related to file writes
Network Indicators:
- Inbound traffic to Vembu web port (default 6060) with POST requests to /sgwebservice_o.php
- Outbound connections from Vembu server to unknown IPs post-exploitation
SIEM Query:
source="vembu_logs" AND (url="/sgwebservice_o.php" OR message="logFilePath")🔗 References
- https://csirt.divd.nl/2021/05/11/Vembu-zero-days/
- https://csirt.divd.nl/cases/DIVD-2020-00011/
- https://csirt.divd.nl/cves/CVE-2021-26473/
- https://www.wbsec.nl/vembu
- https://csirt.divd.nl/2021/05/11/Vembu-zero-days/
- https://csirt.divd.nl/cases/DIVD-2020-00011/
- https://csirt.divd.nl/cves/CVE-2021-26473/
- https://www.wbsec.nl/vembu
