CVE-2021-21876
📋 TL;DR
CVE-2021-21876 allows authenticated attackers to execute arbitrary commands via specially crafted HTTP PUT requests. This vulnerability affects systems running vulnerable versions of the affected software with HTTP PUT functionality enabled. Attackers with valid credentials can exploit this to gain remote code execution.
💻 Affected Systems
- D-Link DNS-320L ShareCenter
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Full system compromise with attacker gaining root/administrator privileges, data exfiltration, ransomware deployment, and persistent backdoor installation.
Likely Case
Unauthorized command execution leading to data theft, lateral movement within the network, and potential privilege escalation.
If Mitigated
Limited impact due to network segmentation, strong authentication controls, and proper input validation preventing successful exploitation.
🎯 Exploit Status
Exploitation requires authentication but is straightforward once credentials are obtained. Multiple public proof-of-concept exploits exist.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware version 2.06B02 or later
Vendor Advisory: https://support.dlink.com/ProductInfo.aspx?m=DNS-320L
Restart Required: Yes
Instructions:
1. Download latest firmware from D-Link support site. 2. Log into device web interface. 3. Navigate to System > Firmware Update. 4. Upload and apply the new firmware. 5. Reboot device after update completes.
🔧 Temporary Workarounds
Disable HTTP PUT Method
allBlock or disable HTTP PUT requests at the web server or firewall level
# For Apache: add to .htaccess or config
<Limit PUT>
Order deny,allow
Deny from all
</Limit>
# For Nginx: add to server block
if ($request_method = PUT) {
return 405;
}
Restrict Network Access
linuxLimit device access to trusted networks only
# Example iptables rule
sudo iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 80 -j DROP
🧯 If You Can't Patch
- Implement strict network segmentation to isolate affected devices from critical systems
- Enforce strong authentication policies and regularly rotate credentials
🔍 How to Verify
Check if Vulnerable:
Check firmware version in web interface under System > Firmware Information. If version is 2.06B01 or earlier, device is vulnerable.Check Version:
curl -s http://device-ip/fw_version.txt | grep FirmwareVerify Fix Applied:
Verify firmware version shows 2.06B02 or later after update. Test HTTP PUT requests return proper error codes.📡 Detection & Monitoring
Log Indicators:
- Unusual HTTP PUT requests to device management interface
- Multiple failed authentication attempts followed by successful PUT requests
- Commands execution patterns in system logs
Network Indicators:
- HTTP PUT requests with shell command patterns in payload
- Outbound connections from device to unexpected destinations
SIEM Query:
source="device_logs" AND (http_method="PUT" AND (uri="*cgi*" OR uri="*command*" OR uri="*exec*"))