CVE-2021-20042
📋 TL;DR
CVE-2021-20042 allows unauthenticated remote attackers to use SonicWall SMA 100 series appliances as unintended proxies to bypass firewall rules. This affects SMA 200, 210, 400, 410, and 500v appliances, enabling attackers to route malicious traffic through these devices undetected.
💻 Affected Systems
- SonicWall SMA 200
- SonicWall SMA 210
- SonicWall SMA 400
- SonicWall SMA 410
- SonicWall SMA 500v
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Attackers could route any malicious traffic (ransomware, data exfiltration, command-and-control) through the SMA appliance, bypassing all firewall protections and remaining undetected while compromising internal networks.
Likely Case
Attackers use the SMA appliance as a proxy to bypass firewall rules, potentially accessing internal resources or launching attacks from what appears to be a trusted device.
If Mitigated
With proper network segmentation and monitoring, the impact is limited to potential unauthorized access through the proxy, but detection would be challenging without specific monitoring.
🎯 Exploit Status
The vulnerability requires no authentication and has a simple exploitation path, making it attractive for attackers.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 10.2.1.2-34sv and later
Vendor Advisory: https://psirt.global.sonicwall.com/vuln-detail/SNWLID-2021-0026
Restart Required: Yes
Instructions:
1. Log into SMA management interface. 2. Navigate to System > Settings > Upgrade. 3. Upload firmware version 10.2.1.2-34sv or later. 4. Apply the update and restart the appliance.
🔧 Temporary Workarounds
Restrict Management Interface Access
allLimit access to the SMA web management interface to trusted IP addresses only.
Configure firewall rules to restrict access to SMA management IP/port to specific source IPs
Disable Unnecessary Services
allDisable any unnecessary services on the SMA appliance that might be exposed.
Review and disable non-essential services in SMA configuration
🧯 If You Can't Patch
- Isolate SMA appliances in a dedicated network segment with strict firewall rules
- Implement network monitoring for unusual proxy-like traffic patterns from SMA devices
🔍 How to Verify
Check if Vulnerable:
Check SMA firmware version via web interface: System > Settings > Status. If version is below 10.2.1.2-34sv, the device is vulnerable.Check Version:
No CLI command available; check via web interface at System > Settings > StatusVerify Fix Applied:
Verify firmware version is 10.2.1.2-34sv or later in System > Settings > Status.📡 Detection & Monitoring
Log Indicators:
- Unusual proxy connections in SMA logs
- Multiple failed authentication attempts followed by successful proxy usage
- Traffic patterns showing SMA device acting as intermediary
Network Indicators:
- Unexpected traffic routing through SMA appliances
- SMA devices communicating with unusual external IPs
- Traffic bypassing normal firewall paths
SIEM Query:
source="sma_logs" AND (event_type="proxy_connection" OR event_type="unauthorized_access")