CVE-2021-3297
📋 TL;DR
This vulnerability allows unauthenticated attackers to gain administrator access to Zyxel NBG2105 routers by setting a login cookie value to 1. It affects users of Zyxel NBG2105 devices with specific firmware versions, enabling complete device compromise.
💻 Affected Systems
- Zyxel NBG2105
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device takeover allowing attackers to reconfigure network settings, intercept traffic, install malware, or use the device as a pivot point into internal networks.
Likely Case
Unauthorized administrative access leading to network configuration changes, DNS hijacking, or credential theft from connected devices.
If Mitigated
Limited impact if device is behind firewall with restricted administrative access and network segmentation.
🎯 Exploit Status
Simple cookie manipulation attack requiring minimal technical skill; exploit details are publicly available.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check vendor advisory for latest patched version
Vendor Advisory: https://www.zyxel.com/support/SupportLandingSR.shtml?c=gb&l=en&kbid=M-01490&md=NBG2105
Restart Required: Yes
Instructions:
1. Access Zyxel support portal. 2. Download latest firmware for NBG2105. 3. Log into router admin interface. 4. Navigate to firmware update section. 5. Upload and apply new firmware. 6. Reboot device.
🔧 Temporary Workarounds
Restrict Administrative Access
allLimit administrative interface access to specific IP addresses or disable remote administration.
Network Segmentation
allPlace router on isolated management network segment with strict firewall rules.
🧯 If You Can't Patch
- Replace affected device with supported model
- Implement network monitoring for unauthorized administrative access attempts
🔍 How to Verify
Check if Vulnerable:
Attempt to access admin interface with 'login=1' cookie set; if successful without credentials, device is vulnerable.Check Version:
Log into router admin interface and check firmware version in system status pageVerify Fix Applied:
After patching, attempt same cookie manipulation; access should be denied without proper authentication.📡 Detection & Monitoring
Log Indicators:
- Unauthorized admin login attempts
- Successful logins from unusual IP addresses
- Configuration changes without authorized user activity
Network Indicators:
- HTTP requests with 'login=1' cookie to router admin interface
- Unusual administrative traffic patterns
SIEM Query:
source="router_logs" AND (cookie="login=1" OR "admin" AND "unauthorized")🔗 References
- https://codeberg.org/nieldk/vulnerabilities/src/branch/main/zyxel%20nbg2105/Admin%20bypass
- https://github.com/nieldk/vulnerabilities/blob/main/zyxel%20nbg2105/Admin%20bypass
- https://www.zyxel.com/support/SupportLandingSR.shtml?c=gb&l=en&kbid=M-01490&md=NBG2105
- https://www.zyxel.com/us/en/support/security_advisories.shtml
- https://github.com/nieldk/vulnerabilities/blob/main/zyxel%20nbg2105/Admin%20bypass
- https://www.zyxel.com/support/SupportLandingSR.shtml?c=gb&l=en&kbid=M-01490&md=NBG2105
- https://www.zyxel.com/us/en/support/security_advisories.shtml
