CVE-2021-34203
📋 TL;DR
D-Link DIR-2640-US routers running firmware version 1.01B04 have an incorrect access control vulnerability in their PPPoE configuration. When PPPoE is enabled, the router starts the Quagga routing daemon with default credentials and exposes it to the entire network, allowing attackers to gain administrative access via telnet. This affects all users of this specific router model with the vulnerable firmware.
💻 Affected Systems
- D-Link DIR-2640-US
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete network compromise: attacker gains full router control, can redirect all traffic, perform DNS hijacking, intercept sensitive data, and launch phishing attacks against all connected devices.
Likely Case
Network traffic monitoring and manipulation: attacker can view all network traffic, modify routing tables, and potentially redirect users to malicious sites.
If Mitigated
Limited impact if telnet access is blocked at network perimeter and strong network segmentation is in place.
🎯 Exploit Status
Exploitation requires telnet access to the router's internal network. Default credentials are hardcoded and cannot be changed by users.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check D-Link security bulletin for latest patched version
Vendor Advisory: https://www.dlink.com/en/security-bulletin/
Restart Required: Yes
Instructions:
1. Visit D-Link support site. 2. Download latest firmware for DIR-2640-US. 3. Log into router admin interface. 4. Navigate to firmware update section. 5. Upload and install new firmware. 6. Reboot router.
🔧 Temporary Workarounds
Disable PPPoE
allIf PPPoE is not required, disable it to prevent Quagga daemon from starting with vulnerable configuration.
Block Telnet Access
linuxConfigure firewall rules to block telnet (port 23) access to the router from untrusted networks.
iptables -A INPUT -p tcp --dport 23 -j DROP
🧯 If You Can't Patch
- Segment the router on an isolated network segment with strict access controls
- Implement network monitoring for telnet connections to the router's IP on port 23
🔍 How to Verify
Check if Vulnerable:
Check if router is running firmware version 1.01B04 and has PPPoE enabled. Attempt telnet connection to router IP on port 23 with default credentials.Check Version:
Log into router web interface and check firmware version in System Status or Administration section.Verify Fix Applied:
After firmware update, verify telnet access is no longer available or requires proper authentication. Check that Quagga daemon is not running with default credentials.📡 Detection & Monitoring
Log Indicators:
- Telnet connection attempts to router IP
- Quagga daemon startup logs
- Unexpected routing table changes
Network Indicators:
- Telnet traffic (port 23) to router IP
- Unexpected BGP/OSPF routing protocol traffic
SIEM Query:
source_ip="router_ip" AND destination_port=23 AND protocol=TCP🔗 References
- http://d-link.com
- http://dir-2640-us.com
- https://github.com/liyansong2018/CVE/tree/main/2021/CVE-2021-34203
- https://www.dlink.com/en/security-bulletin/
- http://d-link.com
- http://dir-2640-us.com
- https://github.com/liyansong2018/CVE/tree/main/2021/CVE-2021-34203
- https://www.dlink.com/en/security-bulletin/
