There is an authenticated admin RCE in Craft CMS 5.8.21 via Server-Side Template Injection using the create() Twig function combined with a Symfony Process gadget chain.

This bypasses the fix implemented for CVE-2025-57811 (patched in 5.8.7).

Required Permissions

• Administrator permissions or access to System Messages utility
• allowAdminChanges enabled in production (against our security recommendations) or access to System Messages utility

Vulnerability Details

The create() Twig function exposes Craft::createObject(), which allows instantiation of arbitrary PHP classes with constructor arguments. Combined with the bundled symfony/process dependency, this enables RCE.

Attack Vector

Admin panel → Settings → Entry Types → Title Format field

Proof of Concept Payload

{% set p = create("Symfony\\Component\\Process\\Process", [["id"]])
%}{{ p.mustRun.getOutput }}

Steps to Reproduce

1. Log in as admin
2. Navigate to Settings → Entry Types
3. Edit any entry type’s "Title Format" field
4. Insert the payload above
5. Create/edit an entry of that type
6. Command executes, output appears in entry title

Impact

• Authenticated Remote Code Execution
• Runs as web server user (root in default Docker setup)
• Full server compromise

Root Cause

Craft::createObject() allows the instantiation of any class, including
Symfony\Component\Process\Process, which executes shell commands.

Suggested Fix

• Blocklist dangerous classes in createObject() when called from Twig
• Or remove/restrict the create() Twig function
• Or validate class names against an allowlist

References

e31e508