CVE-2026-28695: Craft affected by authenticated RCE via Twig SSTI - create() function + Symfony Process gadget

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

Resources

https://github.com/craftcms/cms/commit/e31e50849ad71638e11ea55fbd1ed90ae8f8f6e0

— GitHub