CVE-2026-46638: Twig: `{% sandbox %}{% include %}` skips checkSecurity() on cached templates (incomplete fix for CVE-2024-45411)

Description

The fix for CVE-2024-45411 / GHSA-6j75-5wfj-gh66 added an explicit $loaded->unwrap()->checkSecurity() call in CoreExtension::include() so that a template already cached in Environment::$loadedTemplates is re-checked when included with sandboxed = true.

The deprecated but still functional {% sandbox %}{% include ... %}{% endsandbox %} tag path was not updated: it compiles to enableSandbox(); yield from $this->load(...)->unwrap()->yield(...); disableSandbox(); with no checkSecurity() re-invocation. If the included template was loaded once outside the sandbox in the same Environment instance, its constructor (and therefore its compiled checkSecurity() call) already ran while isSandboxed() was false, so the tags/filters/functions allowlist enforced by SecurityPolicy::checkSecurity() is never applied.

An attacker who can author the included template gains access to every filter, function and tag registered in the environment, regardless of the sandbox policy.

Resolution

The compiled output of {% sandbox %}{% include %} now calls checkSecurity() on the loaded template, matching the behaviour of CoreExtension::include() with sandboxed = true.

Credits

Twig would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and providing the fix.