Affected versions
Twig versions <=3.26.0 are affected by this security issue.
The issue has been fixed in Twig 3.27.0.
Description
The per-template filter, tag and function allow-list check is compiled into
the checkSecurity() method of each Template subclass and was invoked
once from the constructor, gated by SandboxExtension::isSandboxed($source).
Template instances are then cached on the Environment in
$loadedTemplates, so the verdict computed at construction time was sticky
for the rest of the process.
Any later change of sandbox state on the same Environment left that
cached verdict in place: toggling SandboxExtension::enableSandbox() /
disableSandbox(), swapping the policy via setSecurityPolicy(), a
SourcePolicyInterface decision flip, or simply having a parent, macro or
included template pre-instantiated outside the sandbox before a sandboxed
render reached it. In all of these cases, the filters, tags and functions
used by the affected template kept running with the original (typically
empty) check, bypassing the SecurityPolicy allow-list.
Method, property and __toString allow-lists are not affected: they are
enforced at every call site at runtime through
SandboxExtension::checkMethodAllowed(), checkPropertyAllowed() and
ensureToStringAllowed(), which re-read the current state on every call.
Long-lived workers (FrankenPHP, RoadRunner, Symfony Messenger consumers,
FPM with hot autoloading) that share a single Environment between
sandboxed and non-sandboxed renders are the most exposed: a single
non-sandboxed render of a shared layout pre-warms its Template instance,
after which any later sandboxed render that extends, uses, includes or
imports from that layout silently skips the filter/tag/function allow-list
for the pre-warmed instance.
Resolution
The allow-list check is no longer run from the constructor. Template
gains a public ensureSecurityChecked() method that calls the compiled
checkSecurity() only when SandboxExtension::isSandboxed($source)
returns true for the current source, and it is invoked at every entry point
that can reach a Template instance whose security has not yet been
verified against the current state: Template::yield(),
Template::yieldBlock() (on the resolved block template, which covers
extends, use, traits and parent blocks), Template::getParent()
(which evaluates user code when the parent name is dynamic) and
Template::getTemplateForMacro() (on the resolved macro template).
The explicit checkSecurity() calls previously emitted by IncludeNode
and CoreExtension::include() are removed: the included template's own
yield() now re-runs the check against the current sandbox state. The
compiled checkSecurity() body is a cheap walk over compile-time-static
arrays, so the per-render cost is negligible. Old cached compiled PHP files
keep working unchanged: the constructor-time call they still contain is
idempotent.
Credits
We would like to thank Fabien Potencier for reporting and fixing the issue.