October CMS: PHP Object Injection via Backend Widget Session Storage

The backend SessionMaker trait stored widget session state as base64(serialize(...)) and consumed it with unserialize() without an allowed_classes restriction. Any code path that could write to a widget.* session key with attacker-controlled bytes could trigger PHP object injection the next time the widget read its session state, allowing instantiation of arbitrary classes and reachable PHP gadget chains.

This issue only affects installations running with cms.safe_mode enabled. Safe Mode is a niche opt-in feature, primarily used for demo installations and multi-tenant or shared-editor scenarios where untrusted users are deliberately granted access to the CMS markup editor. In standard production deployments Safe Mode is off, backend access is restricted to trusted administrators, and a markup editor can already execute arbitrary PHP directly. The session-write path that reaches this sink is gated by the Safe Mode sandbox, so installations without Safe Mode enabled are not exposed.

Scope of impact is narrow even with Safe Mode enabled. The standard backend code paths that populate widget session state (search terms, sort options, selected IDs, filter values) wrap the input inside a known array shape before serializing, so user-supplied values never reach unserialize() as a controllable serialized payload. Exploitation requires the Safe Mode session-write path together with a suitable PHP gadget chain reachable from the installed dependency set. The hardening below removes the underlying object-injection sink so the class of issue is closed off regardless.

Impact

• Arbitrary PHP code execution as the web server user, via a gadget chain deserialized from a widget session key, triggered the next time the affected widget is rendered
• Requires cms.safe_mode to be enabled, and a backend user with CMS markup editing access who is not intended to be trusted as a full administrator
• A suitable PHP gadget chain must be reachable from the installed dependency set
• Not exploitable when Safe Mode is disabled

Patches

The vulnerability has been patched in v3.7.17 and v4.2.21. Two changes were applied:

• Backend\Traits\SessionMaker now stores widget session state as plain JSON instead of base64(serialize(...)), eliminating the object-injection sink entirely for new writes. Reads transparently fall back to the legacy format for one upgrade cycle so existing sessions retain their saved widget state.
• The legacy unserialize() fallback path now sets allowed_classes => false, so even values written before the upgrade cannot instantiate objects.

Workarounds

If upgrading immediately is not possible, restrict CMS markup editing access to fully trusted administrators only, the standard October CMS recommendation for any deployment.

References

• Reported by EndlssNightmare