GHSA-2ff2-mx52-q8wp

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