CVE-2026-49400 in October
Summary
by MITRE • 09/14/2026
October System provides the system module for October Content Management System. Prior to versions 3.7.17 and 4.2.21, 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. 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. As a workaround, restrict CMS markup editing access to fully trusted administrators only, the standard October CMS recommendation for any deployment.
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Analysis
by VulDB Data Team • 09/14/2026
The vulnerability identified in October System versions prior to 3.7.17 and 4.2.21 represents a critical PHP Object Injection flaw rooted in improper handling of serialized data within the backend SessionMaker trait. Specifically, the system stored widget session state using base64-encoded serialization and subsequently consumed this data via an unserialize function call without implementing any restrictions on which classes could be instantiated during deserialization. This architectural oversight creates a direct pathway for attackers to achieve arbitrary code execution if they can control the input written to specific session keys associated with widgets. The technical mechanism relies on the fact that PHP's unserialize function, when left unrestricted, will attempt to instantiate objects of any class present in the serialized payload, provided those classes are autoloaded and available within the application environment. This behavior aligns directly with CWE-502, which describes Deserialization of Untrusted Data as a distinct weakness leading to potential remote code execution or other severe impacts depending on the availability of gadget chains.
The operational impact of this vulnerability is heavily contextualized by the specific configuration requirements for exploitation. The flaw only manifests in installations where the cms.safe_mode setting is enabled. Safe Mode is an opt-in feature designed primarily for demo environments, multi-tenant setups, or shared-editor scenarios where untrusted users are explicitly granted access to the CMS markup editor. In standard production deployments, this mode remains disabled by default because backend access is typically restricted to trusted administrators who already possess the ability to execute arbitrary PHP code through other means, such as plugin development or direct file uploads. Consequently, for the vast majority of October CMS installations running in their intended secure configuration, this vulnerability does not present an immediate threat vector. The scope of impact remains narrow even when Safe Mode is active, as standard backend code paths that populate widget session state with user-supplied values like search terms or filter options wrap these inputs within a known array structure before serialization. This wrapping process prevents raw attacker-controlled serialized payloads from reaching the unserialize sink directly through normal administrative workflows.
Exploitation of this vulnerability requires a confluence of specific conditions: the presence of Safe Mode, access to a session-write path that allows writing arbitrary bytes to widget-related keys without proper sanitization or structural constraints, and the existence of reachable PHP gadget chains within the installed dependency set. The attacker must be able to inject data into the session store in a format that survives base64 encoding and triggers object instantiation upon subsequent read operations by the widget system. This scenario is consistent with ATT&CK technique T1059, specifically sub-techniques related to command and script interpreters or code injection via deserialization flaws. The risk is further compounded if third-party plugins introduce vulnerable classes that can be leveraged as part of a gadget chain, allowing an attacker to escalate from simple object instantiation to full system compromise. However, the requirement for Safe Mode significantly limits the attack surface, distinguishing this issue from more widespread vulnerabilities found in default configurations.
To mitigate this vulnerability and close the underlying security gap, October CMS released patches in versions 3.7.17 and 4.2.21 that fundamentally alter how session state is managed. The primary hardening measure involves replacing the legacy base64-serialized storage mechanism with plain JSON encoding for new widget session writes. This change eliminates the object-injection sink entirely because JSON parsing does not instantiate PHP objects, thereby neutralizing the core weakness associated with CWE-502. To ensure backward compatibility during upgrades, reads of existing sessions transparently fall back to the legacy unserialize format for one upgrade cycle, preserving saved widget state without requiring immediate user intervention. Crucially, this fallback path now enforces allowed_classes set to false within the unserialize function call. This restriction ensures that even if an attacker had previously injected malicious serialized data while Safe Mode was enabled and before the patch was applied, those payloads cannot be deserialized into executable objects after the update is installed.
Administrators are advised to implement additional defensive measures beyond applying the software patches. The primary recommendation is to restrict CMS markup editing access exclusively to fully trusted administrators, which aligns with standard security best practices for content management systems and mitigates the risk of untrusted users interacting with session-managing components. By limiting who can write to widget sessions or manipulate backend state through Safe Mode features, organizations reduce the likelihood of encountering exploitable conditions even if residual vulnerabilities exist in other parts of the application stack. Regular updates to October CMS are essential to maintain this hardened posture and ensure that any newly discovered gadget chains or related weaknesses are addressed promptly by the development team.