CVE-2026-109308 in Craft CMS
Summary
by MITRE • 10/12/2026
Craft CMS 4.0.0 before 4.18.7 and 5.0.0 before 5.10.14 passes the raw 'updates' request parameter in AppController::actionCacheUpdates to UpdatesModel without Component::cleanseConfig(), allowing behavior and event-handler injection. Authenticated control panel users with the performUpdates permission or Updates-utility access can supply 'as' or 'on' keys, such as an AttributeTypecastBehavior gadget, to execute arbitrary code.
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Analysis
by VulDB Data Team • 10/13/2026
The vulnerability identified in Craft CMS versions 4.0.0 through 4.18.7 and 5.0.0 through 5.10.14 represents a critical server-side request forgery vector rooted in improper input validation within the application controller logic. Specifically, the AppController::actionCacheUpdates method accepts an updates parameter that is passed directly to the UpdatesModel without undergoing sanitization via Component::cleanseConfig(). This architectural oversight allows authenticated users with specific privileges to inject arbitrary PHP objects into the application's configuration processing pipeline. The core technical flaw lies in the failure to strip or validate keys such as 'as' and 'on', which are recognized by Yii2, the underlying framework powering Craft CMS, as indicators for behavior attachment and event handler registration respectively. By supplying these specific keys within the request payload, an attacker can manipulate how the application instantiates and configures internal components.
This flaw enables behavior injection, a technique that allows attackers to alter the runtime behavior of PHP objects without modifying their source code directly. In this context, authenticated control panel users possessing the performUpdates permission or access to the Updates utility can craft requests containing serialized payloads targeting known gadget chains, such as AttributeTypecastBehavior. When the application processes these malicious inputs, it inadvertently instantiates and executes methods within these gadgets during the configuration cleansing phase that was supposed to be secure but is instead bypassed due to the missing validation step. This mechanism effectively transforms a standard administrative function into an arbitrary code execution vector, allowing the attacker to run PHP commands with the same privileges as the web server process.
The operational impact of this vulnerability is severe, granting remote attackers full control over the compromised system under specific conditions. Because exploitation requires authentication and elevated permissions within the Craft CMS administration panel, it targets users who have been granted significant trust by administrators. However, once authenticated access is achieved with the requisite update privileges, the attacker can execute arbitrary code on the server. This leads to a complete compromise of confidentiality, integrity, and availability for the hosted web application and potentially the underlying infrastructure if further lateral movement or privilege escalation techniques are employed against other services running under the same user context. The ability to inject behaviors means that even complex security controls implemented via middleware or event listeners can be bypassed or overridden by the injected malicious logic.
From a classification perspective, this vulnerability aligns with CWE-94 Improper Control of Generation of Code (Code Injection) and specifically CWE-501 Trust Boundary Violation, as the application fails to properly validate input at a critical security boundary before passing it to internal processing functions. In terms of adversary tactics, this exploitation technique maps directly to ATT&CK Tactic TA0004 Privilege Escalation and Technique T1068 Exploitation for Privilege Escalation, as well as T1203 Exploitation for Client Execution if the code execution leads to client-side impacts. The use of gadget chains like AttributeTypecastBehavior is characteristic of deserialization-based attacks often seen in modern PHP applications relying on complex framework structures.
Mitigation strategies must focus on immediate patching and rigorous input validation practices. Administrators should upgrade Craft CMS to version 4.18.7 or later, or version 5.10.14 or later, where the developers have addressed this specific oversight by ensuring that all incoming configuration data is properly sanitized before being processed by internal models. For environments where immediate patching is not feasible due to compatibility constraints, implementing a Web Application Firewall rule set capable of detecting and blocking requests containing suspicious 'as' or 'on' keys within JSON payloads directed at the updates endpoint can provide temporary relief. Additionally, enforcing strict least-privilege principles for administrative accounts reduces the attack surface by limiting which users possess the performUpdates permission, thereby restricting potential attackers to those with lower levels of access who cannot trigger this specific code path. Regular security audits focusing on input validation in controller actions are essential to prevent similar trust boundary violations in future development cycles.