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Craft CMS Vulnerable to potential authenticated Remote Code Execution via malicious attached Behavior

High severity GitHub Reviewed Published Feb 9, 2026 in craftcms/cms • Updated Feb 9, 2026

Package

composer craftcms/cms (Composer)

Affected versions

>= 5.0.0-RC1, <= 5.8.21
>= 4.0.0-RC1, <= 4.16.17

Patched versions

5.8.22
4.16.18

Description

Relationship to Previously Patched Vulnerability

This vulnerability is in addition to the RCE vulnerability patched in GHSA-255j-qw47-wjh5. That advisory addressed a similar RCE vulnerability that affected two specific routes:

  • /index.php?p=admin%2Factions%2Ffields%2Fapply-layout-element-settings
  • /index.php?p=admin%2Factions%2Ffields%2Frender-card-preview

This one addresses some additional endpoints that were not covered in the GHSA-255j-qw47-wjh5.

The patched vulnerability used a malicious AttributeTypecastBehavior with a wildcard event listener ("on *": "self::beforeSave") and __construct() syntax to trigger RCE via the typecastBeforeSave callback. The fix was implemented in commits:

This vulnerability follows the same attack pattern (behavior injection via "as <behavior>" syntax) but affects a different code path (assembleLayoutFromPost() in Fields.php) that was not patched in those commits. The attack vector uses typecastAfterValidate instead of typecastBeforeSave and does not require the wildcard event listener syntax, demonstrating that multiple entry points exist for this type of vulnerability.

Executive Summary

A Remote Code Execution (RCE) vulnerability exists in Craft CMS where the assembleLayoutFromPost() function in src/services/Fields.php fails to sanitize user-supplied configuration data before passing it to Craft::createObject(). This allows authenticated administrators to inject malicious Yii2 behavior configurations that execute arbitrary system commands on the server. This vulnerability represents an unpatched variant of the behavior injection vulnerability addressed in GHSA-255j-qw47-wjh5, affecting different endpoints through a separate code path.

Vulnerability Details

Attack Prerequisites

  • Authentication: Admin-level access required
  • Network Access: Access to admin panel (/admin)

Location

  • File: src/services/Fields.php
  • Function: assembleLayoutFromPost() (lines 1125-1143)
  • Root Cause: Missing cleanseConfig() call on user-supplied fieldLayout POST parameter

Vulnerable Code Path

// src/services/Fields.php:1125-1133
public function assembleLayoutFromPost(?string $namespace = null): FieldLayout
{
    $paramPrefix = $namespace ? rtrim($namespace, '.') . '.' : '';
    $request = Craft::$app->getRequest();
    $config = JsonHelper::decode($request->getBodyParam("{$paramPrefix}fieldLayout"));
    // ... additional config values added ...
    $layout = $this->createLayout($config);  // <-- No cleanseConfig() call!
    // ...
}

// src/services/Fields.php:1089-1093
public function createLayout(array $config): FieldLayout
{
    $config['class'] = FieldLayout::class;
    return Craft::createObject($config);  // <-- Untrusted data passed directly
}

Attack Chain

The exploitation leverages Yii2's object configuration system and behavior attachment mechanism:

  1. Behavior Injection: Attacker includes 'as rce' key in the fieldLayout JSON POST parameter
  2. Object Creation: Craft::createObject() processes the config through Yii2's BaseYii::configure()
  3. Behavior Attachment: Yii2's Component::__set() detects the 'as ' prefix and attaches the behavior
  4. RCE Trigger: When validate() is called on the model, EVENT_AFTER_VALIDATE fires
  5. Command Execution: AttributeTypecastBehavior calls the configured typecast function (ConsoleProcessus::execute) with the uid attribute value as the command

RCE Gadget Chain

FieldLayout POST parameter
    → Craft::createObject()
    → Yii2 Component::__set() with 'as rce' key
    → AttributeTypecastBehavior attached
    → Model::validate() called
    → EVENT_AFTER_VALIDATE triggered
    → typecastAfterValidate → typecastAttributes()
    → call_user_func(['Psy\Readline\Hoa\ConsoleProcessus', 'execute'], $command)
    → Shell command execution

Affected Controllers

The assembleLayoutFromPost() function is called by multiple admin controllers:

Controller Action Permission Required
TagsController actionSaveTagGroup() Admin
CategoriesController actionSaveGroup() Admin
EntryTypesController actionSave() Admin
GlobalsController actionSaveSet() Admin
VolumesController actionSave() Admin
UsersController actionSaveUserFieldLayout() Admin
AddressesController actionSaveAddressFieldLayout() Admin

References

References

@angrybrad angrybrad published to craftcms/cms Feb 9, 2026
Published by the National Vulnerability Database Feb 9, 2026
Published to the GitHub Advisory Database Feb 9, 2026
Reviewed Feb 9, 2026
Last updated Feb 9, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required High
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(50th percentile)

Weaknesses

Use of Externally-Controlled Input to Select Classes or Code ('Unsafe Reflection')

The product uses external input with reflection to select which classes or code to use, but it does not sufficiently prevent the input from selecting improper classes or code. Learn more on MITRE.

CVE ID

CVE-2026-25498

GHSA ID

GHSA-7jx7-3846-m7w7

Source code

Credits

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