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Server-Side Template Injection (SSTI) with Grav CMS security sandbox bypass

High
rhukster published GHSA-c9gp-64c4-2rrh Mar 21, 2024

Package

Grav CMS

Affected versions

<= 1.7.44

Patched versions

1.7.45

Description

Summary

Grav CMS is vulnerable to a Server-Side Template Injection (SSTI), which allows any authenticated user (editor permissions are sufficient) to execute arbitrary code on the remote server bypassing the existing security sandbox.

Details

The Grav CMS implements a custom sandbox to protect the powerful Twig methods "registerUndefinedFunctionCallback()" and "registerUndefinedFilterCallback()", in order to avoid SSTI attacks by denying the calling of dangerous PHP functions into the Twig template directives (such as: "exec()", "passthru()", "system()", etc.).
The current defenses are based on a blacklist of prohibited functions (PHP, Twig), checked through the "isDangerousFunction()" method called in the file "system/src/Grav/Common/Twig.php":

...
$this->twig = new TwigEnvironment($loader_chain, $params);

$this->twig->registerUndefinedFunctionCallback(function (string $name) use ($config) {
    $allowed = $config->get('system.twig.safe_functions');
    if (is_array($allowed) && in_array($name, $allowed, true) && function_exists($name)) {
        return new TwigFunction($name, $name);
    }
    if ($config->get('system.twig.undefined_functions')) {
        if (function_exists($name)) {
            if (!Utils::isDangerousFunction($name)) {
                user_error("PHP function {$name}() was used as Twig function. This is deprecated in Grav 1.7. Please add it to system configuration: `system.twig.safe_functions`", E_USER_DEPRECATED);

                return new TwigFunction($name, $name);
            }

           /** @var Debugger $debugger */
           $debugger = $this->grav['debugger'];
           $debugger->addException(new RuntimeException("Blocked potentially dangerous PHP function {$name}() being used as Twig function. If you really want to use it, please add it to system configuration: `system.twig.safe_functions`"));
        }

        return new TwigFunction($name, static function () {});
    }

    return false;
});

$this->twig->registerUndefinedFilterCallback(function (string $name) use ($config) {
    $allowed = $config->get('system.twig.safe_filters');
    if (is_array($allowed) && in_array($name, $allowed, true) && function_exists($name)) {
        return new TwigFilter($name, $name);
    }
    if ($config->get('system.twig.undefined_filters')) {
        if (function_exists($name)) {
            if (!Utils::isDangerousFunction($name)) {
                user_error("PHP function {$name}() used as Twig filter. This is deprecated in Grav 1.7. Please add it to system configuration: `system.twig.safe_filters`", E_USER_DEPRECATED);
                return new TwigFilter($name, $name);
            }
...

In the code above it can be seen that the calls of the "isDangerousFunction()" are not performed when the method/filter in the "$name" variable has been considered safe. A function can be defined safe only by an administrator user, by adding it into the configuration properties "system.twig.safe_functions" and/or "system.twig.safe_filters" (a sort of whitelists that by default are empty) of the configuration file "system/config/system.yaml".

It is to note that within the "system/src/Grav/Common/Twig.php" file a Twig class is defined (with its constructor, methods and attributes) and in particular the Twig object (and environment) is instantiated on it:

/**
 * Class Twig
 * @package Grav\Common\Twig
 */
class Twig
{
    /** @var Environment */
    public $twig;
    /** @var array */
    public $twig_vars = [];
    /** @var array */
    public $twig_paths;
    /** @var string */
    public $template;
...
   /**
     * Constructor
     *
     * @param Grav $grav
     */
    public function __construct(Grav $grav)
    {
        $this->grav = $grav;
        $this->twig_paths = [];
    }

    /**
     * Twig initialization that sets the twig loader chain, then the environment, then extensions
     * and also the base set of twig vars
     *
     * @return $this
     */
    public function init()
    {
        if (null === $this->twig) {
            /** @var Config $config */
            $config = $this->grav['config'];
...

Since the security sandbox does not protect the Twig object it is possible to interact with it (e.g. call its methods, read/write its attributes) through opportunely crafted Twig template directives injected on a web page.
Then an authenticated editor user could be able to add arbitrary functions into the Twig attributes "system.twig.safe_functions" and "system.twig.safe_filters" in order to circumvent the Grav CMS sandbox.

PoC

An authenticated user with the permissions to edit a page (having Twig processing enabled) on the Grav CMS admin console, could create/edit a web page containing a malicious template directive to execute arbitrary OS commands on the remote web server.
For instance, in order to abuse the vulnerability and execute the prohibited "system('id')" code, bypassing the sandbox, the editor could generate a web page containing the following template directives:

{% set arr = {'1':'system', '2':'foo'} %}
{{ var_dump(grav.twig.twig_vars['config'].set('system.twig.safe_functions', arr)) }}
{{ system('id') }}

Once saved the malicious page could be accessed by unauthenticated users to execute the "system('id')" code on the remote server hosting the vulnerable Grav CMS.

Impact

It is possible to execute remote code on the underlying server and compromise it.

Tested version

Grav CMS v1.7.43

Reported by

Maurizio Siddu

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 v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

CVE ID

CVE-2024-28116

Credits