🛡️ CVE-2023-34252 — grav

Grav Server-side Template Injection (SSTI) via Twig Default Filters

Hi,

actually we have sent the bug report to [security@getgrav.org](mailto:[email protected]) on 27th March 2023 and on 10th April 2023.

# Grav Server-side Template Injection (SSTI) via Insufficient Validation in filterFilter

Summary:

| Product | Grav CMS |

| ----------------------- | --------------------------------------------- |

| Vendor | Grav |

| Severity | High - Users with login access to Grav Admin panel and page creation/update permissions are able to obtain remote code/command execution |

| Affected Versions | <= [v1.7.40](https://github.com/getgrav/grav/tree/1.7.40) (Commit [685d762](https://github.com/getgrav/grav/commit/685d76231a057416651ed192a6a2e83720800e61)) (Latest version as of writing) |

| Tested Versions | v1.7.40 |

| Internal Identifier | STAR-2023-0007 |

| CVE Identifier | TBD |

| CWE(s) | CWE-20: Improper Input Validation, CWE-1336: Improper Neutralization of Special Elements Used in a Template Engine |

CVSS3.1 Scoring System:

Base Score: 7.2 (High)

Vector String: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H

| Metric | Value |

| ---------------------------- | --------- |

| Attack Vector (AV) | Network |

| Attack Complexity (AC) | Low |

| Privileges Required (PR) | High |

| User Interaction (UI) | None |

| Scope (S) | Unchanged |

| Confidentiality \(C) | High |

| Integrity (I) | High |

| Availability (A) | High |

Product Overview:

Grav is a PHP-based flat-file content management system (CMS) designed to provide a fast and simple way to build websites. It supports rendering of web pages written in Markdown and Twig expressions, and provides an administration panel to manage the entire website via an optional Admin plugin.

Vulnerability Summary:

There is a logic flaw in the GravExtension.filterFilter() function whereby validation against a denylist of unsafe functions is only performed when the argument passed to filter is a string. However, passing an array as a callable argument allows the validation check to be skipped. Consequently, a low privileged attacker with login access to Grav Admin panel and page creation/update permissions is able to inject malicious templates to obtain remote code execution.

Vulnerability Details:

The vulnerability can be found in the GravExtension.filterFilter() function declared in [/system/src/Grav/Common/Twig/Extension/GravExtension.php](https://github.com/getgrav/grav/blob/1.7.40/system/src/Grav/Common/Twig/Extension/GravExtension.php#L1692-L1698):

~~~php

...

class GravExtension extends AbstractExtension implements GlobalsInterface

{

...

/**

• Return a list of all filters.

*

• @return array

*/

public function getFilters(): array

{

return [

...

// Security fix

new TwigFilter('filter', [$this, 'filterFilter'], ['needs_environment' => true]),

];

}

...

/**

• @param Environment $env
• @param array $array
• @param callable|string $arrow
• @return array|CallbackFilterIterator
• @throws RuntimeError

*/

function filterFilter(Environment $env, $array, $arrow)

{

if (is_string($arrow) && Utils::isDangerousFunction($arrow)) { // [1]

throw new RuntimeError('Twig |filter("' . $arrow . '") is not allowed.');

}

return \twig_array_filter($env, $array, $arrow); // [2]

}

}

~~~

At [1], the $arrow parameter contains the argument supplied to the filter. For example, it may refer to "funcname" in {{ array|filter("funcname") }} or the closure (a.k.a. arrow function) el => el != 'exclude' in {{ array|filter(el => el != 'exclude') }}. Observe that Utils::isDangerousFunction($arrow) is only invoked if $arrow is a string. As such, non-string arguments may be passed to twig_array_filter() at [2] due to the absence of type enforcement at [1].

The implementation of the twig_array_filter() function can be found in [/src/Extension/CoreExtension.php](https://github.com/twigphp/Twig/blob/v1.44.7/src/Extension/CoreExtension.php) within Twig's codebase:

~~~php

function twig_array_filter(Environment $env, $array, $arrow)

{

if (!twig_test_iterable($array)) {

throw new RuntimeError(sprintf('The "filter" filter expects an array or "Traversable", got "%s".', \is_object($array) ? \get_class($array) : \gettype($array)));

}

if (!$arrow instanceof Closure && $env->hasExtension('\Twig\Extension\SandboxExtension') && $env->getExte