CVE-2025-66294

Grav is a file-based Web platform. Prior to 1.8.0-beta.27, a Server-Side Template Injection (SSTI) vulnerability exists in Grav that allows authenticated attackers with editor permissions to execute arbitrary commands on the server and, under certain conditions, may also be exploited by unauthenticated attackers. This vulnerability stems from weak regex validation in the cleanDangerousTwig method. This vulnerability is fixed in 1.8.0-beta.27.

Grav is a file-based Web platform. Prior to 1.8.0-beta.27, a Server-Side Template Injection (SSTI) vulnerability exists in Grav that allows authenticated attackers with editor permissions to execute arbitrary commands on the server and, under certain conditions, may also be exploited by unauthenticated attackers. This vulnerability stems from weak regex validation in the cleanDangerousTwig method. This vulnerability is fixed in 1.8.0-beta.27.

Summary

A Server-Side Template Injection (SSTI) vulnerability exists in Grav that allows authenticated attackers with editor permissions to execute arbitrary commands on the server and, under certain conditions, may also be exploited by unauthenticated attackers. This vulnerability stems from weak regex validation in the cleanDangerousTwig method.

Important

• First of all this vulnerability is due to weak sanitization in the method clearDangerousTwig, so any other class that calls it indirectly through for example $twig->processString to sanitize code is also vulnerable.

• For this report, we will need the official Form and Admin plugin installed, also I will be chaining this with another vulnerability to allow an editor which is a user with only pages permissions to edit the process section of a form.

• I made another report for the other vulnerability which is a Broken Access Control which allows a user with full permission for pages to change the process section by intercepting the request and modifying it.

Permissions Needed

• The main case for this vulnerability is an editor which can unconditionally takeover the whole system through creating a vulnerable form.
• Second case is as an unauthenticated user, so if the form exists already and accepts user input and puts it through evaluate_twig, a guest can takeover the system.

Details

When we make a form with a process section and a message action, when the form is submitted we get to deal with onFormProcess in form.php through the message case:

            case 'message':
                $translated_string = $this->grav['language']->translate($params);
                $vars = array(
                    'form' => $form
                );

                /** @var Twig $twig */
                $twig = $this->grav['twig'];
                $processed_string = $twig->processString($translated_string, $vars);

                $form->message = $processed_string;
                break;

Which takes our parameters as in our action values, like in our case the value of our message action and sends it to processString which then calls the method cleanDangerousTwig from Security.php, now here's where we find the vulnerability is caused by two things:

• First of all is weak regex which doesn't account for nested function calls, which allows us to bypass this function's sanitization
• Second issue which is the evaluate and evaluate_twig functions which are allowed, and since we can call Twig syntax from inside them, it will lead to nested function calls which we can bypass and thus execute arbitrary payloads.

    public static function cleanDangerousTwig(string $string): string
    {
        if ($string === '') {
            return $string;
        }

        $bad_twig = [
            'twig_array_map',
            'twig_array_filter',
            'call_user_func',
            'registerUndefinedFunctionCallback',
            'undefined_functions',
            'twig.getFunction',
            'core.setEscaper',
            'twig.safe_functions',
            'read_file',
        ];
         
        // This allows for a payload like {{ evaluate("read_file('/etc/passwd')") }}
        $string = preg_replace('/(({{\s*|{%\s*)[^}]*?(' . implode('|', $bad_twig) . ')[^}]*?(\s*}}|\s*%}))/i', '{# $1 #}', $string);
        return $string;
    }

PoC

First to showcase how the function handles the payload, I built a small php program that replicates the behavior of cleanDangerousTwig:

<?php

function cleanDangerousTwig(string $string): string
{
    if ($string === '') {
        return $string;
    }

    $bad_twig = [
        'twig_array_map',
        'twig_array_filter',
        'call_user_func',
        'registerUndefinedFunctionCallback',
        'undefined_functions',
        'twig.getFunction',
        'core.setEscaper',
        'twig.safe_functions',
        'read_file',
    ];
    $string =…

Grav is a file-based Web platform. Prior to 1.8.0-beta.27, a Server-Side Template Injection (SSTI) vulnerability exists in Grav that allows authenticated attackers with editor permissions to execute arbitrary commands on the server and, under certain conditions, may also be exploited by unauthenticated attackers. This vulnerability stems from weak regex validation in the cleanDangerousTwig method. This vulnerability is fixed in 1.8.0-beta.27.

Grav is a file-based Web platform. Prior to 1.8.0-beta.27, a Server-Side Template Injection (SSTI) vulnerability exists in Grav that allows authenticated attackers with editor permissions to execute arbitrary commands on the server and, under certain conditions, may also be exploited by unauthenticated attackers. This vulnerability stems from weak regex validation in the cleanDangerousTwig method. This vulnerability is fixed in 1.8.0-beta.27.

Summary

A Server-Side Template Injection (SSTI) vulnerability exists in Grav that allows authenticated attackers with editor permissions to execute arbitrary commands on the server and, under certain conditions, may also be exploited by unauthenticated attackers. This vulnerability stems from weak regex validation in the cleanDangerousTwig method.

Important

• First of all this vulnerability is due to weak sanitization in the method clearDangerousTwig, so any other class that calls it indirectly through for example $twig->processString to sanitize code is also vulnerable.

• For this report, we will need the official Form and Admin plugin installed, also I will be chaining this with another vulnerability to allow an editor which is a user with only pages permissions to edit the process section of a form.

• I made another report for the other vulnerability which is a Broken Access Control which allows a user with full permission for pages to change the process section by intercepting the request and modifying it.

Permissions Needed

• The main case for this vulnerability is an editor which can unconditionally takeover the whole system through creating a vulnerable form.
• Second case is as an unauthenticated user, so if the form exists already and accepts user input and puts it through evaluate_twig, a guest can takeover the system.

Details

When we make a form with a process section and a message action, when the form is submitted we get to deal with onFormProcess in form.php through the message case:

            case 'message':
                $translated_string = $this->grav['language']->translate($params);
                $vars = array(
                    'form' => $form
                );

                /** @var Twig $twig */
                $twig = $this->grav['twig'];
                $processed_string = $twig->processString($translated_string, $vars);

                $form->message = $processed_string;
                break;

Which takes our parameters as in our action values, like in our case the value of our message action and sends it to processString which then calls the method cleanDangerousTwig from Security.php, now here's where we find the vulnerability is caused by two things:

• First of all is weak regex which doesn't account for nested function calls, which allows us to bypass this function's sanitization
• Second issue which is the evaluate and evaluate_twig functions which are allowed, and since we can call Twig syntax from inside them, it will lead to nested function calls which we can bypass and thus execute arbitrary payloads.

    public static function cleanDangerousTwig(string $string): string
    {
        if ($string === '') {
            return $string;
        }

        $bad_twig = [
            'twig_array_map',
            'twig_array_filter',
            'call_user_func',
            'registerUndefinedFunctionCallback',
            'undefined_functions',
            'twig.getFunction',
            'core.setEscaper',
            'twig.safe_functions',
            'read_file',
        ];
         
        // This allows for a payload like {{ evaluate("read_file('/etc/passwd')") }}
        $string = preg_replace('/(({{\s*|{%\s*)[^}]*?(' . implode('|', $bad_twig) . ')[^}]*?(\s*}}|\s*%}))/i', '{# $1 #}', $string);
        return $string;
    }

PoC

First to showcase how the function handles the payload, I built a small php program that replicates the behavior of cleanDangerousTwig:

<?php

function cleanDangerousTwig(string $string): string
{
    if ($string === '') {
        return $string;
    }

    $bad_twig = [
        'twig_array_map',
        'twig_array_filter',
        'call_user_func',
        'registerUndefinedFunctionCallback',
        'undefined_functions',
        'twig.getFunction',
        'core.setEscaper',
        'twig.safe_functions',
        'read_file',
    ];
    $string =…