Craft CMS vulnerable to Remote Code Execution via unrestricted file extension

Summary

Unrestricted file extension lead to a potential Remote Code Execution (Authenticated, ALLOW_ADMIN_CHANGES=true)

Details

Vulnerability Cause :

If the name parameter value is not empty string('') in the View.php's doesTemplateExist() -> resolveTemplate() -> _resolveTemplateInternal() -> _resolveTemplate() function, it returns directly without extension verification, so that arbitrary extension files are rendered as twig templates (even if they are not extensions set in defaultTemplateExtensions = ['html', 'twig'])

    /**
     * Searches for a template files, and returns the first match if there is one.
     *
     * @param string $basePath The base path to be looking in.
     * @param string $name The name of the template to be looking for.
     * @param bool $publicOnly Whether to only look for public templates (template paths that don’t start with the private template trigger).
     * @return string|null The matching file path, or `null`.
     */
    private function _resolveTemplate(string $basePath, string $name, bool $publicOnly): ?string
    {
        // Normalize the path and name
        $basePath = FileHelper::normalizePath($basePath);
        $name = trim(FileHelper::normalizePath($name), '/');

        // $name could be an empty string (e.g. to load the homepage template)
        if ($name !== '') {
            if ($publicOnly && preg_match(sprintf('/(^|\/)%s/', preg_quote($this->_privateTemplateTrigger, '/')), $name)) {
                return null;
            }

            // Maybe $name is already the full file path
            $testPath = $basePath . DIRECTORY_SEPARATOR . $name;

            if (is_file($testPath)) {
                return $testPath;
            }

            foreach ($this->_defaultTemplateExtensions as $extension) {
                $testPath = $basePath . DIRECTORY_SEPARATOR . $name . '.' . $extension;

                if (is_file($testPath)) {
                    return $testPath;
                }
            }
        }

        foreach ($this->_indexTemplateFilenames as $filename) {
            foreach ($this->_defaultTemplateExtensions as $extension) {
                $testPath = $basePath . ($name !== '' ? DIRECTORY_SEPARATOR . $name : '') . DIRECTORY_SEPARATOR . $filename . '.' . $extension;

                if (is_file($testPath)) {
                    return $testPath;
                }
            }
        }

        return null;
    }

When attacker with admin privileges on the DEV or Misconfigured STG, PROD, they can exploit this vulnerability to remote code execution (ALLOW_ADMIN_CHANGES=true)

PoC

Step 1) Create a new filesystem. Base Path: /var/www/html/templates

Step 2) Create a new asset volume. Asset Filesystem: template