Craft CMS: save_images_Asset graphql mutation can be abused to exfiltrate AWS credentials of underlying host

Summary

• The save_images_Asset graphql mutation allows a user to give a url of an image to download. (Url must use a domain, not a raw IP.)
• Attacker sets up domain attacker.domain with an A record of something like 169.254.169.254 (special AWS metadata IP)
• Attacker invokes save_images_Asset with url: http://attacker.domain/latest/meta-data/iam/security-credentials and filename "foo.txt"
• Craft fetches sensitive information on attacker's behalf, and makes it available for download at /assets/images/foo.txt
• Normal checks to verify that image is valid are bypassed because of .txt extension
• Normal checks to verify that url is not an IP address are bypassed because user provided a valid domain that resolves to a sensitive internal IP address

Details

handleUpload() in src/gql/resolvers/mutations/Assets.php contains the code that processes the save_images_Asset mutation.

It has some basic validation logic for the url parameter (source of the image) and filename parameter (what to save image as):

   } elseif (!empty($fileInformation['url'])) {
            $url = $fileInformation['url'];

            // make sure the hostname is alphanumeric and not an IP address
            $hostname = parse_url($url, PHP_URL_HOST);
            if (
                !filter_var($hostname, FILTER_VALIDATE_DOMAIN, FILTER_FLAG_HOSTNAME) ||
                filter_var($hostname, FILTER_VALIDATE_IP)
            ) {
                throw new UserError("$url contains an invalid hostname.");
            }

            if (empty($fileInformation['filename'])) {
                $filename = AssetsHelper::prepareAssetName(pathinfo(UrlHelper::stripQueryString($url), PATHINFO_BASENAME));
            } else {
                $filename = AssetsHelper::prepareAssetName($fileInformation['filename']);
            }

            $extension = strtolower(pathinfo($filename, PATHINFO_EXTENSION));
            if (is_array($allowedExtensions) && !in_array($extension, $allowedExtensions, true)) {
                throw new AssetDisallowedExtensionException(Craft::t('app', "“{$extension}” is not an allowed file extension."));
            }

The upshot of this validation is that url must contain a hostname, not an IP, and filename must contain an allowed extension. If the allowed extension is a typical image extension, further validation will be done downstream to verify that the downloaded content is in fact an image.

An authenticated attacker can trick this mutation into fetching sensitive AWS metadata, or other sensitive information from the craft instance's internal network.

• First, the attacker must register a domain -- e.g. attacker.domain.