GHSA-287w-mxq6-x2cp Craft CMS: Sensitive File Disclosure / Server-Side File Read

• Packagist craftcms/cms: >= 4.0.0-RC1, < 4.18.0 → fixed in 4.18.0 • Packagist craftcms/cms: >= 5.0.0-RC1, < 5.10.0 → fixed in 5.10.0

The `dataUrl()` Twig function is included in Craft’s Twig sandbox allowlist, allowing any control panel user granted the `utility:system-messages` permission to embed a file-reading payload into system email templates. When those emails are sent, the server reads the target file and returns its contents as a base64-encoded data URL embedded in the email body. The .env file, which typically contains the database password, CRAFT_SECURITY_KEY, and third-party API keys, passes all of Craft’s existing `dataUrl()` protection checks and is fully exfiltrated. Obtaining CRAFT_SECURITY_KEY enables an attacker to forge session tokens and escalate to full admin account takeover. Details Affected versions: Craft CMS 4.x, 5.x (confirmed against 5.9.19) The vulnerability arises from the combination of three code facts: 1. dataUrl is in the Twig sandbox allowlist src/config/twig-sandbox.php, line 115: php'allowedFunctions' => [ ... 'dataUrl', // ← allows file reading inside sandboxed templates ... ], 2. Html::dataUrl() does not block dotfiles src/helpers/Html.php, lines 1065–1090. The function applies four checks before reading a file: Must be within the project root .env is at the root Must not be in a system directory (config/, vendor/, storage/, templates/) .env is not Must not be a .php file .env has no extension File must exist .env always exists in a Craft install There is no check for dotfiles or specifically for .env. All four checks pass silently and file_get_contents() is called, with the result returned as data:text/plain;base64,.... 3. System message body is rendered via renderSandboxedString() src/mail/Mailer.php, lines 181–183: php$subject = $view->renderSandboxedString($systemMessage->subject, $variables); $textBody = $view->renderSandboxedString($systemMessage->body, $variables); $htmlBody = $view->renderSandboxedString($systemMessage->body, $variables, escapeHtml: true); Any body content saved to a system message is executed inside the Twig sandbox when the email renders. Because dataUrl is in allowedFunctions, the sandbox policy permits its execution without restriction. Access control: The utility:system-messages permission is a non-admin CP permission grantable to any user group via Settings > Users > Groups. It is not restricted to admins. Impact Vulnerability type: Sensitive File Disclosure / Server-Side File Read Who is impacted: Any Craft CMS 4.x or 5.x installation where at least one non-admin control panel user has been granted the utility:system-messages permission, and email sending is configured. Resources: • https://github.com/craftcms/cms/pull/18559