Posted by Rob Mann, Security Engineering Manager

[Cross-posted from the Google Cloud Platform Blog]

Deploying a new build is a thrill, but every release should be scanned for security vulnerabilities. And while web application security scanners have existed for years, they’re not always well-suited for Google App Engine developers. They’re often difficult to set up, prone to over-reporting issues (false positives)—which can be time-consuming to filter and triage—and built for security professionals, not developers.

Today, we’re releasing Google Cloud Security Scanner in beta. If you’re using App Engine, you can easily scan your application for two very common vulnerabilities: cross-site scripting (XSS) and mixed content.

While designing Cloud Security Scanner we had three goals:

1. Make the tool easy to set up and use
2. Detect the most common issues App Engine developers face with minimal false positives
3. Support scanning rich, JavaScript-heavy web applications

To try it for yourself, select Compute > App Engine > Security scans in the Google Developers Console to run your first scan, or learn more here.

So How Does It Work?

Crawling and testing modern HTML5, JavaScript-heavy applications with rich multi-step user interfaces is considerably more challenging than scanning a basic HTML page. There are two general approaches to this problem:

1. Parse the HTML and emulate a browser. This is fast, however, it comes at the cost of missing site actions that require a full DOM or complex JavaScript operations.
2. Use a real browser. This approach avoids the parser coverage gap and most closely simulates the site experience. However, it can be slow due to event firing, dynamic execution, and time needed for the DOM to settle.

Cloud Security Scanner addresses the weaknesses of both approaches by using a multi-stage pipeline. First, the scanner makes a high speed pass, crawling, and parsing the HTML. It then executes a slow and thorough full-page render to find the more complex sections of your site.

While faster than a real browser crawl, this process is still too slow. So we scale horizontally. Using Google Compute Engine, we dynamically create a botnet of hundreds of virtual Chrome workers to scan your site. Don’t worry, each scan is limited to 20 requests per second or lower.

Then we attack your site (again, don’t worry)! When testing for XSS, we use a completely benign payload that relies on Chrome DevTools to execute the debugger. Once the debugger fires, we know we have JavaScript code execution, so false positives are (almost) non-existent. While this approach comes at the cost of missing some bugs due to application specifics, we think that most developers will appreciate a low effort, low noise experience when checking for security issues—we know Google developers do!

As with all dynamic vulnerability scanners, a clean scan does not necessarily mean you’re security bug free. We still recommend a manual security review by your friendly web app security professional.

Ready to get started? Learn more here. Cloud Security Scanner is currently in beta with many more features to come, and we’d love to hear your feedback. Simply click the “Feedback” button directly within the tool.