Original Post: Exploiting dynamic rendering engines to take control of web apps
Summary:
Dynamic rendering is a technique that serves pre-rendered web pages to crawlers for better SEO. Popular tools for dynamic rendering are Rendertron and Prerender, but they can introduce vulnerabilities if not configured properly. The author exploited a vulnerability in Rendertron to take over a production web application, earning $5,000 through a bug bounty program.
Modern websites often use JavaScript frameworks that are not SEO-friendly since bots can’t render JavaScript. Dynamic rendering addresses this by serving HTML content to crawlers. Security concerns arise when headless browsers running in production are exploited through dynamic rendering applications. The author discovered many dynamic rendering tools with vulnerabilities using Semgrep.
Key Points:
- Dynamic rendering helps bots index JavaScript-heavy websites by pre-rendering content server-side.
- Popular tools like Rendertron and Prerender are prone to vulnerabilities that can lead to SSRF and other attacks.
- The author used Semgrep to identify vulnerable instances and exploited an open redirect in a bug bounty program.
- Safeguards like restricting local network requests and proper configurations can mitigate these risks.
- Both attackers and defenders should be vigilant about headless browser security issues.
Conclusion:
Dynamic rendering is beneficial for modern websites but poses security risks if not managed correctly. Tools like Semgrep can help detect vulnerabilities early in development.
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