Next.js server actions represent a modern server-side function invocation mechanism where all server actions are POSTed to a single endpoint, distinguished only by hashed identifiers sent in the Next-Action header. These hashes are opaque and change with each build, making manual penetration testing difficult because testers cannot easily determine which hash corresponds to which server-side function. However, when the Next.js application is built with productionBrowserSourceMaps enabled, the JavaScript bundles contain source maps that map these hashes back to the original function names. The NextjsServerActionAnalyzer Burp extension leverages this by scanning proxy history for JavaScript chunks containing createServerReference calls and extracting mappings between hash IDs and function names using regex patterns. This allows penetration testers to identify which server actions correspond to which functions, track usage across builds, and detect unused or hidden server actions that might be vulnerable or expose sensitive functionality. The extension also automates the creation of test requests for these actions, significantly improving testing efficiency. The presence of productionBrowserSourceMaps in production environments, while useful for debugging, inadvertently leaks internal application structure and function names, increasing the attack surface. This can facilitate targeted attacks such as unauthorized data access, privilege escalation, or remote code execution if vulnerabilities exist in these server actions. The tool was demonstrated on a real-world Next.js application with dozens of server actions, revealing unused actions like exportFinancialData() that could be tested for vulnerabilities. Although no direct exploits are reported, the exposure of detailed server action mappings combined with potential vulnerabilities in these actions represents a significant security risk.

For European organizations, the exposure of server action mappings due to enabled productionBrowserSourceMaps can lead to increased risk of targeted attacks against backend functionality. Attackers or penetration testers can identify sensitive or critical server actions such as deleteUserAccount() or exportUserData() that may not be accessible through the UI but still exist on the server. This can lead to unauthorized data exfiltration, account takeover, or manipulation of critical business logic. The impact is particularly severe for organizations handling sensitive personal data (e.g., GDPR-regulated data), financial information, or critical infrastructure. Additionally, the ease of mapping hashes to function names lowers the barrier for attackers to discover and exploit vulnerabilities in server actions. This threat can undermine confidentiality and integrity of data and services, potentially causing reputational damage, regulatory penalties, and operational disruption. The risk is amplified in environments where productionBrowserSourceMaps are left enabled inadvertently, a common misconfiguration. European companies using Next.js extensively in their web applications, especially those with complex server-side logic, are at heightened risk.

1. Disable productionBrowserSourceMaps in production builds to prevent leaking source map information and function mappings. 2. Conduct regular security reviews and penetration tests focusing on Next.js server actions, using tools like NextjsServerActionAnalyzer to identify and test all server actions, including unused ones. 3. Implement strict access controls and input validation on all server actions to minimize risk of exploitation. 4. Monitor server logs for unusual or unauthorized server action invocations, especially those not triggered by the UI. 5. Use application-layer firewalls or runtime application self-protection (RASP) to detect and block suspicious requests targeting server actions. 6. Educate development teams about the risks of leaving debugging features enabled in production and enforce secure build pipelines. 7. Keep Next.js and related dependencies up to date to benefit from security patches addressing server action vulnerabilities. 8. Consider code audits or static analysis to identify potentially risky server actions that expose sensitive functionality.