RequestPolicy: A Complete Guide for Developers

How RequestPolicy Improves Web Security and Privacy### Introduction

RequestPolicy is a browser extension and a design approach that gives users and site authors fine-grained control over which third-party requests are allowed while loading web pages. By default, modern websites often include content, scripts, images, fonts, analytics, and advertising loaded from many external domains. Each third-party request expands the attack surface and can enable cross-site tracking, resource compromise, and privacy violations. RequestPolicy reduces those risks by blocking unwanted cross-origin requests and empowering explicit, user- or site-defined policies.

What RequestPolicy Does (Core Functions)

• Blocks cross-origin requests by default, including scripts, iframes, images, stylesheets, fonts, and XHR/fetch calls that target different origins than the page’s.
• Allows users to create allowlists (per-site or global) to permit specific third-party domains.
• Offers temporary allow options for one-time needs (e.g., to log in or to use an embedded widget) without creating persistent trust.
• Logs and shows blocked requests so users can make informed decisions and troubleshoot site functionality.

How It Improves Security

1. Reduces third-party script injection risk
   Third-party scripts run with the privileges of the page and can perform malicious actions (exfiltrate data, inject further resources, or execute drive‑by attacks). By blocking third-party script requests unless explicitly allowed, RequestPolicy limits the number of external scripts that can execute in a page’s context.

2. Limits supply-chain attack surface
   Many supply-chain attacks happen when an external dependency or ad network is compromised. Preventing automatic loading of resources from numerous external hosts reduces the likelihood that a compromised third-party will affect the user.

3. Blocks malicious cross-site requests (CSRF/CSRF-like exfiltration)
   Some cross-site requests can be abused to perform actions or leak information. RequestPolicy’s control over outgoing requests reduces unexpected cross-origin interactions, lowering the attack surface for CSRF-like flows.

4. Prevents drive-by downloads and malicious iframes
   Iframes or embedded content from untrusted origins can host malware or phishing pages. Blocking cross-origin iframes and plugin resources by default prevents many drive-by attacks.

How It Improves Privacy

1. Stops cross-site tracking and third-party fingerprinting
   Third-party resources—trackers, CDNs, analytics, advertising networks—tie browsing activity across sites. Blocking those requests by default prevents many common cross-site tracking techniques.

2. Limits identifier leakage via referer and cookies
   When third-party requests are prevented, fewer requests carry referer headers or cookies that would otherwise reveal the user’s browsing context to external domains.

3. Reduces third-party storage access
   Blocking requests to tracking domains prevents scripts from setting or reading third-party cookies, localStorage entries, or other storage mechanisms used for cross-site tracking and fingerprinting.

4. Makes data flows explicit
   Because RequestPolicy requires explicit allow rules, users (or site administrators) must consciously permit data exchange with third parties. This transparency helps users understand which external domains receive their data.

Practical Examples and Use Cases

• Privacy-conscious browsing: Users who want to prevent trackers, ads, and behavioral profiling can use RequestPolicy to block analytics and advertising domains.
• Corporate and enterprise environments: Companies can enforce policies that block access to certain external resources, reducing exfiltration risk and ensuring compliance.
• Developers debugging resource dependencies: Developers can see which external domains a site relies on and temporarily allow them to replicate production behavior in a controlled way.
• Secure kiosk or public workstation setups: Lock down external requests to only required domains to minimize exposure on shared devices.

Usability Considerations and Trade-offs

• Breakage vs. privacy: Aggressively blocking cross-origin requests will often break site functionality (embedded videos, payment widgets, social logins). RequestPolicy mitigates this with temporary allow options and per-site allowlists, but users must balance convenience and privacy.
• Management overhead: Maintaining allowlists can be time-consuming for users who frequently visit complex sites. Rulesets or site profiles can reduce friction.
• False sense of security: RequestPolicy reduces many risks but is not a complete security solution—same-origin vulnerabilities, malicious content served from first-party domains, or browser bugs remain possible attack vectors.

Tips for Effective Use

• Start in learning mode: Log blocked requests for a few days without blocking, to see what sites need.
• Use temporary allows for one-off features and then revoke them.
• Create per-site rules for sites you trust and global rules for commonly used CDNs or essential services.
• Combine RequestPolicy with other defenses: content security policies (CSP), browser sandboxing, tracker-blockers, and script-blockers for layered protection.

Comparison with Related Tools

Limitations and Future Directions

• Automation and rule-sharing: Better rule-sharing (community-maintained allowlists) and machine-learning suggestions could reduce user burden.
• Integration with browser privacy features: Closer integration with builtin browser controls (e.g., automatic referer trimming, cookie partitioning) would provide stronger default privacy.
• Standardization: Server-driven mechanisms like stricter CORS and CSP adoption reduce reliance on extensions, but user-side tools remain valuable where servers don’t implement protections.

Conclusion

RequestPolicy strengthens web security and privacy by making cross-origin requests explicit and controllable. By blocking third-party requests by default and requiring explicit allows, it reduces exposure to supply‑chain attacks and cross‑site tracking while making data flows transparent. Balancing usability with protection, and combining RequestPolicy with other controls, yields the best real-world privacy and security outcomes.