The Permissions-Policy directive (previously known as Feature-Policy) is a crucial part of modern web security, providing web developers with the tools to control which browser features are accessible by their websites.

One of the key directives within this policy is "cross-origin-isolated". This directive is designed to safeguard websites by ensuring that certain powerful features or APIs are only available when the web page is "cross-origin isolated," meaning the site is fully isolated from untrusted cross-origin resources.

History and Origin of Permissions-Policy

The Permissions-Policy header was introduced by Google in 2018 under the name Feature-Policy. Its primary objective was to give developers more granular control over web features, reducing the risk of security and privacy vulnerabilities. As the web evolved, the need for tighter security controls around cross-origin resources became more evident, especially with the rise of sophisticated attacks that exploit cross-origin vulnerabilities.

The cross-origin-isolated directive was introduced to enhance security for certain advanced features, such as SharedArrayBuffers, performance APIs, and others that interact closely with the system. By ensuring that these features are only available to websites that fully isolate themselves from untrusted cross-origin resources, the directive helps prevent serious attacks like Spectre and Meltdown, which can exploit side-channel vulnerabilities in browsers to steal sensitive data.

What Does the Cross-Origin-Isolated Directive Do?

The "cross-origin-isolated" directive ensures that websites cannot access certain powerful APIs unless they are fully isolated from untrusted cross-origin resources. This means that the page must be served in a secure environment where all resources, including iframes, are either from the same origin or have the appropriate security controls, such as CORS (Cross-Origin Resource Sharing) headers, in place.

To achieve a cross-origin-isolated state, a website needs to fulfil two key requirements:

1. Cross-Origin Resource Policy (CORP): This ensures that the page only loads resources (such as images, scripts, or iframes) that explicitly permit sharing with the origin.
2. Cross-Origin Opener Policy (COOP): This policy ensures that documents do not share a browsing context with cross-origin content, preventing issues like cross-origin data leaks.

By meeting these requirements, the website gains access to high-precision APIs and advanced features that would otherwise be restricted in a non-isolated context.

Why Was It Added?

The cross-origin-isolated directive was introduced to address several pressing security concerns, especially those related to side-channel attacks and cross-origin resource vulnerabilities. Key motivations behind its addition include:

1. Mitigating Spectre and Meltdown: These are side-channel attacks that exploit weaknesses in modern CPU architectures. By tricking the browser into sharing data across origins, an attacker could potentially extract sensitive information from other websites or browser tabs. Cross-origin isolation helps prevent this by strictly limiting the interaction between origins.

2. Securing Powerful Web APIs: Certain web APIs, like SharedArrayBuffer and WebAssembly, provide low-level access to hardware or high-precision timing data, which could be used by attackers to carry out side-channel attacks. The cross-origin-isolated directive ensures that these powerful APIs are only available to websites that have taken the necessary steps to isolate themselves from potentially harmful cross-origin resources.

3. Enhancing Data Privacy: The directive also prevents cross-origin data leakage by isolating sensitive data within a single origin. This ensures that if a website is compromised or an attacker attempts to inject malicious scripts, they cannot steal data from another origin.

Use Cases It Guards Against

The cross-origin-isolated directive is designed to protect against several high-risk use cases and security threats:

1. Side-Channel Attacks (e.g., Spectre and Meltdown): These attacks exploit the way modern processors handle speculative execution, allowing attackers to read sensitive data from memory that should be inaccessible. By isolating cross-origin content, the directive mitigates the risk of these attacks by preventing malicious code from accessing sensitive data from other origins.

2. Cross-Origin Resource Sharing (CORS) Vulnerabilities: Without proper isolation, a malicious website could load and exploit resources from another origin. This could lead to data theft or manipulation, especially in cases where websites rely on sensitive APIs. Cross-origin isolation ensures that resources are properly protected by requiring explicit permission for cross-origin sharing.

3. Cross-Site Scripting (XSS): Cross-origin isolation helps reduce the risk of XSS attacks by limiting the interaction between origins. This makes it harder for attackers to inject malicious scripts that can access sensitive data from another origin within the same browsing context.

4. High-Precision Timing Attacks: Certain APIs, such as the performance.now() API, provide high-precision timing data that can be used in timing attacks to infer information about other processes or data. The cross-origin-isolated directive restricts access to these APIs unless the website meets strict isolation requirements.

Why Should You Set Permissions-Policy Cross-Origin-Isolated Correctly?

There are several compelling reasons why website owners should ensure their cross-origin-isolated directive is properly configured:

1. Defence Against Advanced Attacks: With the rise of sophisticated attacks like Spectre and Meltdown, it is essential to ensure that your website is protected. The cross-origin-isolated directive acts as a crucial line of defence against these side-channel attacks, reducing the risk that attackers can exploit vulnerabilities in your site or its resources.

2. Enabling Access to Powerful Web Features: Websites that implement cross-origin isolation can unlock access to powerful web APIs that provide enhanced functionality. For example, SharedArrayBuffer is crucial for advanced web applications, such as games or real-time data processing, and is only available to cross-origin-isolated websites. Ensuring that your site meets the isolation requirements allows you to offer more powerful and efficient features to your users.

3. Protecting User Privacy: By restricting interaction between origins, cross-origin isolation helps safeguard user data and prevents attackers from accessing sensitive information across different browsing contexts. This is particularly important for websites that handle confidential data, such as online banking or healthcare services.

4. Boosting Security Posture: As web security continues to evolve, users expect websites to provide robust protections against a variety of threats. By implementing the cross-origin-isolated directive, website owners demonstrate a strong commitment to security, which helps build trust with users and reinforces the site’s credibility.

5. Compliance with Modern Security Standards: In an era of increasing cyber threats and stringent privacy regulations like GDPR, it’s critical for website owners to follow best practices when it comes to data protection. Cross-origin isolation is a modern security standard that helps ensure your site is compliant with the latest recommendations and guidelines for web security.

Conclusion: Ensuring Security with Cross-Origin Isolation

The Permissions-Policy cross-origin-isolated directive is a vital security measure that helps protect websites from advanced attacks such as Spectre and Meltdown, while also enabling access to powerful web APIs that require a secure, isolated environment. By implementing this directive, website owners can significantly reduce the risk of side-channel attacks, cross-origin resource vulnerabilities, and data leaks, ensuring a safer and more secure browsing experience for users.

In a world where cyber threats are constantly evolving, properly configuring the cross-origin-isolated directive is not only a technical best practice but also a critical step towards maintaining a secure, trustworthy, and high-performing website. Whether you’re handling sensitive user data or simply want to offer advanced features securely, ensuring cross-origin isolation is the key to modern web security.