The Permissions-Policy directive (previously known as Feature-Policy) is a security-focused mechanism that enables website owners to manage which browser features their web pages can access.

One specific directive within this policy is "document-domain", which is used to control whether a website is allowed to modify the document.domain property. This property has long been used to facilitate communication between different subdomains of the same origin, but it also introduces potential security vulnerabilities if misused.

History and Origin of Permissions-Policy

The Permissions-Policy header was introduced by Google in 2018 under the name Feature-Policy. Its primary goal was to give developers more fine-grained control over what features a website can access, reducing the attack surface and preventing unauthorised access to powerful APIs or browser features. Over time, as web applications became more complex and the need for stricter security controls grew, Permissions-Policy evolved, with several new directives being introduced to manage sensitive features.

One such directive is "document-domain", which controls whether the website can modify the document.domain property. This property was originally introduced to enable communication between subdomains (e.g., example.com and sub.example.com) by setting document.domain to a common value. However, this flexibility has created security issues in modern web applications, as it can be exploited in certain attacks. The document-domain directive was added to give developers control over whether or not this functionality can be used, ensuring that websites do not expose themselves to unnecessary risks.

What Does the Document-Domain Directive Do?

The "document-domain" directive determines whether a web page is allowed to set or modify the document.domain property. By default, browsers enforce the same-origin policy, which restricts communication between different origins (i.e., a web page from example.com cannot interact with sub.example.com unless specific permissions are granted). The document.domain property was introduced to loosen this restriction between subdomains by allowing them to communicate if they share a common document.domain.

For example:

• Setting document-domain=() will disallow any modification of the document.domain property, enforcing strict same-origin rules.
• Allowing it (document-domain=*) would permit the page to modify this property, enabling communication between different subdomains.

While this might be useful in some legacy systems or specific web applications, the use of document.domain in modern web development is generally discouraged due to its potential to weaken origin isolation and increase the risk of cross-origin attacks.

Why Was It Added?

The document-domain directive was added to the Permissions-Policy header to address several important security concerns:

1. Strengthening Same-Origin Policy: The same-origin policy is a critical security measure that restricts how scripts from one origin can interact with resources from another origin. By allowing websites to modify document.domain, this policy is weakened, as subdomains can communicate more freely, potentially leading to unintended data exposure or security vulnerabilities. The directive allows developers to enforce strict same-origin policies, preventing such communication when it is not needed.

2. Mitigating Cross-Site Scripting (XSS) Risks: Cross-site scripting (XSS) is a common web vulnerability where malicious scripts are injected into a web page. If an attacker gains control over a subdomain (often the weaker point in a site’s security), they could use the modified document.domain property to attack the primary domain by accessing cookies or other sensitive data. By restricting the ability to modify document.domain, developers can reduce the risk of such attacks.

3. Reducing Attack Surface in Modern Web Applications: In modern web development, there are alternative ways to facilitate secure communication between subdomains, such as using Cross-Origin Resource Sharing (CORS) or the postMessage API. These methods are more secure and avoid the risks associated with modifying document.domain. The directive encourages developers to adopt these modern security practices, reducing the likelihood of legacy vulnerabilities.

Use Cases It Guards Against

The document-domain directive is particularly valuable for protecting against several problematic scenarios:

1. Cross-Site Scripting (XSS) Exploits via Subdomains: A compromised subdomain could allow an attacker to modify the document.domain property, enabling them to access cookies or session data from the primary domain. By preventing the modification of document.domain, the directive protects sensitive data and prevents an attacker from escalating privileges across subdomains.

2. Cross-Origin Data Leakage: Websites with multiple subdomains often handle sensitive user data. If the document.domain property is misused, subdomains that should not have access to specific data could inadvertently gain access, leading to privacy violations. By enforcing a strict same-origin policy, this risk is mitigated.

3. Legacy Applications: Some legacy applications still rely on modifying the document.domain property for inter-subdomain communication. While this might have been a common practice in the past, it is now considered a security risk. The document-domain directive helps phase out this practice by giving developers the ability to block its use, encouraging the adoption of more secure alternatives.

4. Privilege Escalation Attacks: If a less secure subdomain is allowed to modify document.domain, an attacker could potentially use this as an entry point to attack the more secure primary domain. By restricting access to document.domain, developers can ensure that the integrity of the primary domain is maintained, even if a subdomain is compromised.

Why Should You Set Permissions-Policy Document-Domain Correctly?

There are several compelling reasons why website owners should configure the document-domain directive appropriately:

1. Protecting Against Cross-Domain Attacks: One of the primary functions of the same-origin policy is to protect against cross-domain attacks. By preventing modification of document.domain, website owners can ensure that their security boundaries between different subdomains remain intact, reducing the risk of attackers using one subdomain to access data from another.

2. Preventing XSS Vulnerabilities: Cross-site scripting (XSS) is one of the most common web vulnerabilities, and it can be exacerbated when the document.domain property is modified to allow subdomain communication. By enforcing strict policies that prevent modification of this property, developers reduce the potential for XSS attacks, helping to safeguard both user data and website functionality.

3. Encouraging Modern Security Practices: The use of document.domain is considered outdated in modern web development, with more secure alternatives like CORS or postMessage available. By setting the document-domain directive to block modifications, developers can encourage the adoption of more secure methods of inter-domain communication, improving the overall security posture of their websites.

4. Maintaining User Trust: Websites that handle sensitive data—whether personal information, financial details, or otherwise—need to demonstrate strong security practices. By configuring the document-domain directive correctly, website owners show a commitment to security, helping to build trust with their users and protect their reputation.

5. Reducing the Attack Surface: One of the key principles of web security is to reduce the attack surface—i.e., limiting the features and permissions that are available to potential attackers. By restricting the use of document.domain, website owners can reduce the overall attack surface of their site, making it harder for malicious actors to exploit vulnerabilities.

Conclusion: Securing Subdomain Communication with the Document-Domain Directive

The Permissions-Policy document-domain directive is a powerful tool for improving web security by controlling the ability of websites to modify the document.domain property. While this property was historically useful for facilitating communication between subdomains, it also presents significant security risks, particularly in the context of cross-site scripting and data leakage attacks.

By properly configuring the document-domain directive, website owners can enforce strict same-origin policies, reduce their attack surface, and protect against a wide range of vulnerabilities. In today’s digital landscape, where privacy and security are paramount, ensuring the proper use of the document-domain directive is a critical step in safeguarding user data and maintaining trust. Whether you're running a legacy application or a modern web platform, correctly setting this directive is essential for maintaining a secure and resilient online presence.