Which Domains and Subdomains Can Access Browser Cookies?

Setting Cookie Domains

To set cookie domains, web servers use the Set-Cookie HTTP header. This header includes the domain attribute, which specifies the scope of the cookie. When setting a cookie, you can include the domain attribute like this:

Set-Cookie: name=value; Domain=example.com

The domain attribute determines which domains can access the cookie. If you don't specify a domain, the browser uses the current domain as the default. This means the cookie is only accessible by the exact domain that set it.

Accessing Cookies Across Domains and Subdomains

The same-origin policy affects cookie access. This security measure prevents websites from accessing data from other domains. For cookies, this means a website can only access cookies that belong to its own domain or parent domains.

When accessing cookies on the main domain:

• Cookies set for example.com are accessible by example.com
• Cookies set for .example.com (with a leading dot) are accessible by example.com and all its subdomains

For subdomain cookie access:

• Subdomains can access cookies set for their parent domain
• Subdomains cannot access cookies set for other subdomains
• A subdomain like sub.example.com can access cookies set for .example.com, but not those set for other.example.com

These rules help maintain security while allowing flexibility in cookie management across domains and subdomains.

Root Domain Cookie Access

Setting cookies for the root domain, like .example.com, allows access across the domain and its subdomains. To set a cookie for the root domain, use this format:

Set-Cookie: name=value; Domain=.example.com

Root domain cookies are accessible by:

• The main domain (example.com)
• All subdomains (www.example.com, blog.example.com, etc.)

This access makes root domain cookies useful for maintaining user sessions across website parts.

Subdomain-Specific Cookie Access

To set cookies for specific subdomains, use the exact subdomain in the Domain attribute:

Set-Cookie: name=value; Domain=www.example.com

Subdomain-specific cookies have limited access:

• They are only accessible by the specified subdomain
• The main domain cannot access these cookies
• Other subdomains cannot access these cookies

This restriction helps isolate data between website sections, improving security and preventing data sharing.

Cross-Subdomain Cookie Sharing

Sharing cookies between subdomains requires planning. Here are some techniques:

1. Use root domain cookies: Set cookies for .example.com to share them across subdomains.
2. Server-side synchronization: Store data server-side and use subdomain-specific cookies as keys to access shared data.
3. Use document.domain: In some cases, set document.domain to a common parent domain to allow cross-subdomain scripting.

Security considerations for cross-subdomain cookie sharing:

• Limit shared data to reduce data leak risks
• Use HTTPS to encrypt cookie data
• Set the Secure flag on cookies to restrict them to HTTPS connections
• Review and update access policies to maintain data isolation

When implementing cross-subdomain cookie sharing, balance data access needs with the principle of least privilege to maintain security.

Preventing Unauthorized Cookie Access

Best practices for setting cookie domains:

1. Use specific domains: Set cookies for the most specific domain name possible to limit access.

2. Avoid setting cookies for top-level domains: Don't set cookies for domains like .com or .org, as this allows access from any subdomain.

3. Use the HttpOnly flag: This stops client-side scripts from accessing the cookie, reducing the risk of cross-site scripting (XSS) attacks.

4. Set the Secure flag: This makes sure cookies are only sent over HTTPS connections, protecting them from interception.

5. Implement the SameSite attribute: This helps prevent cross-site request forgery (CSRF) attacks by controlling when cookies are sent with cross-site requests.

Risks of broad domain settings:

1. Data leakage: Cookies accessible by multiple subdomains may expose sensitive information to unauthorized parties.

2. Session hijacking: Broad domain settings can make it easier for attackers to steal user sessions.

3. Cross-site tracking: Cookies accessible across multiple domains can be used to track user behavior without their consent.

4. Increased attack surface: The more domains that can access a cookie, the more potential entry points for attackers.

Third-Party Cookie Considerations

Definition of third-party cookies:

Third-party cookies are created by domains other than the one the user is visiting. They are typically used for tracking and advertising purposes across different websites.

Browser policies on third-party cookie access:

1. Default blocking: Many web browsers now block third-party cookies by default to improve user privacy.

2. Intelligent Tracking Prevention (ITP): Some browsers use ITP to limit the lifespan of third-party cookies and restrict their functionality.

3. User controls: Browsers often provide settings for users to manage third-party cookie permissions.

4. Same-site cookie enforcement: Browsers are implementing stricter rules for cross-site cookie access, requiring explicit same-site declarations.

5. Future of third-party cookies: Major browsers plan to phase out support for third-party cookies entirely, pushing for alternative tracking methods.