Web Security Checks in Gecko

Key Concepts and Terminology

Security Principal (nsIPrincipal)

A Security Principal represents the security context for a piece of code or data. Firefox uses four types of principals:

  • ContentPrincipal: Used for typical web pages and can be serialized to an origin URL, e.g., https://example.com/.

  • NullPrincipal: Used for pages that are never same-origin with anything else, such as iframes with the sandbox attribute or documents loaded with a data: URI. This is also known as an opaque origin.

  • SystemPrincipal: Used for the browser’s user interface, commonly referred to as “browser chrome”, and various other background services (OCSP requests, fetching favicons). Pages like about:preferences use the SystemPrincipal.

  • ExpandedPrincipal: Used by browser extensions that need to assume the security context of a website. An ExpandedPrincipal is best understood as a list of principals.


OriginAttributes help in managing and enforcing security policies by distinguishing different security contexts that might otherwise be considered the same based on their Principal. They are used to:

  • Isolate data and resources in private browsing mode.

  • Implement cache isolation.

  • Manage user context identifiers for container tabs.

  • Enforce first-party isolation.


The OriginAttributes class extends the functionality of dom::OriginAttributesDictionary and includes additional methods for setting and managing various attributes.

Key attributes include:

  • FirstPartyDomain: Used to isolate data based on the domain.

  • UserContextId: Identifies different user contexts, such as container tabs.

  • PrivateBrowsingId: Indicates whether a request is made in private browsing mode.

  • PartitionKey: Used to implement cache isolation.

Load Info Object (nsILoadInfo)

The nsILoadInfo object is crucial for security checks. It holds all security-relevant attributes, including security flags indicating what checks need to be performed and the associated Principal.


  • loadingPrincipal: The principal of the document where the result of the load will be used.

  • triggeringPrincipal: The principal that triggered the URL to load.

  • securityFlags: Indicate the type of security checks required.

  • contentPolicyType: Specifies the type of content being loaded, used for security checks like Content Security Policy.

Loading Lifecycle in Firefox

From Request to Response

  1. Request Initiation: A web page initiates a request.

  2. nsIChannel Creation: Firefox creates an nsIChannel object, representing the request.

  3. nsILoadInfo Attachment: An nsILoadInfo object is required for the creation of an nsIChannel and holds security-related information.

  4. Security Checks: Security checks are performed using the ContentSecurityManager.

  5. Request Execution: If all checks pass, the request proceeds.

Role of nsIChannel and nsILoadInfo

  • nsIChannel: Manages the transport algorithm (e.g., HTTP, WebSocket).

  • nsILoadInfo: Holds security relevant meta information of a network load and determines what security checks need to be enforced.

Security Checks During Loading

Pre-Request Checks

  • Same-Origin Policy: Ensures resources are only accessed if they share the same origin.

  • Content Security Policy: Enforces content restrictions based on policies defined by the site.

  • Mixed Content Blocking: Implements the Mixed Content standard, including blocking and upgrading of insecure (HTTP) content on secure (HTTPS) pages.

ContentSecurityManager and doContentSecurityCheck()

  • ContentSecurityManager: Centralized manager for performing security checks.

  • PerformSecurityCheck(): Key function that is invoked to perform all relevant security checks before a request is executed.

Subsumes Concept

  • Definition: A principal subsumes another if it has access to the same resources.

  • Implementation: aPrincipal->Subsumes(aOtherPrincipal) is used to check access permissions.

Code example:

bool subsumes = principal1->Subsumes(principal2);

Subsumption is asymmetrical. One principal subsuming the other does not imply the inverse. A typical example is the SystemPrincipal, which subsumes all other principals.


The interface definition in source code have a lot of detailed comments: