============================ 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 ---------------------------- `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. Attributes ---------- 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. Attributes: ----------- - `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. References ---------- The interface definition in source code have a lot of detailed comments: - The `nsIPrincipal `_ interface definition. - The `nsILoadInfo `_ interface definition. - The `nsIContentSecurityManager `_ interface definition