Microsoft Edge InPrivate Mode — Deep Dive into Software Architecture

Microsoft Edge’s InPrivate mode is built on Chromium’s multi-process architecture, but it introduces additional layers and mechanisms specifically designed for privacy isolation, data control, and security enforcement. The architecture involves complex interactions between browser components, OS-level features, and memory management.

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1. Profile and Session Isolation

Chromium Profile Model

• Normal Mode Profile:

  • Stores user data persistently on disk in a dedicated user profile directory (e.g., cookies, cache, history, localStorage).

  • Shared across all normal browser windows.

• InPrivate Mode Profile:

  • Chromium creates a temporary, in-memory profile for InPrivate sessions.

  • This profile is ephemeral—it exists only for the lifetime of the InPrivate window.

  • Data like cookies, cache, browsing history, localStorage, and session storage are scoped to this profile and kept in volatile memory or temporary storage.

Profile Data Management

• Upon launching an InPrivate window, Edge initiates a separate profile instance with a unique profile identifier.

• The profile manager ensures:

  • No data crosses between normal and InPrivate profiles.

  • Each InPrivate session is isolated from others (i.e., different InPrivate windows do not share storage unless explicitly linked).

• All writes to storage (cookies, IndexedDB, Cache API, localStorage) are intercepted and routed to memory-backed or ephemeral stores.

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2. Multi-Process Architecture and Process Isolation

Processes in Chromium-based Browsers

• Browser Process (UI Process): Manages UI, tab management, profile management, and coordinates all other processes.

• Renderer Processes: Render web content (HTML, CSS, JS) using Blink engine.

• GPU Process: Handles hardware-accelerated graphics.

• Network Process: Manages all network communications.

• Utility Processes: Various isolated tasks (audio, extensions, etc.).

InPrivate Process Isolation

• Dedicated Renderer Processes:

  • Tabs opened in InPrivate mode run in renderer processes associated exclusively with the InPrivate profile.

  • This ensures that renderer processes handling InPrivate content cannot access normal profile data.

• Sandboxing:

  • Chromium’s sandbox model is enhanced for InPrivate:

    • OS-level sandbox restrictions remain.

    • Data access is limited to memory-backed storage.

  • Renderer processes cannot write to persistent storage paths used by normal profiles.

• Process-level Partitioning:

  • The site isolation model is respected within InPrivate:

    • Different sites run in separate processes.

    • Each process inherits the ephemeral profile context.

• Browser Process Awareness:

  • The browser process tracks which renderer processes are associated with InPrivate mode.

  • This enables cleanup of all associated data and processes when InPrivate windows close.

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3. Memory-Only Storage and Data Handling

Ephemeral Storage Components

• Cookies and Storage APIs:

  • Cookies are held in a volatile cookie store that exists only in memory.

  • Access to cookies is intercepted by the network stack to ensure no persistence.

• Cache:

  • HTTP cache and resource cache are stored in-memory or in temporary OS locations (like temp folders) and cleared on session end.

• IndexedDB & LocalStorage:

  • Uses a memory-backed or ephemeral filesystem abstraction.

  • This prevents any data from being persisted or recovered after the session.

• Session Storage:

  • Scoped strictly to the tab and lifetime of the InPrivate session.

Data Flow Control

• When a web page requests to read or write storage, Edge routes these requests to the InPrivate profile’s ephemeral stores.

• Any writes are flagged as non-persistent.

• On closing all InPrivate windows, the browser process explicitly clears all volatile data related to the InPrivate profile.

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4. Network Layer and Privacy Enhancements

Network Process Behavior

• The network stack distinguishes between normal and InPrivate modes by the profile context.

• Cookies and headers are isolated per profile.

• Persistent storage of caches, cookies, or session data is disabled or routed to ephemeral stores for InPrivate.

Tracking Prevention

• InPrivate mode enables Microsoft Edge’s Tracking Prevention at its strictest level by default.

• Uses machine learning and heuristic models to block known trackers, third-party cookies, fingerprinting scripts.

• Tracker blocking decisions are enforced at the network request interception layer.

• SmartScreen continues to protect against phishing and malware sites.

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5. Extension and Plugin Handling

• Extensions are disabled by default in InPrivate mode to avoid leaking information.

• Users can explicitly enable extensions per their choice, but even then:

  • Extension data and state are isolated to the InPrivate profile.

  • Extension APIs respect the ephemeral storage model.

• Plugin processes also run with strict sandboxing and data isolation.

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6. UI and OS Integration

Visual Indicators and UX

• The UI switches to a dark theme with “InPrivate” labels/icons to indicate the privacy context.

• The address bar and tab bars reflect InPrivate status.

• Certain features like form autofill, password saving, and history recording are disabled.

OS-level Privacy and Security

• InPrivate mode uses OS APIs to prevent caching of sensitive data:

  • On Windows, Edge uses Protected Media Path (PMP) and sandboxing features.

  • Temporary profile data uses OS-level temp directories that are cleaned up on session end.

• On mobile platforms, additional platform-specific privacy controls are enforced.

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7. Lifecycle Management

• When the last InPrivate window closes:

  • The browser process triggers a cleanup routine.

  • All InPrivate profile data stored in memory or temporary storage is wiped.

  • Associated renderer and utility processes are terminated.

  • Network caches, cookies, and other ephemeral data are destroyed.

• The user’s normal profile remains unaffected.

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Summary Diagram: Detailed InPrivate Mode Architecture

+-----------------------+
|     Browser Process    |  <-- Manages UI, profiles, lifecycle, cleanup
|  - Tracks normal vs    |  
|    InPrivate sessions  |
+-----------+-----------+
            |
            | (Separate Profiles & Contexts)
+-----------+-----------------+------------------+
|                             |                  |
|    Normal Profile           |   InPrivate Profile (Temporary)  |
|    (Persistent Storage)     |   (Memory-only, ephemeral)       |
|                             |                  |
|  +----------------------+   |  +---------------------------+   |
|  | Renderer Processes    |   |  | Renderer Processes         |   |
|  |  (Normal browsing)    |   |  |  (Isolated, ephemeral)     |   |
|  +----------------------+   |  +---------------------------+   |
|                             |                  |
|  +----------------------+   |  +---------------------------+   |
|  | Network Process       |<--+--| Network Process           |   |
|  | (Persistent cookies,  |      | (Volatile cookies/cache,  |   |
|  | cache, logs)          |      |  strict tracking block)    |   |
|  +----------------------+      +---------------------------+   |
|                             |                  |
|  +----------------------+   |  +---------------------------+   |
|  | Extensions & Plugins  |   |  | Disabled by default or     |   |
|  | (Persistent storage)  |   |  | enabled with ephemeral     |   |
|  +----------------------+   |  +---------------------------+   |
+-----------------------------+-----------------------------+

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Additional Notes and Insights

• Privacy-by-Design: Edge InPrivate mode follows privacy-by-design principles, ensuring no residual data persists after sessions.

• Performance Optimizations: Edge optimizes memory and CPU usage by aggressive cleanup and resource deallocation after InPrivate windows close.

• Enterprise Policies: Enterprises can control whether InPrivate mode is enabled or disabled through group policies.

• Limitations: Some information might still leak through:

  • Browser fingerprinting if trackers are sophisticated.

  • Network-level metadata (IP addresses, DNS).

  • User behavior if logged into online accounts during InPrivate browsing.

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1. The Exact API Hooks Chromium Uses to Isolate Storage

• Chromium uses profile-based partitioning APIs that create temporary, in-memory storage partitions for InPrivate sessions.

• Storage APIs like Cookies, IndexedDB, localStorage, Cache API are hooked to use memory-backed stores scoped to the InPrivate profile.

• These APIs intercept read/write calls and redirect them to volatile, ephemeral storage, preventing writes to disk.

• The profile manager dynamically creates and tears down these isolated storage contexts per InPrivate session.

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2. How the Network Stack Enforces Privacy Policies

• The network stack runs with awareness of the current profile context (normal vs. InPrivate).

• It isolates cookies, caches, and headers to the ephemeral InPrivate session, preventing persistence.

• Tracking prevention modules actively block third-party trackers and fingerprinting scripts by intercepting and filtering requests.

• The SmartScreen filter remains active to block malicious URLs.

• Network logs and diagnostic data are minimized or disabled for InPrivate mode to avoid storing browsing traces.

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3. Memory Management Techniques for Ephemeral Data

• All InPrivate data (cookies, cache, IndexedDB, etc.) is stored in RAM or OS temporary memory-backed filesystems.

• Chromium uses in-memory databases or RAM disks to hold session data.

• Upon session end, these volatile stores are explicitly cleared and freed by the browser process.

• Garbage collection and memory cleanup routines are more aggressive to prevent leaks.

• Temporary storage locations on disk (if used) are cleaned immediately on close.

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4. OS Sandboxing Details Specific to Windows/macOS

• On Windows:

  • Edge uses Windows Sandbox APIs and AppContainer sandboxing to restrict InPrivate processes.

  • File system and registry access are limited to ephemeral areas.

  • Network isolation is enforced via restricted permissions.

• On macOS:

  • Edge uses macOS sandboxing mechanisms based on Seatbelt profiles.

  • Restricts access to user files and limits network capabilities for InPrivate processes.

• Both platforms ensure processes run with least privilege, limiting access to persistent storage and system resources, enforcing strict isolation between InPrivate and normal modes.

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