# Modelo de Amenaza de Tor VPN

### Mike Perry

mikeperry@torproject.org

August 6, 2025

## 0. Introduction

Similar to Tor Browser, and unlike traditional VPNs, Tor VPN gives each application its own route through the Tor network. This feature prevents network adversaries from linking a user's private activity with their non-private activity.

In contrast to Tor Browser, the privacy benefits that Tor VPN provides depend highly upon the behavior of individual applications, the permissions granted to those applications, and the configuration of the base Android system.

For privacy-hostile applications in partial Tor mode, or for applications that users only occasionally use with Tor, Tor VPN provides only censorship circumvention and protection from network surveillance.

When used on properly configured privacy-enhanced Android distributions such as GrapheneOS, Tor VPN can provide additional privacy benefits even for privacy-hostile applications.

This document describes the nature of the benefits that Tor VPN provides and when they apply.

### 0.1. Table of Contents

- Tor VPN Threat Model
- Mike Perry
- 0. Introduction
- 0.1. Table of Contents
- 1. Overview
- 1.1. Threat Model Summary
- 2. Security Properties
- 2.1. Low-level VPN Security Properties
- 2.2. ICTM Security Invariants
- 2.3. ICTM Security Invariant Activation Conditions
- 3. Android App Types and Configurations
- 3.1. Application Types
- 3.2. Device Configurations
- 4. Threat Model Matrices
- 4.1. Tor VPN Per-App Mode Matrix
- 4.2. Tor VPN All-App Mode Matrix
- 5. Known Weaknesses
- 5.1. All Android Versions
- 5.1.1. Network Interface IP Address Visibility
- 5.1.2. Push Notification Linkability
- 5.1.3. Multicast DNS and SSDP Leaks
- 5.1.4. Android ID
- 5.1.5. SIM Card Operator ID
- 5.1.6. WiFi Calling
- 5.2. Stock Android
- 5.2.1. Apps Can Use Cell Network to Bypass VPN
- 5.2.2. Secondary User Profiles Bypass VPN at Start
- 5.2.3. Google Account Linkability
- 5.2.4. Various DNS Leaks
- 5.2.5. VPN Bypass Whitelist
- 5.3. GrapheneOS
- 5.3.1. Android Advertising ID Linkability

## 1. Overview

The Tor VPN threat model uses the ICTM methodology. ICTM specifies security invariants that the user can expect when using the software in a particular way or in a particular environment. An ICTM threat model starts from a point of "No Security" and adds security invariants when the software environment meets particular requirements.

We express what Tor VPN provides in terms of ICTM security invariants, which apply to specific application types and usage scenarios.

In our usage of ICTM, we define five ICTM security invariants that represent levels of privacy that Tor VPN provides. This document links these invariant terms in Capitalized Bold Italic to their definition sections.

These security invariants apply to the combination of four types of Android applications, two modes of Tor VPN (all-app or per-app), and five Android device configurations. This document links these classification terms in Capitalized Bold to their definition sections.

The combined outcome results in two threat model matrices that represent the ICTM security invariant that is active in a given situation.

### 1.1. Threat Model Summary

The level of pseudonymity that Tor VPN provides to the user guides the overall threat model.

Tor VPN provides at least censorship circumvention to all selected apps.

Tor VPN does not magically make all apps anonymous or pseudonymous. If the user used their real name or even created their account without Tor, then they have no pseudonymity.

If the user uses a privacy-enhanced version of Android and is careful about their Google account setup and app usage, then Tor VPN can help protect the usage of a single pseudonym for the user of the device. (The user may try to use different names in different apps, but app behavior can link these pseudonyms to the Google account and/or to each other).

If the user uses privacy-preserving apps with proper configuration, then Tor VPN can enable the usage of separate pseudonyms in those apps. (When properly configured, an adversary cannot link the different pseudonyms used in distinct privacy-preserving apps).

Essentially, this threat model describes the circumstances under which the user obtains these different levels of pseudonymity protection.

## 2. Security Properties

The low-level VPN security properties and the ICTM security invariants support the Tor VPN threat model.

### 2.1. Low-level VPN Security Properties

Two low-level security properties that the OnionMasq networking daemon provides support the ICTM security invariants. These low-level security properties must hold for the ICTM invariants to apply:

1. Network Leak Safety. Apps MUST NOT be able to bypass the VPN when it is enabled. This includes during device startup, if the VPN is not connected yet, or if the network is temporarily disconnected. The VPN industry calls this property a "Kill Switch", "Always On VPN Lockdown", or "On Demand". The Always-on Lockdown feature of Android 8 and above provides this property.

2. Network App Isolation. All network activity of each Tor-routed app MUST be isolated from other apps by using its own dedicated Tor circuit(s). Specific apps MAY bypass Tor entirely if the user chooses to allow this. Las API de ConnectivityManager de Android 10 y superiores proporcionan esta propiedad.

### 2.2. ICTM Security Invariants

These are the security invariants that Tor VPN is capable of providing to individual apps. To simplify activation conditions, some invariants inherit the properties of earlier ones. We list these inherited invariants under each invariant.

The high-level conceptualization to keep in mind is that we are defining distinctions in the level of pseudonymity that the user can expect for an app: no pseudonymity, multi-app shared pseudonymity, per-app pseudonymity, and anonymity.

The next section lists the activation conditions under which they apply.

- No Additional Security
- Invariant Property: Tor VPN provides no additional security.

- Censorship Circumvention
- Invariant Property: A network adversary is unable to block application traffic.

- Network Privacy
- Invariant Property: A local network adversary is unable to determine which applications are used with Tor VPN, and apps are unable to determine the user's current IP address (i.e., approximate current location).
- Inherited Invariants: Censorship Circumvention

- Linkable Pseudonymity
- Invariant Property: App usage has pseudonymity, but this pseudonymity may be linked to other activity on the device or to a sandboxed and pseudonymous Google account. This is also known as Multi-App Shared Pseudonymity.
- Inherited Invariants: Censorship Circumvention, Network Privacy

  > NOTE: Linkable Pseudonymity is a fragile state. As pseudonymous activity is linked together, the odds increase that there is some way to deanonymize some of that activity, thus deanonymizing all of it. One can conceptualize this invariant as providing "Multi-App Shared Pseudonymity," where all apps with Linkable Pseudonymity on the device effectively share the same pseudonym. This is why we require either the Pseudonymous Google Configuration or Google-Free Configuration for apps to gain this invariant. (Pseudonymity would otherwise vanish as soon as it was linked to a non-pseudonymous Google account.)

- Unlinkable Pseudonymity
- Invariant Property: App usage has unique pseudonymity that is not possible to link to other device activity. This is also known as Per-App Pseudonymity.
- Inherited Invariants: Censorship Circumvention, Network Privacy

  > NOTE: One can conceptualize this invariant as providing "Per-App Pseudonymity." Because of Tor VPN's Network App Isolation, when this invariant is active for an app, its activity cannot be linked to non-private activity on the device. This is not the case for traditional VPNs, which allow the VPN operator and VPN endpoint observers to link such activity.

- Full Anonymity
- Invariant Property: App usage is unlinkable to other device activity, as well as to previous activity of that same app.
- Inherited Invariants: Censorship Circumvention, Network Privacy

### 2.3. ICTM Security Invariant Activation Conditions

Our security invariants activate depending on what type of app is in use, the Tor VPN mode (per-app or all-apps), and properties of the base OS and configuration.

The lowest numbered invariant where an app matches these clauses is the invariant for that app.

A device may have multiple apps operating at different security invariant levels, depending on these conditions.

The Threat Model Matrices present this same activation information in table form.

- No Additional Security
- Activation Condition: Active for all apps when the user installs Tor VPN but does not enable it at all.

  > Rationale: Surprisingly, this is a common failure mode for VPN apps among people who do not understand how they work.

- Activation Condition: Active for all apps when the user installs Tor VPN on a device past the end-of-life for Android security fixes.

  > Rationale: We cannot provide any additional security to a device that is already vulnerable to unfixed, publicly disclosed security flaws. We assume these public flaws can subvert any privacy or security properties the user would hope to gain by running Tor VPN.

- Activation Condition: Active for apps that are not selected to use Tor when the user runs Tor VPN in per-app mode.

  > Rationale: In per-app mode, apps that are not selected for Tor use get no protection. (Though they could if we implemented a per-app network block feature).

- Censorship Circumvention
- Activation Condition: Active for Privacy-hostile Apps when the user runs Tor VPN in per-app mode.

  > Rationale: Privacy-hostile applications can trick the user into opening URLs in apps that are not selected to use Tor and can link non-Tor activity to their activity.

- Activation Condition: Active for Privacy-hostile Apps when the user runs Tor VPN in all-app mode and the device uses the IP Address Privacy Configuration.

  > Rationale: On all versions of Android, Privacy-hostile applications can use the `ACCESS_NETWORK_STATE` Android permission to determine the SIM card IP address and can engage in data sharing with the ISP, which removes Network Privacy. On Stock Google Android (but not GrapheneOS), they can bind to the cell network interface to directly bypass the VPN for arbitrary network connections.

- Network Privacy
- Activation Condition: Active for all privacy-hostile and pseudonymous Unmasked Apps not covered by the previous invariant when the user runs Tor VPN in either per-app or all-app mode.

  > Rationale: We assume unmasked apps have no pseudonymity.

- Activation Condition: Active for Privacy-hostile Apps when the user runs Tor VPN in all-app mode and the device uses only the IP Address Privacy Configuration (and not any further unlinkability configuration).

  > Rationale: Privacy-hostile apps can link their activity to the user's real name and Google account unless the user takes further steps.

- Activation Condition: Active for Pseudonym-friendly Apps when the user runs Tor VPN in either all-app mode or per-app mode, when the device uses only the IP Address Privacy Configuration (and not any further unlinkability configuration).

  > Rationale: Pseudonym-friendly apps can be accidentally linked to a user's real name and Google account through push notifications, phone number authentication, and other mechanisms, unless the user takes further steps.

- Linkable Pseudonymity
- Activation Condition: Active for Pseudonym-friendly Apps when the user runs Tor VPN in either all-app mode or per-app mode, and the device uses the Pseudonymous Google Configuration and the user grants push notification permission to the app.

  > Rationale: The usage of Google push notifications links the pseudonymous app to the device's Google account, which is also pseudonymous in this configuration.

- Activation Condition: Active for Privacy-hostile Apps when the user runs Tor VPN in all-app mode and the device uses the Pseudonymous Google Configuration or Google-Free Configuration.

  > Rationale: Privacy-hostile apps can cooperate with other privacy-hostile apps on the phone to link activity, even without a device Google account.

- Unlinkable Pseudonymity
- Activation Condition: Active for Pseudonym-friendly Apps when the user runs Tor VPN in either all-app mode or per-app mode, and has configured the device and app with Pseudonymous App Configuration or Google-Free Configuration.

  > Rationale: These device configurations prevent linkability of the pseudonymous app to the device's Google account.

- Activation Condition: Active for Privacy-hostile Apps when the user runs Tor VPN in all-app mode and has configured the device with Isolated User Profile Configuration.

  > Rationale: This device configuration isolates privacy-hostile apps from the device's Google account and from each other.

- Full Anonymity
- Activation Condition: Active for Anonymity Apps when the user runs Tor VPN in either all-app mode or per-app mode.

  > Rationale: Anonymity apps have no other identifiers other than the current IP address, which is now a Tor exit IP address.

## 3. Android App Types and Configurations

As can be seen, our security invariants depend upon the app type and the Android device configuration. In this section, we define those applications and configurations specifically.

### 3.1. Application Types

We classify Android applications into four types:

- Unmasked Apps Unmasked apps are apps that were previously used outside of Tor, associated with a non-pseudonymous phone number, or associated with a real name/email. In essence, these apps have been unmasked to the user's real name. Unmasked apps can no longer have any strong pseudonymity expectations and can at most gain only Network Privacy from Tor VPN.

  > NOTE: Once an app is unmasked on a device, creating a new pseudonymous account in the same app does not remove this classification. In fact, due to the Android ID identifier behavior, deleting the app and reinstalling it in the same profile still allows a new pseudonym to be linked to the deleted one. A user must use the Isolated User Profile Configuration to create a new device user profile to use the app in an unlinkable way with respect to the current profile's Android ID for that app.

- Examples of unmasked usage include:
- Signal registered with a regular phone number.
- A ProtonMail account accessed over non-Tor.
- An Element account linked to an email address in the user's real name.
- Any app that has been linked with a non-pseudonymous Google account (via push notifications, location usage, etc.).
- Any app that has been linked to a pseudonymous Google account that is later accessed over non-Tor.

- Privacy-hostile Apps Privacy-hostile apps are apps that use every available mechanism to attempt to link their activity to other user activity. They make use of the Android Advertising ID, identifiers, network state, location, sensors, etc. In a stock Android configuration, these apps gain Censorship Circumvention from Tor VPN, but can gain higher Tor VPN invariants when used with privacy-enhancing Android distributions, as referenced in those invariants. See Device Configurations for specific configuration details. Example Apps: Facebook, Instagram, WhatsApp, TikTok, X, and Reddit.

- Pseudonym-friendly Apps These are apps that provide accounts with some degree of pseudonymity, either as an explicit design goal or as a consequence of not actively trying to link user activity together or otherwise track the user. Unfortunately, the Android OS can easily subvert their pseudonymity through misconfiguration or even basic usage. In a default Android configuration, these apps gain Network Privacy from Tor VPN. These apps can gain Unlinkable Pseudonymity from Tor VPN on stock Android, under the device configuration conditions specified in that invariant. Example Apps: Signal, Session, Element, ProtonMail, and TutaMail.

- Anonymity Apps These are apps that do not store long-term identifiers or logins. In some cases, such as private browsers, the app clears identifiers upon restart or otherwise regularly. In other cases, such as Zcash wallets, the app uses Zero-Knowledge proofs rather than trackable identifiers. These apps gain Full Anonymity from Tor VPN. Example Apps: Zashi, Cake Wallet, and private browsers.

### 3.2. Device Configurations

Architecturally, Tor VPN is capable of providing a high degree of unlinkability to applications on the same device, as we can see from the security invariants. Unfortunately, Android is an extremely privacy-invasive platform. The Android ecosystem has been built upon the same advertising model that the web has been built upon. Because Google controls Android and is the primary benefactor (if not sole conduit) of Android advertising revenue, Google designed the platform to maximize the ability to track usage and to link user activity together to build a comprehensive advertising profile of the user. This linkability is problematic even for apps that provide pseudonymity as a design goal, merely through their usage of the system APIs. The primary way that the system subverts pseudonymity is by associating the pseudonym with the Google account, most often through push notifications, but also through associating location information with the Android Advertising ID. On a stock Google Android, the device's Google account gets full device information: serial number, IMEI, IMSI, phone number, WiFi MAC address, full location history, and installed application list. We assume that any linking mechanism to the stock Android Google account provides deanonymization of a pseudonym. In these cases, apps can gain Network Privacy at most. The good news is that there are ways of configuring an Android device that improve the ICTM security invariant that Tor VPN can provide:

- IP Address Privacy Configuration From a VPN provider's perspective, the most unfortunate aspect of Android is that every app on the device typically has the `ACCESS_NETWORK_STATE` permission, which gives access to all IP addresses on the device, along with a lot of other network interface information that can be used to track the user. For a good overview of all the information that is available by default, install Network Analyzer or use the Device ID test in our VPN Test App. Unfortunately, even privacy-preserving Android distributions such as GrapheneOS do not provide the ability to redact non-VPN network information and non-VPN IP addresses when a VPN is active. Additionally, on stock Android, apps can bypass the VPN interface by binding to the cell network interface, regardless of Always-on and Block Connections Android settings. Until those tickets are fixed:
- The device must be in Airplane mode and used with a WiFi hotspot or tethered to another phone.

- Pseudonymous App Configuration As per the app classification, Pseudonym-friendly Apps are privacy-focused apps that provide a pseudonym and do not actively try to deanonymize the user. They also may not necessarily ensure that this pseudonym is fully unlinkable. To ensure that Pseudonym-friendly Apps are unlinkable to other device activity, the user must do the following:
- Do not use the device phone number for app registration. Use an online SMS or VoIP provider over Tor.
- Do not use the device's Gmail account for app registration.
- Disable notifications from within the app (disabling push permission is insufficient).
- For apps that are capable of making voice and video calls, either use IP Address Privacy Configuration, or configure the app to always relay these calls for you.

When these properties are met, the app will not be linked to the device's Google account, which is a common source of deanonymization.

> NOTE: Removing push notification permission is NOT ENOUGH to prevent apps from being able to register with Firebase (Google) to receive a push token that links the app to the Google account. See the Push Notification section of Known Weaknesses for more information. For now, if an app does not provide settings to disable notification registration, then you must use one of the following configurations instead. (For example: Session Messenger allows push registration to be disabled or switched to non-Google; Signal currently offers no such configuration).

- Pseudonymous Google Configuration On stock Android, linking activity to the Google account typically means deanonymization. The Google account is typically in the user's real name, and even if not, it has access to all device identifiers, including cell network and location history. There are two levels of reducing the damage that this linkability can do while still retaining key device functionality:
- Make a new pseudonymous Google account on GrapheneOS.
- Use a GrapheneOS device with the Google Service Framework package installed, but without a Google account.

It turns out that from the point of view of our ICTM security invariants, both of these are equivalent: they both result in Linkable Pseudonymity for both pseudonym-friendly apps and privacy-hostile apps because they significantly reduce the amount of linkable information, but they do not eliminate it. The primary reason is due to the remaining push notification linkability. See the Push Notification Linkability section of Known Weakness for more info. If the push notification linkability weakness is mitigated, then pseudonym-friendly apps will have Unlinkable Pseudonymity in this configuration when using push notifications. The first option accepts linkability through the Google account but mitigates deanonymization through careful management of this Google account. In this configuration, the user creates a new Google account over Tor and relies on the GrapheneOS Google Sandbox to provide pseudonymity for the device's Google account. In this sandbox, the system treats Google as a normal app, with revocable permissions and no access to hardware identifiers. Additionally, GrapheneOS requires the user to manually disable the Android Advertising ID. Doing so eliminates a large source of excess linkability. The Android Advertising ID is heavily used to link pseudonyms to location data. In the second option, the user configures a new privacy-enhanced Android device without a Google account but still uses push notifications with sandboxed Google Play Services on GrapheneOS. The user can use F-Droid or Aurora Store to install apps without a Google account. Without a Google account, there is less linkability risk between apps through other Google APIs other than push.

> NOTE: The device must always use Tor to avoid linking its device push registration to a non-Tor IP address. This may be problematic in censorship scenarios where configuration information is delivered over push notifications. In this case, without a device Google account, it may be possible to clear registration data and re-register to remove linkability. But the effectiveness of this will depend on the data retention duration of the services involved for expired push tokens. See the Push Notification Linkability section of Known Weaknesses.

> - These properties define this configuration:
> - Configure with the IP Address Privacy Configuration to prevent Google Services and hostile apps from using the cell IP address for linkability.
> - Optional Google Account Config:
> - Create a new pseudonymous Google account over Tor for the device, using an SMS or VoIP phone number.
> - The Android Advertising ID of this account has been disabled.
> - Do not use the device phone number or Google account for app registration. Use a different online SMS or VoIP provider over Tor.
> - If Tor VPN is in per-app mode, all Google Play Services and Google apps must be selected to use Tor in this configuration.
> - Disable Private DNS and Connectivity Checks.

- Google-Free Configuration In this configuration, the user installs GrapheneOS without Google Play Services installed at all, yielding a fully "de-Googled" Android device. The user can still use Aurora Store to install apps without a Google account, though many apps may crash or fail without access to Google Play Services. This configuration entirely prevents linkability through push notifications and other Google API usage. For the apps that function, these foot-guns will be completely unavailable. We still assume that privacy-hostile applications can link their activity in this mode if they cooperate on-device or via sensor side channels. This is why these apps are still listed as having Linkable Pseudonymity, but this linkability is significantly less than with a Google account or Google APIs involved.
- These properties define this configuration:
- Install GrapheneOS, but do not install Google Play Services.
- Configure with IP Address Privacy Configuration.
- Use Aurora Store to install apps.
- Do not use the device phone number for app registration. Use a different online SMS or VoIP provider over Tor.

- Isolated User Profile Configuration Privacy-hostile apps are considerably harder to make unlinkable. Because the Android system allows apps to communicate with each other, and because there is evidence of apps willfully abusing this communication to link activity, the only real option to make privacy-hostile apps unlinkable is to install them in their own Android user profile on an Android distribution that supports strong profile isolation. The main downside of this approach is that each user profile must run its own instance of Tor VPN.
- The properties of this configuration are:
- Configure with IP Address Privacy Configuration.
- GrapheneOS is needed to ensure apps from separate profiles cannot communicate or bypass the VPN.
- The profile needs its own installation of Tor VPN.
- The profile should not install Google Play Services.
- Do not use the device phone number or Google account for app registration. Use a different online SMS or VoIP provider over Tor.

## 4. Threat Model Matrices

The active security invariant for a given scenario can be summarized in two matrices: one for Tor VPN per-app mode and one for Tor VPN all-app mode. These two matrices exactly represent the ICTM security invariant clauses from Section 2.3.

### 4.1. Tor VPN Per-App Mode Matrix

App Type / Android Config | Base Android | IP Address Privacy Configuration | Pseudonymous App Configuration | Pseudonymous Google Configuration | Google-Free Configuration | Isolated User Profile Configuration
--- | --- | --- | --- | --- | --- | ---
Unmasked Privacy-hostile Apps | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention
Unmasked Pseudonym-friendly Apps | Network Privacy | Network Privacy | Network Privacy | Network Privacy | Network Privacy | Network Privacy
Privacy-hostile Apps | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention | Censorship Circumvention
Pseudonym-friendly Apps | Network Privacy | Network Privacy | Unlinkable Pseudonymity (without push registration**) | Linkable Pseudonymity (with push registration**) | Unlinkable Pseudonymity | Unlinkable Pseudonymity
Anonymity Apps | Full Anonymity | Full Anonymity | Full Anonymity | Full Anonymity | Full Anonymity | Full Anonymity

### 4.2. Tor VPN All-App Mode Matrix

App Type / Android Config | Base Android | IP Address Privacy Configuration | Pseudonymous App Configuration | Pseudonymous Google Configuration | Google-Free Configuration | Isolated User Profile Configuration
--- | --- | --- | --- | --- | --- | ---
Unmasked Privacy-hostile Apps | Censorship Circumvention | Network Privacy | Network Privacy | Network Privacy | Network Privacy | Network Privacy
Unmasked Pseudonym-friendly Apps | Network Privacy | Network Privacy | Network Privacy | Network Privacy | Network Privacy | Network Privacy
Privacy-hostile Apps | Censorship Circumvention | Network Privacy | Network Privacy | Linkable Pseudonymity | Linkable Pseudonymity | Unlinkable Pseudonymity
Pseudonym-friendly Apps | Network Privacy | Network Privacy | Unlinkable Pseudonymity (without push registration**) | Linkable Pseudonymity (with push registration**) | Unlinkable Pseudonymity | Unlinkable Pseudonymity
Anonymity Apps | Full Anonymity | Full Anonymity | Full Anonymity | Full Anonymity | Full Anonymity | Full Anonymity

## 5. Known Weaknesses

In this section, we list the known weaknesses of both stock Android and privacy-enhancing Android, where fixing a weakness would significantly improve the available security invariants.

For each weakness, we list the Consequences for the user and the mitigating configurations the user must perform, when possible.

We also provide fix Recommendations for developers of GrapheneOS, Android, and pseudonym-friendly apps.

### 5.1. All Android Versions

The following Android privacy issues apply to all Android versions, including GrapheneOS and CalyxOS.

#### 5.1.1. Network Interface IP Address Visibility

In all versions of Android, when apps have the `ACCESS_NETWORK_STATE` permission (which nearly all Android apps have), they can examine properties of all network interfaces, including the IP address. For a good overview of all the information that is available by default, install Network Analyzer or use our VPN Leak Test App.

Consequences: This weakness prevents Tor VPN from providing Network Privacy (or further invariants) to Privacy-hostile apps. This weakness also exposes the user to IP address leaks even for Pseudonym-friendly apps that disclose all device IP addresses during WebRTC calls. In both cases, the user must use their phone with a mobile WiFi hotspot, rather than a SIM card, to mitigate this weakness.

Recommendation: When a VPN is enabled, the Android OS should simulate non-VPN interfaces as disconnected, or it should spoof unused RFC1918 addresses for non-VPN interfaces. If addresses are spoofed, the system should not provide the same values to all apps, to prevent the use of spoofed values as identifiers (which likely already happens with IP address values today). These mitigations are essential because they enable VPNs to provide Network Privacy. Most VPN users naturally want their IP addresses to be unavailable to apps for tracking purposes.

#### 5.1.2. Push Notification Linkability

All versions of Android (and also all versions of iOS) are currently subject to linkability through push notifications, even when the user has denied push notification permission to all apps.

When registering for Google's Firebase push notifications, apps obtain an FCM "token" from Firebase, which they submit to their servers. Their servers use this token to deliver messages to Google, who then uses the token ID to look up the device and send it the message.

Our VPN Leak Test App verifies that this registration ability is possible without any push notification permission, in the Device ID test.

This means that a series of record requests (sent first to the app developers and then to Google) will discover the Google account in use with a particular app. From there, even if there is no device Google account, the other push tokens associated with the device can be used to make further record requests to other app service providers for account data associated with their respective token IDs.

Because Google (and many apps) operate internationally, these requests can come from arbitrary jurisdictions. The Anti-Foreign Sanctions Law in China is one such example of a law that can be used for this purpose, although no direct reports of China using push notifications have been made public. It seems likely that Wyden's concerns did not come from nowhere, though.

This linkability is possible even for apps like Signal that do not send any actual content in the push notification and use it only as a wake-up notification. Because Signal still stores the token ID with the user's account on their servers, this linkability also exists for them.

This makes it impossible to have distinct pseudonyms for Signal, or any other app that registers with FCM, unless the user configures their device without Google Play Services or creates a separate user profile without Google Services for these apps.

Consequences: This weakness makes the Pseudonymous App Configuration impossible for many apps, because FCM push registration cannot actually be disabled by the user by denying push notification permission to an app. It also makes Pseudonymous Google Configuration have far more app linkability to the Google account (or device) than would otherwise be the case, linking many privacy-hostile apps to Google and to other pseudonym-friendly apps. The user has no real mitigation options, because disabling push notifications still allows this registration and resulting linkability.

Recommendations:

There are a few different approaches that can reduce the user's exposure to this linkability:

- Permissions Enforcement for Push Registration: Access to the FCM Push Registration APIs (or their Google Service Framework downstream calls) should be blocked if the user has not granted push permissions to an app, because users expect that denying push notifications prevents an app's ability to register for push notifications with their Google account.
- Sandbox Registration APIs Separately: On GrapheneOS, since it already has a Google sandbox, FCM Registration APIs could be directed to separate instances of the Google Services Framework that are not associated with the device's Google account or any of the device permissions that may have been granted to the main GSF instance (such as SMS permission or Location permission). Separate sandboxing is useful, because it will allow users to register for push notifications without associating push registration with their Google account.
- Blinded FCM Registration Tokens: At Google Firebase itself, either Blind Signatures or Privacy Pass could be used during push registration such that the push server authenticates the registration in the normal way, but the actual token is blinded. The push tokens would then be unlinkable to the actual device or Google account, but are still authenticated themselves through the blind signature. This blinding will be beneficial to both users and to Google, because it will save Google the cost of answering push-related record requests from arbitrary international jurisdictions, since push tokens would not be possible to associate with account or device info.
- App Config Options: In the meantime, privacy-preserving apps can offer the option to disable push registration or use an alternate provider such as Unified Push. Note that Signal already provides the ability to deliver notifications through its own WebSocket mechanism, but this is only used if Google Play Services are absent. There is no exposed user option for WebSocket push for users with Google Services installed. Alternate push configuration will be useful because it will help users have push notifications without associating their Google account to their app accounts, before the above options are available.

#### 5.1.3. Multicast DNS and SSDP Leaks

In our testing, we discovered that apps can bypass the VPN by using mDNS requests and SSDP activity. We have a reproduction case in our VPN Leak Test App. This test leaks on all versions of Android, including GrapheneOS. GrapheneOS has attempted to fix some multicast leaks, but our test app shows that these fixes are insufficient.

Additionally, these multicast leaks enable privacy-hostile apps to communicate on a local network, further linking user pseudonyms to other app activity on that same network.

This behavior is also observed in the wild, with Spotify engaging in communication over the local network through this bypass mechanism.

Consequences: This weakness allows Privacy-hostile apps to communicate on the local network to link their usage to other users on the network.

Recommendation: The Android OS should route this multicast behavior through the VPN interface. This will enable the VPN app to decide whether to allow these local network connections.

#### 5.1.4. Android ID

The Android ID is essentially a 64-bit identifier that is shared by all apps in a user profile with the same signing key.

This identifier persists across re-installations of the same app, which can be verified using our VPN Leak Test App using the Device Identifier test. Users who expect to be able to delete an app and then reinstall it to sign in under a different pseudonym will have their pseudonyms linked by Privacy-hostile apps that make use of this identifier.

Consequences: This weakness prevents users from being able to reinstall apps to change pseudonyms without linking those pseudonyms together. Basically, due to this weakness, once an app is linked to the user's real identity on a profile, that app will always be an Unmasked App, even after reinstallation. Users must install that app in a separate profile to avoid this linkability.

Recommendation: This ID should be randomized with every app installation, rather than persisting after uninstallation. This will allow users to delete an app to change pseudonyms.

#### 5.1.5. SIM Card Operator ID

For some reason, no Android permissions are necessary to determine the cell network ID and the provider name of the user's SIM card. This information can be used to determine the user's rough geographic location, depending on the range of the cell provider (typically at least country-level, but sometimes regional, state, or province-level). The Device ID test of our VPN Leak Test app displays this information. It is also visible in Network Analyzer.

Consequences: This weakness does not directly impact any of our privacy invariants, but it is generally bad for privacy.

Recommendation: These APIs should always return NULL/empty (the default when no eSIMs are configured), because this information is not necessary for app operation.

#### 5.1.6. WiFi Calling

We observed that when WiFi calling is enabled for an activated eSIM, the device makes IPsec connections to the eSIM provider, bypassing the VPN.

Consequences: This weakness does not directly impact any of our privacy invariants, but it is generally bad for privacy in that it exposes the user's WiFi IP address to their cell network.

Recommendation: Allow VPN interfaces control of routing this connection, so that it may be disabled if the user chooses to block it.

### 5.2. Stock Android

Stock Android has a number of edge cases that can leak DNS or multicast, which can remove the Network Privacy invariant by revealing to the local network that a particular app may be in use.

#### 5.2.1. Apps Can Use Cell Network to Bypass VPN

Stock Android allows apps to bind to the cell network interface and then connect to an IP address, bypassing any VPN in place, regardless of Always-On or Block Connections VPN settings. Our VPN Test App has a reproduction case for this as part of the Leak Test. This leak is only possible over the cell network interface.

GrapheneOS fixed this issue in the 2025072700 Release.

Consequences: This leak allows Privacy-hostile apps to remove Network Privacy on stock Android. Users must use the IP Address Privacy Configuration and/or use GrapheneOS to prevent this leak.

Recommendations: Google shouldn't allow rogue apps to bind to the cell interface to bypass the VPN in Lockdown Mode, because VPN users expect that apps can't bypass the VPN in this mode.

#### 5.2.2. Secondary User Profiles Bypass VPN at Start

On stock Android, user profile isolation can leak packets upon first use after restart.

Consequences: This leak prevents stock Android users from successfully using Isolated User Profile Configuration to isolate sensitive app activity. Users have to install GrapheneOS to mitigate this.

Recommendation: These leaks should be fixed, because users expect Always on VPN and Block Connection settings to work in all user profiles.

#### 5.2.3. Google Account Linkability

On stock Android, the device Google account has access to all hardware identifiers, as well as SIM card identifiers and phone number. This access typically means that any form of linking to the device Google account (through push notifications, Gmail use, or Google OAuth sign-in) typically means deanonymization of a pseudonym.

Consequences: It is difficult for a user to maintain a pseudonym even in Pseudonym-friendly apps without careful configuration.

Recommendations: Both privacy-conscious apps and Google should have more transparency with respect to actions that link the app's account to the device Google account, because users tend to expect that pseudonymity is not actively subverted behind their backs. In the meantime, the GrapheneOS Google Sandbox enables Pseudonymous Google Configuration.

#### 5.2.4. Various DNS Leaks

Multiple issues with DNS exist in stock Android. Apps that use `getaddrinfo()` from the C library can bypass the VPN. Our VPN Leak Test app reproduces this leak as part of the main Leak Test.

Additional, DNS leaks can happen if the VPN malfunctions or shuts down.

Consequences: DNS leaks can erode Network Privacy by allowing local ISPs to determine which apps are in use. Users must use GrapheneOS to avoid these leaks.

Recommendations: GrapheneOS has fixed these issues. Google Android should upstream these fixes, because users expect that their DNS activity will not bypass their VPN.

#### 5.2.5. VPN Bypass Whitelist

Android has a secure setting `Settings.Secure.always_on_vpn_lockdown_whitelist`, which allows custom Android builds to define a list of apps that always bypass the VPN, regardless of VPN settings. Google and GrapheneOS both have this setting empty, but third-party Android builds may place their apps in this setting. The ReThink Firewall App has a ticket describing this issue.

Google Android also allows connectivity checks to bypass the VPN. GrapheneOS provides a setting to disable this.

Consequences: On Android rebuilds, various vendor apps may be exempted from the VPN, allowing them to connect outside of it. Users must use GrapheneOS to avoid these leaks.

Recommendations: Users should disable or remove vendor apps that they don't need.

### 5.3. GrapheneOS

The following weaknesses are unique to GrapheneOS. Note that the universal Android weaknesses also apply to GrapheneOS.

#### 5.3.1. Android Advertising ID Linkability

Additionally, the default-on behavior of the Android Advertising ID is another source of linkability between apps. GrapheneOS requires users to manually disable this ID, but this is not prominently documented during setup.

A better default would be to have the Android Advertising ID disabled by default or to provide a prominent setup step that educates users about this privacy risk.

We list this as a GrapheneOS-specific weakness because other privacy-preserving versions of Android randomize this ID automatically.

Consequences: Users who are not aware that they need to configure their Google account to disable this identifier will have all of their Privacy-hostile app activity linked to their Google account and to each other. Users must manually disable the Advertising ID in their Google account settings.

Recomendación: De manera similar a como GrapheneOS hace de proxy para las API de ubicación, esta API debería estar redirigida para devolver un número aleatorio o cero, porque esta API es una fuente extremadamente invasiva de vinculación que permite la agregación de datos de ubicación y uso en todas las aplicaciones.