Testing Network Communication

Practically every network-connected mobile app uses the Hypertext Transfer Protocol (HTTP) or HTTP over Transport Layer Security (TLS), HTTPS, to send and receive data to and from remote endpoints. Consequently, network-based attacks (such as packet sniffing and man-in-the-middle-attacks) are a problem. In this chapter we discuss potential vulnerabilities, testing techniques, and best practices concerning the network communication between mobile apps and their endpoints.

Intercepting HTTP(S) Traffic

In many cases, it is most practical to configure a system proxy on the mobile device, so that HTTP(S) traffic is redirected through an interception proxy running on your host machine. By monitoring the requests between the mobile app client and the backend, you can easily map the available server-side APIs and gain insight into the communication protocol. Additionally, you can replay and manipulate requests to test for server-side vulnerabilities.

Several free and commercial proxy tools are available. Here are some of the most popular:

To use the interception proxy, you'll need run it on your machine and configure the mobile app to route HTTP(S) requests to your proxy. In most cases, it is enough to set a system-wide proxy in the network settings of the mobile device - if the app uses standard HTTP APIs or popular libraries such as okhttp, it will automatically use the system settings.

Intercepting HTTP requests with BURP Suite Pro

Using a proxy breaks SSL certificate verification and the app will usually fail to initiate TLS connections. To work around this issue, you can install your proxy's CA certificate on the device. We'll explain how to do this in the OS-specific "Basic Security Testing" chapters.

Intercepting Traffic on the Network Layer

Dynamic analysis by using an interception proxy can be straight forward if standard libraries are used in the app and all communication is done via HTTP. But there are several cases where this is not working:

  • If mobile application development platforms like Xamarin are used that ignore the system proxy settings;

  • If mobile applications verify if the system proxy is used and refuse to send requests through a proxy;

  • If you want to intercept push notifications, like for example GCM/FCM on Android;

  • If XMPP or other non-HTTP protocols are used.

In these cases you need to monitor and analyze the network traffic first in order to decide what to do next. Luckily, there are several options for redirecting and intercepting network communication:

  • Route the traffic through the host machine. You can set up your machine as the network gateway, e.g. by using the built-in Internet Sharing facilities of your operating system. You can then use Wireshark to sniff any traffic from the mobile device;

  • Sometimes you need to execute a MITM attack to force the mobile device to talk to you. For this scenario you should consider bettercap to redirect network traffic from the mobile device to your host machine (see below);

bettercap is a powerful tool to execute MITM attacks and should be preferred nowadays, instead of ettercap. See also Why another MITM tool? on the bettercap site.

  • On a rooted device, you can use hooking or code injection to intercept network-related API calls (e.g. HTTP requests) and dump or even manipulate the arguments of these calls. This eliminates the need to inspect the actual network data. We'll talk in more detail about these techniques in the "Reverse Engineering and Tampering" chapters;

  • On macOS, you can create a "Remote Virtual Interface" for sniffing all traffic on an iOS device. We'll describe this method in the chapter "Basic Security Testing on iOS".

Simulating a Man-in-the-Middle Attack

bettercap can be used during network penetration tests in order to simulate a man-in-the-middle (MITM) attack. This is achieved by executing ARP poisoning or spoofing to the target machines. When such an attack is successful, all packets between two machines are redirected to a third machine that acts as the man-in-the-middle and is able to intercept the traffic for analysis.

For a full dynamic analysis of a mobile app, all network traffic should be intercepted. To be able to intercept the messages several steps should be considered for preparation.

bettercap Installation

bettercap is available for all major Linux and Unix operating systems and should be part of their respective package installation mechanisms. You need to install it on your machine that will act as the MITM. On macOS it can be installed by using brew.

$ brew install bettercap

For Kali Linux you can install bettercap with apt-get:

$ apt-get update
$ apt-get install bettercap

There are installation instructions as well for Ubuntu Linux 18.04 on LinuxHint.

Network Analyzer Tool

Install a tool that allows you to monitor and analyze the network traffic that will be redirected to your machine. The two most common network monitoring (or capturing) tools are:

Wireshark offers a GUI and is more straightforward if you are not used to the command line. If you are looking for a command line tool you should either use TShark or tcpdump. All of these tools are available for all major Linux and Unix operating systems and should be part of their respective package installation mechanisms.

Network Setup

To be able to get a man-in-the-middle position your machine should be in the same wireless network as the mobile phone and the gateway it communicates to. Once this is done you need the IP address of mobile phone.

ARP Poisoning with bettercap

Start your preferred network analyzer tool first, then start bettercap with the following command and replace the IP address below (X.X.X.X) with the target you want to execute the MITM attack against.

$ sudo bettercap -eval "set arp.spoof.targets X.X.X.X; arp.spoof on; set arp.spoof.internal true; set arp.spoof.fullduplex true;"
bettercap v2.22 (built for darwin amd64 with go1.12.1) [type 'help' for a list of commands]
[19:21:39] [sys.log] [inf] arp.spoof enabling forwarding
[19:21:39] [sys.log] [inf] arp.spoof arp spoofer started, probing 1 targets.

bettercap will then automatically send the packets to the network gateway in the (wireless) network and you are able to sniff the traffic. Beginning of 2019 support for full duplex ARP spoofing was added to bettercap.

On the mobile phone start the browser and navigate to http://example.com, you should see output like the following when you are using Wireshark.

Wireshark

If that's the case, you are now able to see the complete network traffic that is sent and received by the mobile phone. This includes also DNS, DHCP and any other form of communication and can therefore be quite "noisy". You should therefore know how to use DisplayFilters in Wireshark or know how to filter in tcpdump to focus only on the relevant traffic for you.

Man-in-the-middle attacks work against any device and operating system as the attack is executed on OSI Layer 2 through ARP Spoofing. When you are MITM you might not be able to see clear text data, as the data in transit might be encrypted by using TLS, but it will give you valuable information about the hosts involved, the protocols used and the ports the app is communicating with.

Span Port / Port Forwarding

As an alternative to a MITM attack with bettercap, a Wifi Access Point (AP) or router can also be used instead. The setup requires access to the configuration of the AP and this should be clarified prior to the engagement. If it's possible to reconfigure you should check first if the AP supports either:

  • port forwarding or

  • has a span or mirror port.

In both scenarios the AP needs to be configured to point to your machines IP. Tools like Wireshark can then again be used to monitor and record the traffic for further investigation.

Setting a Proxy Through Runtime Instrumentation

On a rooted or jailbroken device, you can also use runtime hooking to set a new proxy or redirect network traffic. This can be achieved with hooking tools like Inspeckage or code injection frameworks like Frida and cycript. You'll find more information about runtime instrumentation in the "Reverse Engineering and Tampering" chapters of this guide.

Example: Dealing with Xamarin

As an example, we will now redirect all requests from a Xamarin app to an interception proxy.

Xamarin is a mobile application development platform that is capable of producing native Android and iOS apps by using Visual Studio and C# as programming language.

When testing a Xamarin app and when you are trying to set the system proxy in the WiFi settings you won't be able to see any HTTP requests in your interception proxy, as the apps created by Xamarin do not use the local proxy settings of your phone. There are two ways to resolve this:

  • Add a default proxy to the app, by adding the following code in the OnCreate() or Main() method and re-create the app:

    WebRequest.DefaultWebProxy = new WebProxy("192.168.11.1", 8080);
  • Use bettercap in order to get a man-in-the-middle position (MITM), see the section above about how to setup a MITM attack. When being MITM we only need to redirect port 443 to our interception proxy running on localhost. This can be done by using the command rdr on macOS:

    $ echo "
    rdr pass inet proto tcp from any to any port 443 -> 127.0.0.1 port 8080
    " | sudo pfctl -ef -

The interception proxy need to listen to the port specified in the port forwarding rule above, which is 8080.

CA Certificates

If not already done, install the CA certificates in your mobile device which will allow us to intercept HTTPS requests:

Intercepting Traffic

Start using the app and trigger it's functions. You should see HTTP messages showing up in your interception proxy.

When using bettercap you need to activate "Support invisible proxying" in Proxy Tab / Options / Edit Interface

Verifying Data Encryption on the Network

Overview

One of the core mobile app functions is sending/receiving data over untrusted networks like the Internet. If the data is not properly protected in transit, an attacker with access to any part of the network infrastructure (e.g., a Wi-Fi access point) may intercept, read, or modify it. This is why plaintext network protocols are rarely advisable.

The vast majority of apps rely on HTTP for communication with the backend. HTTPS wraps HTTP in an encrypted connection (the acronym HTTPS originally referred to HTTP over Secure Socket Layer (SSL); SSL is the deprecated predecessor of TLS). TLS allows authentication of the backend service and ensures confidentiality and integrity of the network data.

Ensuring proper TLS configuration on the server side is also important. SSL is deprecated and should no longer be used. TLS v1.2 and v1.3 are considered secure, but many services still allow TLS v1.0 and v1.1 for compatibility with older clients.

When both the client and server are controlled by the same organization and used only for communicating with one another, you can increase security by hardening the configuration.

If a mobile application connects to a specific server, its networking stack can be tuned to ensure the highest possible security level for the server's configuration. Lack of support in the underlying operating system may force the mobile application to use a weaker configuration.

Cipher Suites Terminology

Cipher suites have the following structure: Protocol_KeyExchangeAlgorithm_WITH_BlockCipher_IntegrityCheckAlgorithm

This structure can be described as follows:

  • The Protocol the cipher uses

  • The key Exchange Algorithm used by the server and the client to authenticate during the TLS handshake

  • The block cipher used to encrypt the message stream

  • Integrity check algorithm used to authenticate messages

Example: TLS_RSA_WITH_3DES_EDE_CBC_SHA

In the example above the cipher suites uses:

  • TLS as protocol

  • RSA Asymmetric encryption for Authentication

  • 3DES for Symmetric encryption with EDE_CBC mode

  • SHA Hash algorithm for integrity

Note that in TLSv1.3 the KeyExchangeAlgorithm is not part of the cipher suite, instead it is determined during the TLS handshake.

In the following listing, we’ll present the different algorithms of each part of the cipher suite.

Protocols:

Key Exchange Algorithms:

Block Ciphers:

Integrity Check Algorithms:

Note that The efficiency of a cipher suite depends on the efficiency of its algorithms.

In the following, we’ll present the updated recommended cipher suites list to use with TLS. These cipher suites are recommended by both IANA in its TLS parameters documentation and OWASP TLS Cipher String Cheat Sheet:

Some Android and iOS versions do not support some of the recommended cipher suites, so for compatibility purposes you can check the supported cipher suites for Android and iOS versions and choose the top supported cipher suites.

Static Analysis

Identify all API/web service requests in the source code and ensure that no plain HTTP URLs are used. Make sure that sensitive information is sent over secure channels by using HttpsURLConnection or SSLSocket (for socket-level communication using TLS).

Be aware that SSLSocket doesn't verify the hostname. Use getDefaultHostnameVerifier to verify the hostname. The Android developer documentation includes a code example.

Verify that the server or termination proxy at which the HTTPS connection terminates is configured according to best practices. See also the OWASP Transport Layer Protection cheat sheet and the Qualys SSL/TLS Deployment Best Practices.

Dynamic Analysis

Intercept the tested app's incoming and outgoing network traffic and make sure that this traffic is encrypted. You can intercept network traffic in any of the following ways:

  • Capture all HTTP(S) and Websocket traffic with an interception proxy like OWASP ZAP or Burp Suite and make sure all requests are made via HTTPS instead of HTTP.

  • Interception proxies like Burp and OWASP ZAP will show HTTP(S) traffic only. You can, however, use a Burp plugin such as Burp-non-HTTP-Extension or the tool mitm-relay to decode and visualize communication via XMPP and other protocols.

Some applications may not work with proxies like Burp and ZAP because of Certificate Pinning. In such a scenario, please check "Testing Custom Certificate Stores and SSL Pinning".

If you want to verify whether your server supports the right cipher suites, there are various tools you can use:

  • nscurl - see Testing Network Communication for iOS for more details.

  • testssl.sh which "is a free command line tool which checks a server's service on any port for the support of TLS/SSL ciphers, protocols as well as some cryptographic flaws".

Making Sure that Critical Operations Use Secure Communication Channels

Overview

For sensitive applications like banking apps, OWASP MASVS introduces "Defense in Depth" verification levels. The critical operations (e.g., user enrolment and account recovery) of such applications are some of the most attractive targets to attackers. This requires implementation of advanced security controls, such as additional channels to confirm user actions without relying on SMS or email.

Note that using SMS as an additional factor for critical operations is not recommended. Attacks like SIM swap scams were used in many cases to attack Instagram accounts, cryptocurrency exchanges and of course financial institutions to bypass SMS verification. SIM swapping is a legitimate service offered by many carriers to switch your mobile number to a new SIM card. If an attacker manages to either convince the carrier or recruits retail workers at mobile shops to do a SIM swap, the mobile number will be transferred to a SIM the attacker owns. As a result of this, the attacker will be able to receive all SMS and voice calls without the victim knowing it.

There are different ways to protect your SIM card, but this level of security maturity and awareness cannot be expected from a normal user and is also not enforced by the carriers.

Also the usage of emails shouldn't be considered as a secure communication channel. Encrypting emails is usually not offered by service providers and even when available not used by the average user, therefore the confidentiality of data when using emails cannot be guaranteed. Spoofing, (spear|dynamite) phishing and spamming are additional ways to trick users by abusing emails. Therefore other secure communication channels should be considered besides SMS and email.

Static Analysis

Review the code and identify the parts that refer to critical operations. Make sure that additional channels are used for such operations. The following are examples of additional verification channels:

  • Token (e.g., RSA token, YubiKey),

  • Push notification (e.g., Google Prompt),

  • Data from another website you have visited or scanned (e.g. QR code) or

  • Data from a physical letter or physical entry point (e.g., data you receive only after signing a document at a bank).

Make sure that critical operations enforce the use of at least one additional channel to confirm user actions. These channels must not be bypassed when executing critical operations. If you're going to implement an additional factor to verify the user's identity, consider also one-time passcodes (OTP) via Google Authenticator.

Dynamic Analysis

Identify all of the tested application's critical operations (e.g., user enrollment, account recovery, and financial transactions). Ensure that each critical operation requires at least one additional verification channel. Make sure that directly calling the function doesn't bypass the usage of these channels.

References

OWASP Mobile Top 10 2016

OWASP MASVS

  • V1.2: "Security controls are never enforced only on the client side, but on the respective remote endpoints."

  • V5.1: "Data is encrypted on the network with TLS. The secure channel is used consistently throughout the app."

  • V5.2: "The TLS settings are in line with current best practices, or as close as possible if the mobile operating system does not support the recommended standards."

  • V5.3: "The app verifies the X.509 certificate of the remote endpoint when the secure channel is established. Only certificates signed by a trusted CA are accepted."

  • V5.5: "The app doesn't rely on a single insecure communication channel (e-mail or SMS) for critical operations such as enrollment and account recovery."

CWE

  • CWE-308 - Use of Single-factor Authentication

  • CWE-319 - Cleartext Transmission of Sensitive Information

Tools

Android

iOS

IANA Transport Layer Security (TLS) Parameters

OWASP TLS Cipher String Cheat Sheet

SIM Swapping attacks

NIST

  • FIPS PUB 186 - Digital Signature Standard (DSS)

SIM Swap Fraud

IETF