Android SSL Vulnerability Detection Tools

Introduction

A set of tools for detecting if Android applications are vulnerable to common SSL certificate validation security vulnerabilities which allow man-in-the-middle attackers to intercept and modify encrypted network traffic.

One tool uses static analysis to try to detect potentially vulnerable SSL certificate validation code. The other tool actually tries to carry out a man-in-the-middle attack to actively exploit certificate validation vulnerabilities.

These tools should be useful for developers who want to check if their own applications use SSL securely on a private network that they own. They are not intended for malicious use on public networks.

I developed these tools as part of my Part II project at Cambridge. Thanks to Dr Alastair Beresford for supervising the project.

Note: I have done some rewriting of the repository with git filter-branch to tidy it up. Some of the commit messages may therefore not make much sense.

Building

Gradle is used as the build system. Java 8 (Oracle Java or OpenJDK) on Linux is required.

Run gradle to build the tools and run the unit tests.

There's also a separate set of integration tests for the man-in-the-middle tool which can be run by typing ./mitm-test/run. Warning: the integration tests will modify your iptables configuration and might not restore it properly (especially if they fail).

Static Analysis

The static analysis tool assumes the Android SDK is installed in /opt/android.

Run ./analysis/static-analyser /path/to/the.apk to analyse an application. If you get SPARK-related exceptions from Soot, you can pass the --paddle option to use Paddle (an alternative to SPARK) which might fix it.

Man-in-the-Middle

The ./mitm/mitm script runs the man-in-the-middle tool. ./mitm/mitm-gui runs it with a GUI, which is useful if many connections are being intercepted at the same time as it makes figuring out which data was sent by which connection easier.

Before running the man-in-the-middle tool for the first time you must generate a trusted and untrusted certificate authority, and install the trust certificate on your phone. Run cd mitm; ./make-ca; ./install-ca to do so. Your phone must be rooted to install the trusted certificate in this manner. The Android SDK's tools and platform-tools directories must also be in your $PATH environment variable.

Several required options must be specified on the command line (even with the GUI mode):

Interception Mode

This is set to indicate how you are passing the intercepted traffic to the MITM program with iptables.

• --nat: if you are using the iptables REDIRECT target.
• --tproxy: if you are using the iptables transparent proxying support.
• --fixed <address>:<port>: if you aren't actually intercepting traffic at all. Allows you to proxy traffic to a fixed address and port for testing purposes.

You'll probably want to use the MITM tool in conjunction with some software such as hostapd, which turns your computer into a WiFi hotspot, or dsniff's arpspoof command, which uses ARP spoofing to intercept traffic on an existing WiFi hotspot or network.

For both the --nat and --tproxy modes you'll need to enable IP forwarding:

sysctl -w net.ipv4.ip_forward=1

(This turns your machine into a router, so you might want to be careful with your configuration if you are connecting to the Internet through a network you don't control or you might annoy your local sysadmin if you make a mistake!)

For IPv6, the equivalent sysctl is:

sysctl -w net.ipv6.conf.all.forwarding=1

Example iptables commands for --nat mode

Assuming hostapd is running on wlan0:

iptables -t nat -A PREROUTING -i wlan0 -p tcp -j REDIRECT --to-port 8443

If you want to intercept local connections from your own machine, then you will need to run the MITM tool as a different user (nobody in this example) to prevent it intercepting the connections it opens itself:

iptables -t nat -A OUTPUT -p tcp -m owner --uid-owner nobody -j ACCEPT
iptables -t nat -A OUTPUT -p tcp -j REDIRECT --to-port 8443

I haven't had any luck in getting ARP spoofing working together with the NAT mode, therefore the tool also supports transparent proxying which I have managed to get working with ARP spoofing.

Change iptables to ip6tables if you want to use IPv6 instead. Note that IPv6 NAT requires Linux 3.7 or above (and a recent enough version of the user-space iptables tools too).

Example iptables commands for --tproxy mode

Assuming 192.168.0.1 is the gateway and 192.168.0.100 is the computer whose traffic you wish to intercept, first start up two arpspoof instances:

arpspoof -t 192.168.0.1 192.168.0.100
arpspoof -t 192.168.0.100 192.168.0.1

Disable reverse path filtering (again, be careful, lest you annoy a sysadmin):

sysctl -w net.ipv4.conf.all.rp_filter=0

Add a separate routing table for 'marked' packets which delivers them locally:

ip rule add fwmark 1 lookup 100
ip route add local default dev lo table 100

Add iptables rules which transparently proxy any incoming connections passing through the machine:

iptables -t mangle -N DIVERT
iptables -t mangle -A DIVERT -j MARK --set-mark 1
iptables -t mangle -A DIVERT -j ACCEPT
iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
iptables -t mangle -A PREROUTING -p tcp -j TPROXY --tproxy-mark 0x1/0x1 --on-port 8443

The transparent proxying configuration is tricky to set up, the Squid website has some tips which are applicable. If possible, stick with --nat mode.

As with --nat mode, you can replace iptables with ip6tables if you want to use IPv6. You'll also need to pass the -6 flag to the ip command. Changing the rp_filter sysctl is not required for IPv6.

Certificate Hostname Mode

This is set to determine the value of the Common Name and Subject Alternative Name fields in the generated certificates.

• --matching-hostname: use the same CN and SAN as the real certificate.
• --unmatching-hostname: use a CN which does not match the one in the real certificate.

Certificate Trust Mode

This is set to determine if the generate certificates are signed with the trusted certificate authority (whose certificate is installed on the phone) or the untrusted certificate authority (whose certificate is not installed on the phone).

• --trusted
• --untrusted

Types of Vulnerability

For each combination of hostname and trust mode, if the client accepts a connection which the MITM tool has intercepted then the following vulnerability is present:

Dependencies

The following Java libraries are used by the tools:

• Soot
• Paddle (optional)
• Jedd (optional)
• JOpt Simple
• Bouncy Castle
• Java Native Access

License

The tools are available under Version 2.0 of the Apache License. The full terms of the Apache License are available in the LICENSE file.