Direito - Unip - DNA 2,03/08

sexta-feira, 2 de junho de 2023

Introduction To Reversing Golang Binaries

Golang binaries are a bit hard to analyze but there are some tricks to locate the things and view what is doing the code.

Is possible to list all the go files compiled in the binary even in an striped binaries, in this case we have only one file gohello.go this is a good clue to guess what is doing the program.

On stripped binaries the runtime functions are not resolved so is more difficult to locate the user algorithms:

If we start from the entry point, we will found this mess:

The golang string initialization are encoded and is not displayed on the strings window.

How to locate main? if its not stripped just bp on [package name].main for example bp main.main, (you can locate the package-name searching strings with ".main")

And here is our main.main:

The code is:

So in a stripped binary we cant find the string "hello world" neither the initialization 0x1337 nor the comparator 0x1337, all this is obfuscated.

The initialization sequence is:

The procedure for locating main.main in stripped binaries is:

1. Click on the entry point and locate the runtime.mainPC pointer:

2. click on runtime.main function (LAB_0042B030):

3. locate the main.main call after the zero ifs:

4. click on it and here is the main:

The runtime is not obvious for example the fmt.Scanf() call perform several internal calls until reach the syscall, and in a stripped binary there are no function names.

In order to identify the functions one option is compile another binary with symbols and make function fingerprinting.

In Ghidra we have the script golang_renamer.py which is very useful:

After applying this plugin the main looks like more clear:

This script is an example of function fingerprinting, in this case all the opcodes are included on the crc hashing:

# This script fingerprints the functions
#@author: sha0coder
#@category fingerprinting

print "Fingerprinting..."

import zlib


# loop through program functions
function = getFirstFunction()
while function is not None:
    name = str(function.getName())
    entry = function.getEntryPoint()
    body = function.getBody()
    addresses = body.getAddresses(True)

    if not addresses.hasNext():
        # empty function
        continue 

    ins = getInstructionAt(body.getMinAddress())
    opcodes = ''
    while ins and ins.getMinAddress() <= body.getMaxAddress():
        for b in ins.bytes:
            opcodes += chr(b & 0xff)
        ins = getInstructionAfter(ins)
    
    
    crchash = zlib.crc32(opcodes) & 0xffffffff

    print name, hex(crchash)


    function = getFunctionAfter(function)

quinta-feira, 1 de junho de 2023

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BYOPPP - Build Your Own Privacy Protection Proxy

I have read a blog post, where you can build your own privacy proxy server built on Raspberry PI. The post got me thinking about how I can use this to protect my privacy on my Android phone, and also get rid of those annoying ads.

Since I own a Samsung Galaxy S3 LTE with Android 4.3 (with a HW based Knox counter), rooting the phone now means you break Knox, and loose warranty. Past the point of no return ...

This means I have to solve this without root. Luckily newer Androids support VPN without rooting, but setting a mandatory system-wide proxy is still not possible without root.

But thanks to some iptables magic and Privoxy, this is not a problem anymore :)

The ingredients to build your own privacy protection proxy:

One (or more) cheap VPS server(s)
a decent VPN program
Privoxy
iptables

VPS server

To get the cheap VPS server, I recommend using Amazon EC2, but choose whatever you like. The micro instance is very cheap (or even free), and has totally enough resources for this task. I'm using the Ubuntu free tier now and it works like a charm. And last but not least Amazon has two-factor authentication! You can set up an Ubuntu server under 10 minutes. Use the AWS region nearest to you, e.g. I choose EU - Ireland.

VPN

For the VPN program, I recommend the free version of the OpenVPN AS (EDIT: be sure to use OpenVPN AS 2.0.6 or later, both on the server and the client). Easy to set-up quick start guide is here, GUI based configuration, and one-click client installer for Android, iOS, Windows, Linux, OSX. The Ubuntu installer packages are here.

The most important settings:

I prefer to use the TCP 443 and UDP 53 ports for my OpenVPN setup, and let the user guess why.
For good performance, UDP is preferred over TCP.
VPN mode is Layer 3 (routing/NAT).
Don't forget to allow the configured VPN ports in the AWS firewall (security groups).

Other VPN settings:

Should VPN clients have access to private subnets (non-public networks on the server side)? - Yes
Should client Internet traffic be routed through the VPN? - Yes

Privoxy

The next component we have to install and configure is Privoxy. As usual, "apt-get install privoxy" just works. The next step is to configure privoxy via /etc/privoxy/config file, there are two options to change:

listen-address your.ip.add.ress:8118
accept-intercepted-requests 1

Beware not to allow everyone accessing your Privoxy server in the AWS EC2 security groups, be sure it is reachable only to VPN users!

After everything is set, start privoxy with "service privoxy start", and add it to the autostart "update-rc.d privoxy defaults".

Iptables

And the final step is to configure your iptables chain to forward every web traffic from the VPN clients to the Privoxy server:

iptables -t nat -A PREROUTING -s 5.5.0.0/16 -p tcp -m multiport --dports 80,8080,81 -j DNAT --to-destination your.ip.add.ress:8118

Optionally you can block access to all other ports as well, and what does not go through your Privoxy won't be reachable.

Based on your Linux distribution and preference, you might make this rule persistent.

Final test

Now you can connect to the VPN server from your Android device.

After logging in from a client, you get the following nice packages to install on your device:

After connecting, the final results can be seen in the following screenshots. And yes, there is a reason I chose Angry Birds as an example.

Angry Birds without Privoxy

Angry Birds with Privoxy

Stupid flashlight app with ad

Stupid flashlight app with Privoxy

Spoiler alert

If you are afraid of NSA tracking you, this post is not for you. If you want to achieve IP layer anonymity, this post is not for you. As long as you are the only one using that service, it should be trivial to see what could possibly go wrong with that.

Known issues

Whenever the Internet connection (Wifi, 3G) drops, the VPN connection drops as well, and your privacy is gone ...

Sites breaking your privacy through SSL can still do that as long as the domain is not in the Privoxy blacklist.

Additional recommendation

If you are using OSX or Windows, I can recommend Aviator to be used as your default browser. It is just great, give it a try!

PS: There are also some adblock apps removed from the official store which can block some ads, but you have to configure a proxy for every WiFi connection you use, and it is not working over 3G.

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