Archive for July, 2013

Google Chromecast – released yesterday, hacked today

Posted in Exploits with tags , , on July 29, 2013 by keizer


On Wednesday, July 24th Google launched the Chromecast. As soon as the source code hit GTV Hacker began their audit. Within a short period of time they had multiple items to look at for when their devices arrived. Then they received their Chromecasts the following day and were able to confirm that one of the bugs existed in the build Chromecast shipped with. From that point on they began building what you are now seeing as our public release package.

Exploit Package:

GTV Hacker Chromecast exploit package will modify the system to spawn a root shell on port 23. This will allow researchers to better investigate the environment as well as give developers a chance to build and test software on their Chromecasts. For the normal user this release will probably be of no use, for the rest of the community this is just the first step in opening up what has just been a mysterious stick up to this point. GTV Hacker hope that following this release the community will have the tools they need to improve on the shortfalls of this device and make better use of the hardware.

File:Chromecast dirty side1.jpgIs it really ChromeOS?

No, it’s not. GTV Hacker had a lot of internal discussion on this, and have concluded that it’s more Android than ChromeOS. To be specific, it’s actually a modified Google TV release, but with all of the Bionic / Dalvik stripped out and replaced with a single binary for Chromecast. Since the Marvell DE3005 SOC running this is a single core variant of the 88DE3100, most of the Google TV code was reused. So, although it’s not going to let you install an APK or anything, its origins: the bootloader, kernel, init scripts, binaries, are all from the Google TV.


How does the exploit work?

Lucky for GTV Hacker, Google was kind enough to GPL the bootloader source code for the device. So they could identify the exact flaw that allows us to boot the unsigned kernel. By holding down the single button, while powering the device, the Chromecast boots into USB boot mode. USB boot mode looks for a signed image at 0×1000 on the USB drive. When found, the image is passed to the internal crypto hardware to be verified, but after this process the return code is never checked! Therefore, they could execute any code at will.

ret = VerifyImage((unsigned int)k_buff, cpu_img_siz, (unsigned int)k_buff);

The example above shows the call made to verify the image, the value stored in ret is never actually verified to ensure that the call to “VerifyImage” succeeded. From that, they were able to execute our own kernel. Hilariously, this was harder to do than our initial analysis of exploitation suggested. This was due to the USB booted kernel needing extra modifications to allow us to modify /system as well as a few other tweaks. GTV Hacker then built a custom ramdisk which, when started, began the process of modifying the system by performing the following steps:

  • Mount the USB drive plugged in to the chromecast.
  • Erase the /system partition (mtd3).
  • Write the new custom system image.
  • Reboot.

Note: /system is squashfs as opposed to normally seen EXT4/YAFFS2.

The system image installed from our package is a copy of the original with a modified /bin/clear_crash_counter binary. This binary was modified to perform its original action as well as spawn a telnet server as root.

After the above process, the only modification to the device is done to spawn a root shell. No update mitigations are performed which means that theoretically, an update could be pushed at any moment patching our exploit. Even with that knowledge, having an internal look at the device is priceless and they hope that the community will be able to leverage this bug in time.

Downloads and instructions for exploitation can be found on their wiki at: GTVHacker Wiki: Google Chromecast

Rooting SIM cards

Posted in Encryption, Mobile, Network Security, Valnurability with tags , , on July 23, 2013 by keizer

SIM cards are the de facto trust anchor of mobile devices worldwide. The cards protect the mobile identity of subscribers, associate devices with phone numbers, and increasingly store payment credentials, for example in NFC-enabled phones with mobile wallets.

With over seven billion cards in active use, SIMs may well be the most widely used security token in the world. Through over-the-air (OTA) updates deployed via SMS, the cards are even extensible through custom Java software. While this extensibility is rarely used so far, its existence already poses a critical hacking risk.

Cracking SIM update keys. OTA commands, such as software updates, are cryptographically-secured SMS messages, which are delivered directly to the SIM. While the option exists to use state-of-the-art AES or the somewhat outdated 3DES algorithm for OTA, many (if not most) SIM cards still rely on the 70s-era DES cipher. DES keys were shown to be crackable within days using FPGA clusters, but they can also be recovered much faster by leveraging rainbow tables similar to those that made GSM’s A5/1 cipher breakable by anyone.

To derive a DES OTA key, an attacker starts by sending a binary SMS to a target device. The SIM does not execute the improperly signed OTA command, but does in many cases respond to the attacker with an error code carrying a cryptographic signature, once again sent over binary SMS. A rainbow table resolves this plaintext-signature tuple to a 56-bit DES key within two minutes on a standard computer.

Deploying SIM malware. The cracked DES key enables an attacker to send properly signed binary SMS, which download Java applets onto the SIM. Applets are allowed to send SMS, change voicemail numbers, and query the phone location, among many other predefined functions. These capabilities alone provide plenty of potential for abuse.

In principle, the Java virtual machine should assure that each Java applet only accesses the predefined interfaces. The Java sandbox implementations of at least two major SIM card vendors, however, are not secure: A Java applet can break out of its realm and access the rest of the card. This allows for remote cloning of possibly millions of SIM cards including their mobile identity (IMSI, Ki) as well as payment credentials stored on the card.

Defenses. The risk of remote SIM exploitation can be mitigated on three layers:

  1. Better SIM cards. Cards need to use state-of-art cryptography with sufficiently long keys, should not disclose signed plaintexts to attackers, and must implement secure Java virtual machines. While some cards already come close to this objective, the years needed to replace vulnerable legacy cards warrant supplementary defenses.
  2. Handset SMS firewall. One additional protection layer could be anchored in handsets: Each user should be allowed to decide which sources of binary SMS to trust and which others to discard. An SMS firewall on the phone would also address other abuse scenarios including “silent SMS.”
  3. In-network SMS filtering. Remote attackers rely on mobile networks to deliver binary SMS to and from victim phones. Such SMS should only be allowed from a few known sources, but most networks have not implemented such filtering yet. “Home routing” is furthermore needed to increase the protection coverage to customers when roaming. This would also provide long-requested protection from remote tracking.

Thanks to: Security Research Labs who published this!

This research will be presented at BlackHat on Jul 31st and at the OHM hacking camp on Aug 3rd 2013

Uncovering Android Master Key – 99% of devices are vulnerable!

Posted in Encryption, Malware, Mobile, Valnurability with tags , , , on July 4, 2013 by keizer

The Bluebox Security research team –  recently discovered a vulnerability in Android’s security model that allows a hacker to modify APK code without breaking an application’s cryptographic signature, to turn any legitimate application into a malicious Trojan, completely unnoticed by the app store, the phone, or the end user. The implications are huge! This vulnerability, around at least since the release of Android 1.6 (codename: “Donut” ), could affect any Android phone released in the last 4 years1 – or nearly 900 million devices2– and depending on the type of application, a hacker can exploit the vulnerability for anything from data theft to creation of a mobile botnet.

While the risk to the individual and the enterprise is great (a malicious app can access individual data, or gain entry into an enterprise), this risk is compounded when you consider applications developed by the device manufacturers (e.g. HTC, Samsung, Motorola, LG) or third-parties that work in cooperation with the device manufacturer (e.g. Cisco with AnyConnect VPN) – that are granted special elevated privileges within Android – specifically System UID access.

Installation of a Trojan application from the device manufacturer can grant the application full access to Android system and all applications (and their data) currently installed. The application then not only has the ability to read arbitrary application data on the device (email, SMS messages, documents, etc.), retrieve all stored account & service passwords, it can essentially take over the normal functioning of the phone and control any function thereof (make arbitrary phone calls, send arbitrary SMS messages, turn on the camera, and record calls). Finally, and most unsettling, is the potential for a hacker to take advantage of the always-on, always-connected, and always-moving (therefore hard-to-detect) nature of these “zombie” mobile devices to create a botnet.

How it works:

The vulnerability involves discrepancies in how Android applications are cryptographically verified & installed, allowing for APK code modification without breaking the cryptographic signature.

All Android applications contain cryptographic signatures, which Android uses to determine if the app is legitimate and to verify that the app hasn’t been tampered with or modified. This vulnerability makes it possible to change an application’s code without affecting the cryptographic signature of the application – essentially allowing a malicious author to trick Android into believing the app is unchanged even if it has been.

Details of Android security bug 8219321 were responsibly disclosed through Bluebox Security’s close relationship with Google in February 2013. It’s up to device manufacturers to produce and release firmware updates for mobile devices (and furthermore for users to install these updates). The availability of these updates will widely vary depending upon the manufacturer and model in question.

The screenshot below demonstrates that Bluebox Security has been able to modify an Android device manufacturer’s application to the level that we now have access to any (and all) permissions on the device. In this case, we have modified the system-level software information about this device to include the name “Bluebox” in the Baseband Version string (a value normally controlled & configured by the system firmware).

Screenshot of HTC Phone After Exploit



  • Device owners should be extra cautious in identifying the publisher of the app they want to download.
  • Enterprises with BYOD implementations should use this news to prompt all users to update their devices, and to highlight the importance of keeping their devices updated.
  • IT should see this vulnerability as another driver to move beyond just device management to focus on deep device integrity checking and securing corporate data.

Thanks Bluebox Security