Author: Matthias Luft

Update #1: Slides are available for download here.

In the course of our ongoing cloud security research, we’re continuously thinking about potential attack vectors against public cloud infrastructures. Approaching this enumeration from an external customer’s (speak: attacker’s 😉 ) perspective, there are the following possibilities to communicate with and thus send malicious input to typical cloud infrastructures:

• Management interfaces
• Guest/hypervisor interaction
• Network communication
• File uploads

As there are already several successful exploits against management interfaces (e.g. here and here) and guest/hypervisor interaction (see for example this one; yes, this is the funny one with that ridiculous recommendation “Do not allow untrusted users access to your virtual machines.” ;-)), we’re focusing on the upload of files to cloud infrastructures in this post. According to our experience with major Infrastructure-as-a-Service (IaaS) cloud providers, the most relevant file upload possibility is the deployment of already existing virtual machines to the provided cloud infrastructure. However, since a quick additional research shows that most of those allow the upload of VMware-based virtual machines and, to the best of our knowledge, the VMware virtualization file format was not analyzed as for potential vulnerabilities yet, we want to provide an analysis of the relevant file types and present resulting attack vectors.

As there are a lot of VMware related file types, a typical virtual machine upload functionality comprises at least two file types:

• VMX
• VMDK

The VMX file is the configuration file for the characteristics of the virtual machine, such as included devices, names, or network interfaces. VMDK files specify the hard disk of a virtual machine and mainly contain two types of files: The descriptor file, which describes the specific setup of the actual disk file, and several disk files containing the actual file system for the virtual machine. The following listing shows a sample VMDK descriptor file:


# Disk DescriptorFile
version=1
encoding="UTF-8"
CID=a5c61889
parentCID=ffffffff
isNativeSnapshot="no"
createType="vmfs"
# Extent description
RW 33554432 VMFS "machine-flat01.vmdk"
RW 33554432 VMFS "machine-flat02.vmdk"
[...]


For this post, it is of particular importance that the inclusion of the actual disk file containing the raw device data allows the inclusion of multiple files or devices (in the listing, the so-called “Extent description”). The deployment of these files into a (public) cloud/virtualized environment can be broken down into several steps:

1. Upload to the cloud environment: e.g. by using FTP, web interfaces, $WEB_SERVICE_API (such as the Amazon SOAP API, which admittedly does not allow the upload of virtual machines at the moment).
2. Move to the data store: The uploaded virtual machine must be moved to the data store, which is typically some kind of back end storage system/SAN where shares can be attached to hypervisors and guests.
3. Deployment on the hypervisor (“starting the virtual machine”): This can include an additional step of “cloning” the virtual machine from the back end storage system to local hypervisor hard drives.

To analyze this process more thoroughly, we built a small lab based on VMware vSphere 5 including

• an ESXi5 hypervisor,
• NFS-based storage, and
• vCenter,

everything fully patched as of 2012/05/24. The deployment process we used was based on common practices we know from different customer projects: The virtual machine was copied to the storage, which is accessible from the hypervisor, and was deployed on the ESXi5 using the vmware-cmd utility utilizing the VMware API. Thinking about actual attacks in this environment, two main approaches come to mind:

• Fuzzing attacks: Given ERNW’s long tradition in the area of fuzzing, this seems to be a viable option. Still this is not in scope of this post, but we’ll lay out some things tomorrow in our workshop at #HITB2012AMS.
• File Inclusion Attacks.

Focusing on the latter, the descriptor file (see above) contains several fields which are worth a closer look. Even though the specification of the VMDK descriptor file will not be discussed here in detail, the most important field for this post is the so-called Extent Description. The extent descriptions basically contain paths to the actual raw disk files containing the file system of the virtual machine and were included in the listing above.

The most obvious idea is to change the path to the actual disk file to another path, somewhere in the ESX file system, like the good ol’ /etc/passwd:


# Disk DescriptorFile
version=1
encoding="UTF-8"
CID=a5c61889
parentCID=ffffffff
isNativeSnapshot="no"
createType="vmfs"
# Extent description
RW 33554432 VMFS "machine-flat01.vmdk"
RW 0 VMFSRAW "/etc/passwd"


Unfortunately, this does not seem to work and results in an error message as the next screenshot shows:

As we are highly convinced that a healthy dose of perseverance (not to say stubbornness 😉 ) is part of any hackers/pentesters attributes, we gave it several other tries. As the file to be included was a raw disk file, we focused on files in binary formats. After some enumeration, we were actually able to include gzip-compressed log files. Since we are now able to access files included in the VMDK files inside the guest virtual machine, this must be clearly stated: We have/can get access to the log files of the ESX hypervisor by deploying a guest virtual machine – a very nice first step! Including further compressed log files, we also included the /bootbank/state.tgz file. This file contains a complete backup of the /etc directory of the hypervisor, including e.g. /etc/shadow – once again, this inclusion was possible from a GUEST machine! As the following screenshot visualizes, the necessary steps to include files from the ESXi5 host include the creation of a loopback device which points to the actual file location (since it is part of the overall VMDK file) and extracting the contents of this loopback device:

The screenshot also shows how it is possible to access information which is clearly belonging to the ESXi5 host from within the guest system. Even though this allows a whole bunch of possible attacks, coming back to the original inclusion of raw disk files, the physical hard drives of the hypervisor qualify as a very interesting target. A look at the device files of the hypervisor (see next screenshot) reveals that the device names are generated in a not-easily-guessable-way:

Using this knowledge we gathered from the hypervisor (this is heavily noted at this point, we’re relying on knowledge that we gathered from our administrative hypervisor access), it was also possible to include the physical hard drives of the hypervisor. Even though we needed additional knowledge for this inclusion, the sheer fact that it is possible for a GUEST virtual machine to access the physical hard drives of the hypervisor is a pretty big deal! As you still might have our stubbornness in mind, it is obvious that we needed to make this inclusion work without knowledge about the hypervisor. Thus let’s provide you with a way to access to any data in a vSphere based cloud environment without further knowledge:

1. Ensure that the following requirements are met:
• ESXi5 hypervisor in use (we’re still researching how to port these vulnerabilities to ESX4)
• Deployment of externally provided (in our case, speak: malicious 😉 ) VMDK files is possible
• The cloud provider performs the deployment using the VMware API (e.g. in combination with external storage, which is, as laid out above, a common practice) without further sanitization/input validation/VMDK rewriting.
2. Deploy a virtual machine referencing /scratch/log/hostd.0.gz
3. Access the included /scratch/log/hostd.0.gz within the guest system and grep for ESXi5 device names 😉
4. Deploy another virtual machine referencing the extracted device names
5. Enjoy access to all physical hard drives of the hypervisor 😉

It must be noted that the hypervisor hard drives contain the so-called VMFS, which cannot be easily mounted within e.g. a Linux guest machines, but it can be parsed for data, accessed using VMware specific tools, or exported to be mounted on another hypervisor under our own administrative control.

Summarizing the most relevant and devastating message in short:

VMware vSphere 5 based IaaS cloud environments potentially contain possibilities to access other customers’ data…

We’ll conduct some “testing in the field” in the upcoming weeks and get back to you with the results in a whitepaper to be found on this blog. In any case this type of attacks might provide yet-another path for accessing other tenants’ data in multi-tenant environments, even though more research work is needed here. If you have the opportunity you might join our workshop at #HITB2012AMS.

Have a great day,

Pascal, Enno, Matthias, Daniel