System Administration on System Overlord

OpenSSH Two Factor Authentication (But Not Service Accounts)

david@systemoverlord.com (David Tomaschik) — Sat, 03 Mar 2018 00:00:00 +0000

Very often, people hear “SSH” and “two factor authentication” and assume you’re talking about an SSH keypair that’s got the private key protected with a passphrase. And while this is a reasonable approximation of a two factor system, it’s not actually two factor authentication because the server is not using two separate factors to authenticate the user. The only factor is the SSH keypair, and there’s no way for the server to know if that key was protected with a passphrase. However, OpenSSH has supported true two factor authentication for nearly 5 years now, so it’s quite possible to build even more robust security.

PSA: Typos in mkfs commands are painful

david@systemoverlord.com (David Tomaschik) — Mon, 20 Oct 2014 14:19:40 +0000

TL;DR: I apparently typed mkfs.vfat /dev/sda1 at some point. Oops.

So I rarely reboot my machines, and last night, when I rebooted my laptop (for graphics card weirdness) Grub just came up with:

Error: unknown filesystem.
grub rescue>

WTF, I wonder how I borked my grub config? Let’s see what happens when we ls my /boot partition.

grub rescue>ls (hd0,msdos1)
unknown filesystem

Hrrm, that’s no good. An ls on my other partition isn’t going to be very useful, it’s a LUKS-encrypted LVM PV. Alright, time for a live system. I grab a Kali live USB (not because Kali is necessarily the best option here, it’s just what I happen to have handy) and put it in the system and boot from that. file tells me its an x86 boot sector, which is not at all what I’m expecting from an ext4 boot partition. It slowly dawns on me that at some point, intending to format a flash drive or SD card, I must’ve run mkfs.vfat /dev/sda1 instead of mkfs.vfat /dev/sdb1. That one letter makes all the difference. Of course, it turns out it’s not even a valid FAT filesystem… since the device was mounted, the OS had kept writing to it like an ext4 filesystem, so it was basically a mangled mess. fsck wasn’t able to restore it, even pointing to backup superblocks: it seems as though, among other things, the root inode was destroyed.

The Machine Inside the Machine

david@systemoverlord.com (David Tomaschik) — Tue, 13 May 2014 04:24:00 +0000

Imagine this scenario:

One of your employees visits a site offering a program to download videos from a popular video site. Because they’d like to throw some videos on their phone, they download and install it, but it comes with a hitchhiker: a RAT, or remote access trojan. So Trudy, an attacker, has a foothold, but the user isn’t an administrator, so she starts looking at the network for a place to pivot. Scanning a private subnet, she finds a number of consecutive IP addresses all offering webservers, FTP servers, and even telnet! Connecting to one, the attacker suddenly realizes she has just found her golden ticket…