HackedThat: Breaking in to a hardened server via the back door

Tue, 16 Aug 2016 22:39:10 +0000

Earlier this summer, the team at Inversoft published a comprehensive and sophisticated guide to user data security. The guide spans from hardening servers from provisioning, up through the IP and SSH layers, and all the way to application-level techniques for password hashing, SQL injection protection, and intrusion detection. As proof that they stood behind their advice, the Inversoft team provisioned a pair of Linode hosts, a web server and database server, and gave them the hardening treatment. Inversoft offered up a fully-loaded MacBook to anyone who could break in, taunting all comers by naming the hardened web server hackthis.inversoft.com.

Game on.

False Attempts

So one morning I started poking around. It only took a few minutes to verify that the target servers were hardened just as described in the whitepaper; SSH access via public keys only, no additional ports open other than HTTP/HTTPS. Fingerprinting the web-facing host with nmap showed that it was running the latest Ubuntu version, and revealed nothing about the HTTP server running (though I knew from the whitepaper that it was a recent version of Express).

Darkness washed over the Dude, as I realized that there would be no easy way in. I didn’t have high hopes for a SQL injection attack, so instead I spent some time trying various XSS payloads on their basic web app, all to no avail. I didn’t spend too much time on this vector; even if I were able to get an XSS working, it would at best allow me to read data from other users; a real security issue to be sure, but insufficient to win the coveted MacBook.

The user database under protection was Inversoft’s Passport product, so as my next approach I decided to read a little bit more about it, particularly its API docs. What’s that, hiding behind that last link? The API docs (as well as the documentation for Inversoft’s other products) live in a Confluence wiki. If Inversoft uses Confluence so heavily for customer-facing docs, I wondered, maybe they keep important internal information there as well?

New Avenues

I spent a few minutes gathering usernames from the public-facing Confluence pages and guessing at passwords to no avail. Knowing that Confluence is complicated self-hosted software that often goes un-updated, I looked at the version in a CVE database. There’s something! CVE-2015-8399 allows unauthenticated users to browse and read files from disk that are accessible to the Confluence user, and docs.inversoft.com was vulnerable to it. I spent quite some time looking at basically every file accessible to Confluence, but the team had managed to keep any secrets out of the various Confluence configuration files. I was hoping to snag database credentials or an administrator password, but neither were to be found.

Speaking of database credentials, where was Confluence storing its wiki data? I went back to nmap and took a look at docs.inversoft.com. This was much more interesting; several HTTP servers (mostly Java / Tomcat), Postgres and MySQL databases, Elasticsearch, and some unknown services, as well as SSH, were listening for connections on this host. I determined with a little more digging that this machine was also the host for www.inversoft.com, and a number of internal and external services. I had a lot more to explore now.

I started trying to fingerprint and otherwise gather version information for the services on this host. The server was running an old version of Ubuntu (12.04), but it seemed to be fully-patched (I verified later that it was). The Elasticsearch version running on the server, however, was old enough to be vulnerable to CVE-2015-1427. Go take a look at that link, as it contains any security researcher’s three favorite letters:

R. C. E.

Elasticsearch, it seems, allows API users to specify custom scoring functions that can be used to rank results. Those scoring functions can be written as Groovy code. Elasticsearch implements a sandbox that attempts to prevent any malicious code, but the sandboxing has multiple flaws, and it’s relatively trivial to send a “scoring function” that in fact calls Runtime.getRuntime().exec() with arbitrary shell commands. Given that the Elasticsearch port was open to the world, and no authentication was required to run one of these custom-scored searches, I had all the ingredients I needed to run shell commands. Actually putting together a working PoC and a working reverse shell (connecting out to a new EC2 box I provisioned) took some grunt work, but I ultimately succeeded and found myself staring at a command prompt.

whoami

What could I do with my new shell? First, I determined that I was not running as root, but instead as an application user, conveniently named inversoft. This user could of course read anything that Elasticsearch could, but it turned out there was no useful information in the Elasticsearch cache. I turned my attention back to Confluence; conveniently, it was also running under the inversoft account. With this information, it didn’t take long to find the database credentials used by Confluence to talk to Postgres, and start pulling a full database dump of the Confluence DB for offline perusal. There was a massive amount of data, and I called in my colleagues to help me sift through and look for anything useful.

Of course, I also had to think about detection here. I wasn’t yet to my goal, nor did I have a clear path to it, but I was disrupting two services (Elasticsearch and Postgres) with my activity, and probably starting to leave fingerprints. I determined that, as far as I could tell, I wasn’t harming any production resources or over-taxing their servers with my exploration, and I continued to proceed with caution.

The Confluence database seemed, for a minute, to be the promised land. It was used for internal documentation; not just docs, but also various shared secrets, passwords, and other keys! Best of all, I found a username and password for a Linode account! I knew from the original whitepaper that the “HackThis” machines had been provisioned with Linode, so I signed in and prepared to claim my reward.

It was the wrong Linode account. A real one, with real servers, but not the servers that would win me my prize.

A Plan Emerges

Could there be multiple linked Linode accounts? Might the other account use the same password, or a variant? No, and no. I was able to recover the username of the Linode account I was targeting from the original whitepaper’s screen shots, so I tried several other passwords found in the wiki, and none worked.

I hadn’t yet succeeded, but I could see the way forward. Gain access to the Linode account, and use the web-based console to get root on the target servers. I just needed a password.

How would I get it? I spent some time looking at the postfix server also running on the machine I had reached; could I intercept a password reset email from Linode? Nope, the postfix server wasn’t in use; the team used Google Apps for email. Could I fashion a convincing phishing attack using my privileged position? I couldn’t think of a clever way to do it, and I knew the Inversoft team would be on high alert given the challenge they had issued. I spent some time trying to elevate my privileges to root at the shell (for no good reason) but found that the team had religiously applied Ubuntu’s LTS patches and none of the Linux elevation tricks I could find would work on their kernel.

I continued to browse around the filesystem, searching for any lead. I finally found the way forward in the first place I should have looked; the inversoft user’s home directory. It turns out that not only was this account used to run several services on the box, but it was also used by humans as a shared account for various projects and one-offs. And one of those projects was provisioning the HackThis machines. There, in ~inversoft/.linodecli/config? A Linode API key.

A call to the “list hosts” API in Linode revealed the exact two hosts I had in my sights, and confirmed that now I had the correct key. Time to get a root console, right? Nope. You can do all sorts of things with the Linode API, but getting a console is not one of them; you can only do that on the web with a regular username and password, and I still didn’t have one of those. The next thought was to try and export the disk image from one of the Linode machines, but the API does not provide an “export” function.

Smash and Grab

After lots of messing around with APIs, my colleague Anton had an idea; what if we spin up a new, “intruder” machine in the Linode account with a root password that we know and then connect the target application server’s disk to our intruder server instead? Looking through the API docs, this seemed like it would be a working plan, but it would also be obvious and destructive; our intruder machine would appear on the Linode dashboard, and as soon as we unmounted the volume, the machines we were targeting would start failing.

To pull this off, we’d have to grab the MySQL credentials and exfiltrate the DB as quickly as possible, before the Inversoft operators could detect us and shut us down. Since we were so close to the prize, and since the machines weren’t “real” production machines, rather honeypots designed to be hacked, we decided this was a reasonable and ethical course of action. At this point it was about 8pm local time at the Inversoft offices in Denver, so we hoped nobody would be at their desks, potentially buying us a few extra minutes.

With me at the shell of the intruder host, Anton used the API to attach the application server disk image to my machine, from which I quickly retrieved the Passport API keys and the MySQL credentials to the other host. Anton started enumerating all the data he could from Passport using our API key while I triggered a mysqldump to geth the DB - but I couldn’t connect! We should have seen this coming; the MySQL server had a firewall rule that permitted access only from the application server; this was one of the hardening measures from the whitepaper.

For most attackers, this bit of defense-in-depth would have been a dead-end, but thinking quickly and using our superpowers (Linode API keys), we performed a private IP swap between the application server and our intruder server. Rebooted the intruder server to get the new IP and it was all over: mysqldump connected, the data was SCPed off to my machine, and we had beaten the challenge. Our haste was warranted; once we reported the attack to Inversoft, they let us know that they had received notification emails for every Linode action we had taken (create server, connect disk, swap IP, etc), and had already started investigating just as we had finished downloading the database dump.

The Recap

After discovering an unpatched, unfirewalled Elasticsearch instance using nmap, we gained shell access on a utility server used for various functions at Inversoft. On there, we found API keys for Linode left behind by a human operator. Those keys allowed us to detach disks from running servers and attach them to servers we controlled, stealing sensitive user data (all to win a prize).

What could Inversoft have done differently to prevent this? Their hardening guide was and remains correct; there was no way we were getting through the front door of their servers (SSH or HTTPS). The course we took was a common one in targeted attacks; gain access to secrets used by humans, sometimes in ancillary systems, and use that access to bypass security via operator consoles or other magic. The most frequently seen version of this in the wild is to steal access to an email inbox that can be used to reset a password, and although this attack was slightly different, it’s a great reminder that attackers are far more likely to go around your defenses than through them.

The other weakness was the “jack-of-all-trades” Elasticsearch server that we discovered and exploited. It’s an example of a utility box that runs various random services - maybe acts as a bastion host or testing ground - and nobody quite manages it or knows what it is used for. This server is as weak as its weakest service; and because it is not purpose-managed, it can be difficult to keep track of what is running on it and ensure all services are patched and secured. If you have one of these servers floating around somewhere, you might want to think twice about keeping it - it may very well be the chink in your armor.

Thanks to Inversoft for the effort they put in to writing their security guide and sponsoring the “HackThis” challenge, and of course the prize of a MacBook (which they quickly delivered). Their security guide remains an excellent resource, and we hope the practical lessons learned from this post will help your organization identify less obvious risks risks and secure your infrastructure.

Bradley Buda is a Managing Partner at Polynome and previously the co-founder and CTO at Meldium.