Post-mortem and remediations for the Matrix.org security breach

https://matrix.org/blog/2019/05/08/post-mortem-and-remediations-for-apr-11-security-incident/

by Arathorn • 7 years ago

183 64 7 years ago

Arathorn17• 7 years ago

Author here - hopefully the level of detail here will let others learn from our mistakes (and hopefully benefit from how we've chosen to fix them going forwards). Happy to answer any/all questions or comments.

TL;DR: keep your services patched; lock down SSH; partition your network; and there's almost never a good reason to use SSH agent forwarding.

Arathorn17• 7 years ago

I should probably also link to the HN thread from when this happened, which has a lot of interesting discussion about the security issues in question: https://news.ycombinator.com/item?id=19642554

Also, https://news.ycombinator.com/item?id=19643227 is an excellent tl;dr.

vector_spaces• 7 years ago

Thanks so much for this write-up, really appreciate the candidness and detail.

I've learned a lot from it and will be adding some of these practices to the infrastructure that I manage.

voltagex_3• 7 years ago

The attacker put an SSH key on the box, which was unfortunately exposed to the internet via a high-numbered SSH port for ease of admin by remote users, and placed a trap which waited for any user to SSH into the jenkins user, which would then hijack any available forwarded SSH keys to try to add the attacker’s SSH key

You could also fund/donate to/advocate for a better SSH agent.

I use both Pageant and ssh-agent in my home network for ease of ssh'ing into boxes, especially Unifi gear and some dev VMs. I don't think I will stop using agents, but I probably wouldn't use them at work.

Why couldn't there be an agent that required you to touch a Yubikey before it'd allow keys to be forwarded? Why couldn't you add prompting and timeouts to an agent?

JoachimSchipper• 7 years ago

ssh-agent has prompting and you can set up a Yubikey with ssh.

The problem here was agent forwarding, which you should almost always replace with opening a new connection via ssh -J (or equivalent.)

32032141• 7 years ago

Just use ProxyJump. You basically should never be using agent forwarding.

nealmcb2• 7 years ago

That is an excellent and very helpful writeup!

I'm particularly disappointed to hear that Google doesn't provide any way to rotate the signing key for an app. Is there an issue for that file with them anywhere, or more discussion?

Some day, I hope reputable services have migrated to The Update Framework, which has been pointing out and solving these and other problems related to software updates for several years now.

https://theupdateframework.github.io/

Actually, a quick search leads to this - is it indeed possible to rotate your key, at least for Android's Pie version?

  https://www.androidpolice.com/2018/08/13/android-pie-includes-key-rotation-way-developers-change-app-signatures/

Arathorn17• 7 years ago

So yes, Google Play has let you rotate your key for a few years now, but a) Riot/Android was set up before that was a thing, b) It gives Google the ability to push their own updates to your app, which some of the more paranoid users might object to. So we set it back up with our own key again this time, but this time will protect it with our lives...

Edit: https://developer.android.com/studio/publish/app-signing#app... is the type of key rotation i was talking about here.

Arathorn17• 7 years ago

actually, the mechanism described in https://www.androidpolice.com/2018/08/13/android-pie-include... sounds different to this, but given it mandates Android 9.0, we can't use that either yet. (Our minimum Android is still 4.1...)

trishankdatadog• 7 years ago

Hi, former lead researcher at NYU for The Update Framework (TUF) here, now security engineer at Datadog taking TUF further. Planning to help PyPA apply TUF to PyPI. Happy to help answer questions, just reach out to me here, thanks.

altmind• 7 years ago

[The attacker] placed a trap which waited for any user to SSH into the jenkins user, which would then hijack any available forwarded SSH keys to try to add the attacker’s SSH key to root@ on as many other hosts as possible.

Can the system you log into via ssh just dump your forwarded PRIVATE key? That easily?

Or this was about ssh client being patched on jenkins box to add malicious keys wherever the devops ssh'd from jenkins box?

bpye• 7 years ago

You cannot dump it but you can sign data with it.

Arathorn17• 7 years ago

sorry, I think I could have been clearer here.

When you log into a host with SSH agent forwarding turned on, the private key data itself isn't available to the host you're logging into. However, when you try to SSH onwards from that host, agent forwarding means that the authentication handshake is forwarded through to the agent running on your client, which of course has access to your private keys.

So, even though the private key data itself isn't directly available to the host, any code running which can inspect the SSH_AUTH_SOCK environment variable of the session that just logged in can use that var to silently authenticate with other remote systems on your behalf.

If you've already found a list of candidate hosts (e.g. by inspecting ~/.ssh/known_hosts) then your malware can simply loop over the list, trying to log in as root@ (or user@) and compromising them however you like. Which is what happened here, by copying a malicious authorized_keys2 file with a malicious key onto the target hosts. You don't need to patch the ssh client; it's just ssh agent forwarding doing its thing. :|

mikekchar• 7 years ago

There's another scary implication. If the user has set up SSH on their machines and added their own key as a valid key for logging in (for instance if you have 2 machines and you use the same SSH key for logging into everything), the attacker can log into the users own machine and compromise it. This makes remediation even more difficult because even after you have secured all the servers, the attacker can get back in through an affected user's machine.

voltagex_3• 7 years ago

A simple yet clever attack. I wonder how you'd protect against it without banning SSH forwarding, which has almost certainly saved me from (some) RSI.

zajd• 7 years ago

This might be the best post-mortem I've seen. Bravo Matrix.org team, informative and inspires confidence in your process.

dreamcompiler2• 7 years ago

SSH agent forwarding should be disabled.
SSH should not be exposed to the general internet.
If you need to copy files between machines, use rsync rather than scp.

Great. Just great. I still remember when SSH was described as the solution to fix telnet and rcp. And now we can't use it any more. Fan-freaking tastic.

theamk3• 7 years ago

Eh, I think your misunderstood this. There are still no alternatives to SSH. And if you want to expose something to an open internet, SSH is way better than telnet and rcp.

In particular, rsync command that they are talking about is still using SSH as an underlying transport.

acct17714• 7 years ago

mosh?

Arathorn17• 7 years ago

SSH is fine :) But agent forwarding is the biggest footgun imaginable, and scp sadly has design flaws some of which it literally inherited verbatim from rcp.

But using SSH as a shell is fine. And rewiring your fingers to type rsync rather than scp isn't too bad either - plus you get resumption etc for free :) (And yes, I appreciate the parent is being slightly tongue in cheek).

Edit: of course, if we'd been using xrsh and xrcp from XNS rather than this newfangled TCP/IP stuff none of this would probably ever have happened...

sciurus2• 7 years ago

How does using rsync instead of scp help? Isn't the default behavior of rsync to use SSH for transport, just like ssh does? Thus you'd still rely on forwarding keys or another ssh authentication method.

dreamcompiler2• 7 years ago

Sorry for my snarky tone. I'm dealing with an intrusion of my own right now and your writeup was actually quite helpful, so thanks for doing it.

acct17714• 7 years ago

Is SSH fine..?

mosh dev and users think no.

theamk3• 7 years ago

I applaud the decision to get rid of Jenkins.

The way Jenkins works, with each plugin being able to implement arbitrary endpoint, it is almost inevitable that it would have many security vulnerabilities.

No Jenkins masters should be exposed to the internet, ever -- and if there is really no other way, then set up a proxy in front of it with strict whitelist of allowed URLs.

ummonk• 7 years ago

Great post-mortem, with a candid examination of the decisions that contributed to lax security on the homeserver. While a security breach is never great, this kind of honest post-mortem improves my estimation of the chances that the matrix.org team is likely to get things right in the future.

gruez• 7 years ago

In terms of remediation, designing a secure build process is surprisingly hard, particularly for a geo-distributed team. What we have landed on is as follows:
We then perform all releases from a dedicated isolated release terminal.
We physically store the device securely.

Why didn't they go with a HSM?

Arathorn17• 7 years ago

This approach isn't incompatible with an HSM, as per:

The signing keys (hardware or software) are kept exclusively on this device.

We still want to make very sure that the build environment itself hasn't been tampered with, hence keeping the build machine itself isolated too.

A much better approach would be to use reproducible builds and sign the hash of a build with a hardware key, but we didn't want to block an improved build setup on reproducibilizing everything.

Edit: we may be missing an HSM trick, though, in which case please elaborate :)

cyphar3• 7 years ago

I'm not sure I understand what you mean by it being incompatible -- a HSM is a hardware device which generates and stores its keys separately from your computer's main memory such that getting the keys (even if the machine is compromised) should be impossible. In fact, it would eliminate the issues with

Since Android signing keys are just PKCS #8, and GPG keys are supported by most HSMs, a HSM would definitely be usable (even if you just used an addon HSM card that you added to your "release terminal"). Unfortunately in order to safely use the HSM you'd need to re-generate your keys again from within the HSM -- which obviously is a problem on Android. In addition, HSMs are quite expensive and might be prohibitively so in your case. But I would definitely recommend looking into it if you're really stuck on doing distribution yourselves.

Reproducible builds are a useful thing separately, but using a HSM doesn't require reproducible builds -- after all signing a hash of a binary is the same as just signing the binary. The main benefit of reproducible builds is that people can independently verify that the published source code is actually what was used to build the binary (which means it's an additional layer of verification over signatures).

One question I have is how are going to handle the case where the release terminal fails? Will you have to (painfully) rotate the keys again?

EGreg• 7 years ago

Package management and microservices is the reason for this.

wockaflocka2• 7 years ago

I have never used matrix.org service but I had heard of them previously from their website I could see there was the word ‘security’ or ‘secure’ used a lot.

Reading the blog post I wonder how many security specialists this organisation really has as they would never allow these fundamental errors to be made even with the explanation that they setup their infra in a rush. Dedicated security teams would have surely fixed these basic errors.

I would advise anybody looking for ‘secure’ applications to stay away from these organisations who knows how many possible flaws are deeply embedded in their systems like zero days, memory leaks and more they did not even have a basic security policy system in place... please don’t use the word secure

fartcannon• 7 years ago

Isn't a lot of it/all of it reviewable on their github? Does that not help you make a decision on their quality?

wockaflocka2• 7 years ago

Dosent really help if I can just go into their system and introduce my own code into their SDKs or just sign my own release of a build. It just makes me question how secure their build process is? Without security people you cannot claim to be secure?

alanfranz• 7 years ago

The writeup is interesting. As a security conscious developer (and with quite a lot of experience with deployments of multi-server systems) I wonder if there's a comprehensive, coherent guide in order to do The Right Thing security-wise in such scenarios. Multiple interacting servers, multiple developers, continuous delivery... I think that Google's BeyondCorp approach is rather different than this (and SSH would be publicly exposed) but has an inherent level of complexity which would be hard to cope with in a small org.

cyberpip• 7 years ago

Check out https://infosec.mozilla.org/guidelines/openssh for a nice overview of best-practices on SSH.

rlpb2• 7 years ago

One of the failures here is that they weren't able to keep deployed software up to date for security fixes, even when those security fixes were publicly known.

They have acknowledged this in their section "Keeping patched".

However, there is one thing I think they have omitted to consider. The more reliance on third party software not from the server distribution they are using, the more disparate and unreliable sources for security fixes become.

Careful choice of production software dependencies is therefore a factor. Usually it is unavoidable for some small number of dependencies that are central to the mission. But in general, I wonder if they have any kind of policy to favour distribution-supplied dependencies over any other type.

Another way of looking at this: we already have a community that comes together to provide integrated security updates that can be automatically installed, and you already have access to it. Not using this source compromises that ability. If some software isn't available through Debian, it is usually because there is some practical difficulty in packaging it, and I argue that security maintenance difficulty arises from the same root cause.

On a similar note, I'm curious about their choice to switch from cgit to GitLab. Both are packaged in Debian, but I believe that even Debian doesn't use the packaged GitLab for Debian's own GitLab instance. Assuming that Debian GitLab package's version is therefore not practical, wouldn't cgit be better from a "receives timely security updates through the distribution" perspective?

Arathorn17• 7 years ago

This is an excellent point.

In the (distant) past, we tended to prefer to wrap our own stuff for critical services (e.g. apache, linux kernel) rather than use distribution-maintained packages. The reason was pretty much one of being control freaks: wanting to be able to patch and tweak the config precisely as it came from the developers rather than having to work out how to coerce Debian's apache package to increase the hardcoded accept backlog limit or whatever today's drama might happen to be.

However, this clearly comes at the expense of ease of keeping things patched and up-to-date, and one of the things we got right (albeit probably not for the right reasons at the time) when we did the initial rushed built-out of the legacy infrastructure in 2017 was to switch to using Debian packages for the majority of things.

Interestingly, cgit was not handled by Debian (because we customised it a bunch), and so definitely was a security liability.

Gitlab is a different beast altogether, given it's effectively a distro in its own right, so we treat it like an OS which needs to be kept patched just like we do Debian.

For what it's worth, I think by far the hardest thing to do here is to maintain the discipline to go around keeping everything patched on a regular basis - especially for small teams who lack dedicated ops people. I don't know of a good solution here other than trying to instil the fear of God into everyone when it comes to keeping patched, and throwing more $ and people at it.

Or I guess you can do https://wiki.debian.org/UnattendedUpgrades and pray nothing breaks.

rlpb2• 7 years ago

Or I guess you can do https://wiki.debian.org/UnattendedUpgrades and pray nothing breaks.

Better than getting compromised! I used to have a very conservative approach to changing anything, including great caution with security updates and the desire to avoid automatic security updates with a plan to carefully gate everything.

In practice though, security fixes are cherry-picked and therefore limited in scope, and outages caused by other factors are orders of magnitude more common than outages caused by security updates. Better to remain patched, in my opinion, and risk a non-security outage, than to get compromised by not applying them immediately.

A better way to mitigate the risk is to apply the CI philosophy to deployments. Every deployment component should come with a test to make sure it works in production. Add CI for that. Then automate security updates in production gated on CI having passed. If your security update fails, then it's your test that needs fixing.

nh2• 7 years ago

The Nix package manager can help keeping packages that are not available for your distribution updated and customised (https://nixos.org/nix/).

In the past I used to install newer, or customised, versions of e.g. `git` than were available on my Ubuntu into my home directory using e.g. `./configure --prefix=$HOME/opt`. That got me the features I wanted, but of course made me miss out on security updates, and I would have to remember each software I installed this way.

With nix, I can update them all in one go with `nix-env --upgrade`.

Nix also allows to declaratively apply custom patches to "whatever the latest version is".

That way I can have things like you mentioned (e.g. hardcoded accept backlock for Apache, hardening compile flags) without the mentioned "expense of ease of keeping things patched and up-to-date". I found that very hard to do with .deb packages.

It's not as good as just using unattended-upgrades from your main distro, because you still have to run the one `nix-env --ugprade` command every now and then, but that can be easily automated.

sytse2• 7 years ago

BTW have you considered using GitLab CI instead of buildkite?

deadbunny• 7 years ago

TL;DR Don't let devs manage your systems.

squid000• 7 years ago

I wonder why no one uses SSH certificate based authentication. Yes SSH support certificate signed keys to allow login. No keys need to be at server at all.

Post-mortem and remediations for the Matrix.org security breach

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