Louwrentius

Systemd is a more modern replacement of sysvinit and its in the process of being integrated into most mainstream Linux distributions. I'm a bit troubled by one of it's features.

I'd like to discuss the Forward Secure Sealing (FSS) feature for log files that is part of systemd. FSS cryptographically signs the local system logs, so you can check if log files have been altered. This should make it more difficult for an attacker to hide his or her tracks.

Regarding log files, an attacker can do two things:

1. delete them
2. alter them (remove / change incriminating lines)

The FSS feature does not prevent any of these risks. But it does help you detect that there is something fishy going on if you would verify the signatures regularly. So basically FSS acts a bit like Tripwire.

FSS can only tell you wether or not a log file has been changed. It cannot tell you anything else. More specifically, it cannot tell you the reason why. So I wonder how valuable this feature is.

There is also something else. Signing (sealing) a log file is done every 15 minutes by default. This gives an attacker ample time to alter or delete the most recent log events, often exactly those events that need to be altered/deleted. Even lowering this number to 10 seconds would allow an attacker to delete (some) initial activities using automation. So how useful is this?

What may help in determining what happened to a system is the unaltered log contents themselves. What FSS cannot do by principle is protect the actual contents of the log file. If you want to preserve log events the only secure option is to send them to an external log host (assumed not accessible by an attacker).

However, to my surprise, FSS is presented as an alternative to external logging. Quote from Lennart Poettering:

Traditionally this problem has been dealt with by having an external secured log server 
to instantly log to, or even a local line printer directly connected to the log system. 
But these solutions are more complex to set up, require external infrastructure and have 
certain scalability problems. With FSS we now have a simple alternative that works without 
any external infrastructure.

This quote is quite troubling because it fails to acknowledge one of the raison d'être of external log hosts. It seems to suggest that FSS provides an alternative for external logging, where in fact it does not and cannot do so on principle. It can never address the fact that an attacker can alter or delete logs, whereas external logging can mitigate this risk.

It seems to me that systemd now also wants to play the role as some crude intrusion detection system. It feels a bit like scope creep to me.

Personally I just wonder what more useful features could have been implemented instead of allowing you to transfer a log file verification key using a QR code to your smartphone (What the hell?).

This whole observation is not original, in the comments of the systemd author's blogpost, the same argument is made by Andrew Wyatt (two years earlier). The response from the systemd author was to block him. (see the comments of Lennart Poettering's blogpost I linked to earlier).

Update: Andrew Wyatt behaved a bit immature towards Lennart Poettering at first so I understand some resentment from his side, but Andrews criticism was valid and never addressed by him.

If the systemd author would just have implemented sending log events to an external log server, that would have been way more useful security-wise, I think. Until then, this may do...

IPsec is a well-known and widely-used VPN solution. It seems that it's not widely known that Niels Ferguson and Bruce Schneier performed a detailed security analysis of IPsec and that the results were not very positive.

We strongly discourage the use of IPsec in its current form for protection of any kind of valuable information, and hope that future iterations of the design will be improved.

I conveniently left out the second part:

However, we even more strongly discourage any current alterantives, and recommend IPsec when the alternative is an insecure network. Such are the realities of the world.

To put this in context: keep in mind that this paper was released in 2003 and the actual research may even be older (1999!). OpenVPN, an open-source SSL-based VPN solution was born in 2001 and was still maturing in 2003. So there actually was no real alternative back then.

It worries me that this research done by Ferguson and Schneier is more than a decade old. I've been looking for more recent articles on the current security status of IPsec, but I couldn't find much. There have been some new RFCs been published about IPsec but I'm not familiar enough with the material to understand the implications. They make a lot of recommendations in the paper to improve IPsec security, but are they actually implemented?

I did find a presentation from 2013 by Peter Gutmann (University of Auckland). Based on his Wikipedia page, he seems to 'have some knowledge' about cryptography. The paper adresses the Snowden leaks about the NSA and also touches on IPsec. He basically relies on the paper written by Ferguson and Schneier.

But let's think about this: Ferguson and Schneier criticises the design of IPsec. It is flawed by design. That's one of the worst criticisms any thing related to cryptography can get. That design has probably not changed much, from what I understand. So if their critique on IPsec is still mostly valid, all the more reason not to use IPsec.

So this is part of the conclusion and it doesn't beat around the bush:

We have found serious security weaknesses in all major components of IPsec.
As always in security, there is no prize for getting 90% right; you have to get
everything right. IPsec falls well short of that target, and will require some major
changes before it can possibly provide a good level of security.
What worries us more than the weaknesses we have identified is the complexity
of the system. In our opinion, current evaluation methods cannot handle
systems of such a high complexity, and current implementation methods are not
capable of creating a secure implementation of a system as complex as this.

So if not IPsec, what should you use? I would opt to use an SSL/TLS-based VPN solution like OpenVPN.

I can't vouch for the security for OpenVPN, but a well-known Dutch security firm Fox-IT has released a stripped-down version of the OpenVPN software (removed features) that they consider fit for (Dutch) governmental use. Not to say that you should use that particular OpenVPN version: the point is that OpenVPN is deemed secure enough to be used for governmental usage. For whatever that's worth.

At least, SSL-based VPN solutions have the benefit that they use SSL/TLS, which may have it's own problems, but is at least not as complex as IPsec.