Hanno's blog

In discussions around the PGP ecosystem one thing I often hear is that while PGP has its problems, it's an important tool for package signatures in Linux distributions. I therefore want to highlight a few issues I came across in this context that are rooted in problems in the larger PGP ecosystem.

Let's look at an example of the use of PGP signatures for deb packages, the Ubuntu Linux installation instructions for HHVM. HHVM is an implementation of the HACK programming language and developed by Facebook. I'm just using HHVM as an example here, as it nicely illustrates two attacks I want to talk about, but you'll find plenty of similar installation instructions for other software packages. I have reported these issues to Facebook, but they decided not to change anything.

The instructions for Ubuntu (and very similarly for Debian) recommend that users execute these commands in order to install HHVM from the repository of its developers:



The crucial part here is the line starting with apt-key. It fetches the key that is used to sign the repository from the Ubuntu key server, which itself is part of the PGP keyserver network.

Attack 1: Flooding Key with Signatures

The first possible attack is actually quite simple: One can make the signature key offered here unusable by appending many signatures.

A key concept of the PGP keyservers is that they operate append-only. New data gets added, but never removed. PGP keys can sign other keys and these signatures can also be uploaded to the keyservers and get added to a key. Crucially the owner of a key has no influence on this.

This means everyone can grow the size of a key by simply adding many signatures to it. Lately this has happened to a number of keys, see the blog posts by Daniel Kahn Gillmor and Robert Hansen, two members of the PGP community who have personally been affected by this. The effect of this is that when GnuPG tries to import such a key it becomes excessively slow and at some point will simply not work any more.

For the above installation instructions this means anyone can make them unusable by attacking the referenced release key. In my tests I was still able to import one of the attacked keys with apt-key after several minutes, but these keys "only" have a few ten thousand signatures, growing them to a few megabytes size. There's no reason an attacker couldn't use millions of signatures and grow single keys to gigabytes.

Attack 2: Rogue packages with a colliding Key Id

The installation instructions reference the key as 0xB4112585D386EB94, which is a 64 bit hexadecimal key id.

Key ids are a central concept in the PGP ecosystem. The key id is a truncated SHA1 hash of the public key. It's possible to either use the last 32 bit, 64 bit or the full 160 bit of the hash.

It's been pointed out in the past that short key ids allow colliding key ids. This means an attacker can generate a different key with the same key id where he owns the private key simply by bruteforcing the id. In 2014 Richard Klafter and Eric Swanson showed with the Evil32 attack how to create colliding key ids for all keys in the so-called strong set (meaning all keys that are connected with most other keys in the web of trust). Later someone unknown uploaded these keys to the key servers causing quite some confusion.

It should be noted that the issue of colliding key ids was known and discussed in the community way earlier, see for example this discussion from 2002.

The practical attacks targeted 32 bit key ids, but the same attack works against 64 bit key ids, too, it just costs more. I contacted the authors of the Evil32 attack and Eric Swanson estimated in a back of the envelope calculation that it would cost roughly $ 120.000 to perform such an attack with GPUs on cloud providers. This is expensive, but within the possibilities of powerful attackers. Though one can also find similar installation instructions using a 32 bit key id, where the attack is really cheap.

Going back to the installation instructions from above we can imagine the following attack: A man in the middle network attacker can intercept the connection to the keyserver - it's not encrypted or authenticated - and provide the victim a colliding key. Afterwards the key is imported by the victim, so the attacker can provide repositories with packages signed by his key, ultimately leading to code execution.

You may notice that there's a problem with this attack: The repository provided by HHVM is using HTTPS. Thus the attacker can not simply provide a rogue HHVM repository. However the attack still works.

The imported PGP key is not bound to any specific repository. Thus if the victim has any non-HTTPS repository configured in his system the attacker can provide a rogue repository on the next call of "apt update". Notably by default both Debian and Ubuntu use HTTP for their repositories (a Debian developer even runs a dedicated web page explaining why this is no big deal).

Attack 3: Key over HTTP

Issues with package keys aren't confined to Debian/APT-based distributions. I found these installation instructions at Dropbox (Link to Wayback Machine, as Dropbox has changed them after I reported this):



It should be obvious what the issue here is: Both the key and the repository are fetched over HTTP, a network attacker can simply provide his own key and repository.

Discussion

The standard answer you often get when you point out security problems with PGP-based systems is: "It's not PGP/GnuPG, people are just using it wrong". But I believe these issues show some deeper problems with the PGP ecosystem. The key flooding issue is inherited from the systemically flawed concept of the append only key servers.

The other issue here is lack of deprecation. Short key ids are problematic, it's been known for a long time and there have been plenty of calls to get rid of them. This begs the question why no effort has been made to deprecate support for them. One could have said at some point: Future versions of GnuPG will show a warning for short key ids and in three years we will stop supporting them.

This reminds of other issues like unauthenticated encryption, where people have been arguing that this was fixed back in 1999 by the introduction of the MDC. Yet in 2018 it was still exploitable, because the unauthenticated version was never properly deprecated.

Fix

For all people having installation instructions for external repositories my recommendation would be to avoid any use of public key servers. Host the keys on your own infrastructure and provide them via HTTPS. Furthermore any reference to 32 bit or 64 bit key ids should be avoided.

Update: Some people have pointed out to me that the Debian Wiki contains guidelines for third party repositories that avoid the issues mentioned here.

On Tuesday details about the security vulnerability GHOST in Glibc were published by the company Qualys. When severe security vulnerabilities hit the news I always like to take this as a chance to learn what can be improved and how to avoid similar incidents in the future (see e. g. my posts on Heartbleed/Shellshock, POODLE/BERserk and NTP lately).

GHOST itself is a Heap Overflow in the name resolution function of the Glibc. The Glibc is the standard C library on Linux systems, almost every software that runs on a Linux system uses it. It is somewhat unclear right now how serious GHOST really is. A lot of software uses the affected function gethostbyname(), but a lot of conditions have to be met to make this vulnerability exploitable. Right now the most relevant attack is against the mail server exim where Qualys has developed a working exploit which they plan to release soon. There have been speculations whether GHOST might be exploitable through Wordpress, which would make it much more serious.

Technically GHOST is a heap overflow, which is a very common bug in C programming. C is inherently prone to these kinds of memory corruption errors and there are essentially two things here to move forwards: Improve the use of exploit mitigation techniques like ASLR and create new ones (levee is an interesting project, watch this 31C3 talk). And if possible move away from C altogether and develop core components in memory safe languages (I have high hopes for the Mozilla Servo project, watch this linux.conf.au talk).

GHOST was discovered three times

But the thing I want to elaborate here is something different about GHOST: It turns out that it has been discovered independently three times. It was already fixed in 2013 in the Glibc Code itself. The commit message didn't indicate that it was a security vulnerability. Then in early 2014 developers at Google found it again using Address Sanitizer (which – by the way – tells you that all software developers should use Address Sanitizer more often to test their software). Google fixed it in Chrome OS and explicitly called it an overflow and a vulnerability. And then recently Qualys found it again and made it public.

Now you may wonder why a vulnerability fixed in 2013 made headlines in 2015. The reason is that it widely wasn't fixed because it wasn't publicly known that it was serious. I don't think there was any malicious intent. The original Glibc fix was probably done without anyone noticing that it is serious and the Google devs may have thought that the fix is already public, so they don't need to make any noise about it. But we can clearly see that something doesn't work here. Which brings us to a discussion how the Linux and free software world in general and vulnerability management in particular work.

The “Never touch a running system” principle

Quite early when I came in contact with computers I heard the phrase “Never touch a running system”. This may have been a reasonable approach to IT systems back then when computers usually weren't connected to any networks and when remote exploits weren't a thing, but it certainly isn't a good idea today in a world where almost every computer is part of the Internet. Because once new security vulnerabilities become public you should change your system and fix them. However that doesn't change the fact that many people still operate like that.

A number of Linux distributions provide “stable” or “Long Time Support” versions. Basically the idea is this: At some point they take the current state of their systems and further updates will only contain important fixes and security updates. They guarantee to fix security vulnerabilities for a certain time frame. This is kind of a compromise between the “Never touch a running system” approach and reasonable security. It tries to give you a system that will basically stay the same, but you get fixes for security issues. Popular examples for this approach are the stable branch of Debian, Ubuntu LTS versions and the Enterprise versions of Red Hat and SUSE.

To give you an idea about time frames, Debian currently supports the stable trees Squeeze (6.0) which was released 2011 and Wheezy (7.0) which was released 2013. Red Hat Enterprise Linux has currently 4 supported version (4, 5, 6, 7), the oldest one was originally released in 2005. So we're talking about pretty long time frames that these systems get supported. Ubuntu and Suse have similar long time supported Systems.

These systems are delivered with an implicit promise: We will take care of security and if you update regularly you'll have a system that doesn't change much, but that will be secure against know threats. Now the interesting question is: How well do these systems deliver on that promise and how hard is that?

Vulnerability management is chaotic and fragile

I'm not sure how many people are aware how vulnerability management works in the free software world. It is a pretty fragile and chaotic process. There is no standard way things work. The information is scattered around many different places. Different people look for vulnerabilities for different reasons. Some are developers of the respective projects themselves, some are companies like Google that make use of free software projects, some are just curious people interested in IT security or researchers. They report a bug through the channels of the respective project. That may be a mailing list, a bug tracker or just a direct mail to the developer. Hopefully the developers fix the issue. It does happen that the person finding the vulnerability first has to explain to the developer why it actually is a vulnerability. Sometimes the fix will happen in a public code repository, sometimes not. Sometimes the developer will mention that it is a vulnerability in the commit message or the release notes of the new version, sometimes not. There are notorious projects that refuse to handle security vulnerabilities in a transparent way. Sometimes whoever found the vulnerability will post more information on his/her blog or on a mailing list like full disclosure or oss-security. Sometimes not. Sometimes vulnerabilities get a CVE id assigned, sometimes not.

Add to that the fact that in many cases it's far from clear what is a security vulnerability. It is absolutely common that if you ask the people involved whether this is serious the best and most honest answer they can give is “we don't know”. And very often bugs get fixed without anyone noticing that it even could be a security vulnerability.

Then there are projects where the number of security vulnerabilities found and fixed is really huge. The latest Chrome 40 release had 62 security fixes, version 39 had 42. Chrome releases a new version every two months. Browser vulnerabilities are found and fixed on a daily basis. Not that extreme but still high is the vulnerability count in PHP, which is especially worrying if you know that many webhosting providers run PHP versions not supported any more.

So you probably see my point: There is a very chaotic stream of information in various different places about bugs and vulnerabilities in free software projects. The number of vulnerabilities is huge. Making a promise that you will scan all this information for security vulnerabilities and backport the patches to your operating system is a big promise. And I doubt anyone can fulfill that.

GHOST is a single example, so you might ask how often these things happen. At some point right after GHOST became public this excerpt from the Debian Glibc changelog caught my attention (excuse the bad quality, had to take the image from Twitter because I was unable to find that changelog on Debian's webpages):



What you can see here: While Debian fixed GHOST (which is CVE-2015-0235) they also fixed CVE-2012-6656 – a security issue from 2012. Admittedly this is a minor issue, but it's a vulnerability nevertheless. A quick look at the Debian changelog of Chromium both in squeeze and wheezy will tell you that they aren't fixing all the recent security issues in it. (Debian already had discussions about removing Chromium and in Wheezy they don't stick to a single version.)

It would be an interesting (and time consuming) project to take a package like PHP and check for all the security vulnerabilities whether they are fixed in the latest packages in Debian Squeeze/Wheezy, all Red Hat Enterprise versions and other long term support systems. PHP is probably more interesting than browsers, because the high profile targets for these vulnerabilities are servers. What worries me: I'm pretty sure some people already do that. They just won't tell you and me, instead they'll write their exploits and sell them to repressive governments or botnet operators.

Then there are also stories like this: Tavis Ormandy reported a security issue in Glibc in 2012 and the people from Google's Project Zero went to great lengths to show that it is actually exploitable. Reading the Glibc bug report you can learn that this was already reported in 2005(!), just nobody noticed back then that it was a security issue and it was minor enough that nobody cared to fix it.

There are also bugs that require changes so big that backporting them is essentially impossible. In the TLS world a lot of protocol bugs have been highlighted in recent years. Take Lucky Thirteen for example. It is a timing sidechannel in the way the TLS protocol combines the CBC encryption, padding and authentication. I like to mention this bug because I like to quote it as the TLS bug that was already mentioned in the specification (RFC 5246, page 23: "This leaves a small timing channel"). The real fix for Lucky Thirteen is not to use the erratic CBC mode any more and switch to authenticated encryption modes which are part of TLS 1.2. (There's another possible fix which is using Encrypt-then-MAC, but it is hardly deployed.) Up until recently most encryption libraries didn't support TLS 1.2. Debian Squeeze and Red Hat Enterprise 5 ship OpenSSL versions that only support TLS 1.0. There is no trivial patch that could be backported, because this is a huge change. What they likely backported are workarounds that avoid the timing channel. This will stop the attack, but it is not a very good fix, because it keeps the problematic old protocol and will force others to stay compatible with it.

LTS and stable distributions are there for a reason

The big question is of course what to do about it. OpenBSD developer Ted Unangst wrote a blog post yesterday titled Long term support considered harmful, I suggest you read it. He argues that we should get rid of long term support completely and urge users to upgrade more often. OpenBSD has a 6 month release cycle and supports two releases, so one version gets supported for one year.

Given what I wrote before you may think that I agree with him, but I don't. While I personally always avoided to use too old systems – I 'm usually using Gentoo which doesn't have any snapshot releases at all and does rolling releases – I can see the value in long term support releases. There are a lot of systems out there – connected to the Internet – that are never updated. Taking away the option to install systems and let them run with relatively little maintenance overhead over several years will probably result in more systems never receiving any security updates. With all its imperfectness running a Debian Squeeze with the latest updates is certainly better than running an operating system from 2011 that stopped getting security fixes in 2012.

Improving the information flow

I don't think there is a silver bullet solution, but I think there are things we can do to improve the situation. What could be done is to coordinate and share the work. Debian, Red Hat and other distributions with stable/LTS versions could agree that their next versions are based on a specific Glibc version and they collaboratively work on providing patch sets to fix all the vulnerabilities in it. This already somehow happens with upstream projects providing long term support versions, the Linux kernel does that for example. Doing that at scale would require vast organizational changes in the Linux distributions. They would have to agree on a roughly common timescale to start their stable versions.

What I'd consider the most crucial thing is to improve and streamline the information flow about vulnerabilities. When Google fixes a vulnerability in Chrome OS they should make sure this information is shared with other Linux distributions and the public. And they should know where and how they should share this information.

One mechanism that tries to organize the vulnerability process is the system of CVE ids. The idea is actually simple: Publicly known vulnerabilities get a fixed id and they are in a public database. GHOST is CVE-2015-0235 (the scheme will soon change because four digits aren't enough for all the vulnerabilities we find every year). I got my first CVEs assigned in 2007, so I have some experiences with the CVE system and they are rather mixed. Sometimes I briefly mention rather minor issues in a mailing list thread and a CVE gets assigned right away. Sometimes I explicitly ask for CVE assignments and never get an answer.

I would like to see that we just assign CVEs for everything that even remotely looks like a security vulnerability. However right now I think the process is to unreliable to deliver that. There are other public vulnerability databases like OSVDB, I have limited experience with them, so I can't judge if they'd be better suited. Unfortunately sometimes people hesitate to request CVE ids because others abuse the CVE system to count assigned CVEs and use this as a metric how secure a product is. Such bad statistics are outright dangerous, because it gives people an incentive to downplay vulnerabilities or withhold information about them.

This post was partly inspired by some discussions on oss-security

Recently I had a discussion with someone about the security of various linux distributions where he claimed Debian stable to be very secure and that they use old versions where they backport all occurances of security issues. This is a common assumption, too bad that it's wrong.
I want to document this to demonstrate the dangerousness of opinions like »stay with the old software«, »never touch a running system« and alike that are not limited but often found in the Debian community.

I had a look at the security policy on a package recently very popular for it's vast number of issues, namely php. Their last php-update on php5 was in july. They have a heavily patched version of php 5.2.0 and according to their changelog the last thing they patched was CVE-2007-1864.

Now that probably means that CVE-2007-3996, CVE-2007-3378, CVE-2007-3997, CVE-2007-4652, CVE-2007-4658, CVE-2007-4659, CVE-2007-4670, CVE-2007-4657, CVE-2007-4662, CVE-2007-3998, CVE-2007-5898, CVE-2007-5899, CVE-2007-5900 are unfixed. Oh, and before you ask: This list certainly is incomplete, it's just what I found without much hassle.

Not only looking at php5, php4 is still officially distributed by many vendors, some hosters still use it. You can't really blame them, as php.net announced it to be supported until 2007-12-31. But that's theory, in fact, nobody looks if the various recent issues also apply to php 4, most recent advisories don't mention it. Fact is, at least CVE-2007-3378, CVE-2007-3997, CVE-2007-4657, CVE-2007-3998 also apply to php 4 and are unfixed in their latest release 4.4.7 (not neccessary to mention that they're not fixed in the debian stable php4 package).

If someone prefers to stay with old software, it's up to them. But be serious. It's probably a hell-job to get all the recent security issues in an app like php backported to some ancient version. A quick check showed me that debian isn't able to do that. If you can't do that, don't claim it's in any way »supported«. Because doing that put's other people in dangerous situations.

Eines der interessanteren Modelle der gestrigen Installparty war ein Pentium 75 Notebook, welches sich überschwänglich »Multimedia Notebook Computer« nannte.

Das erste Problem bei einem solchen Gerät ist ja, wie kriegt man da überhaupt Daten drauf. Weder Netzwerk, noch USB, zwar ein integriertes CD-ROM, doch nicht bootfähig. PCMCIA-Netzwerkkarte war auch nicht aufzufinden. Die wohl eher scherzhaft gemeinte Idee, die Festplatte in einen anderen Rechner zu bauen und dort zu installieren, nahm ich beim Wort.

Während der Installparty haben wir dann zwar keine Installation mehr vollbracht, jedoch bot ich an, das Gerät zuhause nochmal genauer unter die Lupe zu nehmen. Nachdem ich wie bereits angedeutet in einem Fremd-Laptop die Festplatte mit einem Debian Etch versehen hatte, der Versuch das ganze zu booten. Schlug fehl, mir fiel auf dass das Kernelimage im Bootmenü ein -686-suffix hatte - Debian erkennt scheinbar automatisch einen neueren Prozessor und installiert ein passendes Kernelimage. Also Platte wieder zurückgebaut, 486-kernel installiert und neu gebootet.

Fährt hoch, X start schlägt wie zu erwarten war fehl (bei gewechselter Hardware). Mit lspci die Hardware betrachtet. Die Grafikkarte war ein Modell von »Chips and Technologies«, was ich zwar noch nie gehört hatte, aber xorg schon. Es existiert ein chips-Treiber, mit dem die Karte dann auch (640x480 bei 16 bpp) lief, zwar mit schwarzen Rändern am Rand, aber immerhin. Das Touchpad war sogar ein Synaptics, Scrolling am Rand funktioniert tadellos. Da jedes moderne Desktop-Environment die Kiste wohl überfordert hätte, musste icewm herhalten.
CD-ROM und Floppy liefen problemlos.

Beim Soundchip musste ich leider passen. Zwar fand ich qua google heraus, dass es sich wohl um einen Opti 930 handelt, das snd-opti93x Modul meldete mir jedoch »no OPTi 82C93x soundcard found«. Möglicherweise mit entsprechenden Parametern für das Kernelmodul lauffähig.

Nun wäre natürlich eine Netzwerkverbindung interessant gewesen. Zwar hatte ich einige PCMCIA-Karten rumliegen, die einzige mit integriertem Anschluss war jedoch eine Cardbus, was unsere alte Hardware nicht verkraftete. Eine D-Link wurde korrekt erkannt und eingebunden, jedoch fehlt mir für diese ein Anschlussteil, der den flachen Stecker in einen Netzwerkstecker konvertiert (die Teile, die immer so schnell kaputtgingen).
Mein letzter Versuch, eine Orinoco WLAN-Karte, führte dann jedoch überraschenderweise zum gewünschten Erfolg. WLAN auf einem Pentium 75, wer kann schon behaupten, sowas schonmal betrieben zu haben? ;-)

Update: Tuxmobil brachte mir noch diese Seite zu dem Laptop.