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During April–June 2006, I was an intern at Microsoft Research, Cambridge. My project, supervised by Tuomas Aura and Michael Roe, was to improve the privacy and security of mobile computer users. A paper summarizing our work was published at SecureComm 2007, but I’ve only just released the paper online: “Securing Network Location Awareness with Authenticated DHCP”.

How a computer should behave depends on its network location. Existing security solutions, like firewalls, fail to adequately protect mobile users because they assume their policy is static. This results in laptop computers being configured with fairly open policies, in order to facilitate applications appropriate for a trustworthy office LAN (e.g. file and printer sharing, collaboration applications, and custom servers). When the computer is taken home or roaming, this policy leaves an excessively large attack surface.

This static approach also harms user privacy. Modern applications broadcast a large number of identifiers which may leak privacy sensitive information (name, employer, office location, job role); even randomly generated identifiers allow a user to be tracked. When roaming, a laptop should not broadcast identifiers unless necessary, and on moving location either pseudonymous identifiers should be re-used or anonymous ones generated.

Both of these goals require a computer to be able to identify which network it is on, even when an attacker is attempting to spoof this information. Our solution was to extend DHCP to include an network location identifier, authenticated by a public-key signature. I built a proof-of-concept implementation for the Microsoft Windows Server 2003 DHCP server, and the Vista DHCP client.

A scheme like this should ideally work on both small PKI-less home LANs and still permit larger networks to aggregate multiple access points into one logical network. Achieving this requires some subtle naming and key management tricks. These techniques, and how to implement the protocols in a privacy-preserving manner are described in our paper.

I’ve spent the day at the First USENIX Workshop on Offensive Technologies (WOOT07) — an interesting new workshop on attack strategies and technologies. The workshop highlights the tension between the “white” and “black” hats in security research — you can’t design systems to avoid security problems if you don’t understand what they are. USENIX’s take on such a forum is less far down the questionable ethical spectrum than some other venues, but it certainly presented and talked about both new exploits for new vulnerabilities, and techniques for evading current protections in concrete detail.

I presented, “Exploiting Concurrency Vulnerabilities in System Call Wrappers,” a paper on the topic of compromising system call interposition-based protection systems, such as COTS virus scanners, OpenBSD and NetBSD’s Systrace, the TIS Generic Software Wrappers Toolkit (GSWTK), and CerbNG. The key insight here is that the historic assumption of “atomicity” of system calls is falacious, and that on both uniprocessor and multiprocessing systems, it is trivial to construct a race between system call wrappers and malicious user processes to bypass protections. I demonstrated sample exploit code against the Sysjail policy on Systrace, and IDwrappers on GSWTK, but the paper includes a more extensive discussion including vulnerabilities in sudo’s Systrace monitor mode. You can read the paper and see the presentation slides here. All affected vendors received at least six months, and in some cases many years advance notice regarding these vulnerabilities.

The moral, for those unwilling to read the paper, is that system call wrappers are a bad idea, unless of course, you’re willing to rewrite the OS to be message-passing. Systems like the TrustedBSD MAC Framework on FreeBSD and Mac OS X Leopard, Linux Security Modules (LSM), Apple’s (and now also NetBSD’s) kauth(9), and other tightly integrated kernel security frameworks offer specific solutions to these concurrency problems. There’s plenty more to be done in that area.

Concurrency issues have been discussed before in computer security, especially relating to races between applications when accessing /tmp, unexpected signal interruption of socket operations, and distributed systems races, but this paper starts to explore the far more sordid area of OS kernel concurrency and security. Given that even notebook computers are multiprocessor these days, emphasizing the importance of correct synchronization and reasoning about high concurrency is critical to thinking about security correctly. As someone with strong interests in both OS parallelism and security, the parallels (no pun intended) seem obvious: in both cases, the details really matter, and it requires thinking about a proverbial Cartesian Evil Genius. Anyone who’s done serious work with concurrent systems knows that they are actively malicious, so a good alignment for the infamous malicious attacker in security research!

Some of the other presentations have included talks about Google’s software fuzzing tool Flayer based on Valgrind, attacks on deployed SIP systems including AT&T’s product, Bluetooth sniffing with BlueSniff, and quantitative analyses of OS fingerprinting techniques. USENIX members will presumably be able to read the full set of papers online immediately; for others, check back in a year or visit the personal web sites of the speakers after you look at the WOOT07 Programme.