Exploit Lecture: Writing FreeBSD Malware

grarpamp grarpamp at gmail.com
Sat Apr 28 02:40:21 UTC 2018


Without exploit mitigations and with an insecure-by-default design,
writing malware for FreeBSD is a fun task, taking us back to 1999-era
Linux exploit authorship. Several members of FreeBSD's development
team have claimed that Capsicum, a capabilities/sandboxing framework,
prevents exploitation of applications. Our in-depth analysis of the
topics below will show that in order to be effective, applying
Capsicum to existing complex codebases lends itself to wrapper-style
sandboxing. Wrapper-style sandbox is a technique whereby privileged
operations get wrapped and passed to a segregated process, which
performs the operation on behalf of the capsicumized process. With a
new libhijack payload, we will demonstrate that wrapper-style
sandboxing requires ASLR and CFI for effectiveness. FreeBSD supports
neither ASLR nor CFI. Tying into the wrapper-style Capsicum defeat,
we'll talk about advances being made with libhijack, a tool announced
at Thotcon 0x4. The payload developed in the Capsicum discussion will
be used with libhijack, thus making it easy to extend. We will also
learn the Mandatory Access Control (MAC) framework in FreeBSD. The MAC
framework places hooks into several key places in the kernel. We'll
learn how to abuse the MAC framework for writing efficient rootkits.
Attendees of this presentation should walk away with the knowledge to
skillfully and artfully write offensive code targeting both the
FreeBSD userland and the kernel.


Shawn Webb is a cofounder of HardenedBSD, a hardened downstream
distribution of FreeBSD. With over a decade in infosec, he dabbles in
both the offensive and defensive aspects of the industry. On the
advisory board for Emerald Onion, Shawn believes in a more free and
open Internet. His whole house is wired for Tor. Getting on the Tor
network is only a network jack away!


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