Linux

When configuring a new device, achieving an acceptable Lynis hardening score is a challenge most practitioners are familiar with.

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Hardening a Linux client system to an acceptable degree is a time-consuming process, one that demands familiarity with a broad set of configuration parameters, framework recommendations, and the reasoning behind each control.

This post introduces our new Linux client hardening guide (MD, PDF), a comprehensive, publicly available hardening reference for Linux systems.

Motivation and Scope

The guide covers the full breadth of controls needed to significantly raise the security posture of a modern Linux installation while preserving operational usability (this will be very subjective, the guide reflects my opinion of “usable”). It has been developed and validated against Ubuntu 24.04 LTS as the primary reference platform, and cross-tested on Fedora, Debian 12, and Arch Linux as well as on traditionally server-oriented distributions like openSUSE Leap 15.6, Debian 12, Rocky Linux 9, and Red Hat Enterprise Linux 9 while not focussing on those as the guide is created for Linux clients.

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The purpose of this blog post is to explain how Secure Boot works. In particular, we will explain where current implementations of Secure Boot by Linux distributors fall short compared to Microsoft Windows and Apple macOS.

Major distributors like Canonical, Debian, openSUSE, and Red Hat place a high priority on making their operating systems work out of the box. Given the current Linux landscape with out-of-tree drivers and incompatible licenses, providing the end user with all the drivers possibly needed to boot the system can be challenging.

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Many Linux hardening guides focus on well-known protections: full-disk encryption, Secure Boot, and password-protected bootloaders. While these measures are critical, they often overlook a subtle but serious attack vector: the ability to drop into a debug shell via the Initial RAM Filesystem (initramfs). This oversight can enable an attacker with brief physical access to bypass conventional boot protections and inject persistent malware into the system.

In this post, it is demonstrated how this attack works on modern Linux distributions, such as Ubuntu and Fedora, and explained why existing guidance often fails to mention it.

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The X11 Window System has been used since September 1987 for Unix desktop systems, allowing applications to display their windows. Today, one of the server implementations of the protocol is the X.Org X server and XWayland, which both use the same codebase. While reviewing the X server, several legacy security issues were identified. These appear to originate from earlier design stages when security considerations were less prominent. Despite the project’s maturity and widespread use, some of these issues have persisted.

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In this blog post, we quickly look into issues involving character devices. As is typical for Linux, everything is a file, so character devices are referenced as files, such as pseudo terminals (pts) under /dev/pts/. man pty briefly introduces the topic. Essentially, it is used to connect a program, such as a terminal emulator, to a shell. In the end, a pty can read and write like a regular file. A colleague already brought up the topic of ptys and character devices. But more recently a Twitter post and the accompanying advisory piqued my interest.

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After quite some time and work, I’m happy to announce the new release of the Linux Heap Analysis Plugins, which are now part of the Rekall project, but not yet part of an official Rekall release, so you have to grab them manually.
This release fixes several bugs and adds the following features:

• Support for Glibc version 2.26 (tcache chunks) and 2.27
• Heapsearch now includes Rekall’s yara scan functionality
• x86 Glibc versions with a modified MALLOC_ALIGNMENT value of 16 (as done in arch’s glibc package 2.26) are now supported
• Improved retrieval of main_arena and new automated retrieval of malloc_par struct; so for the majority of cases, the corresponding cmd line options are not necessary anymore and hence no debug information have to be retrieved.
• main_arena and malloc_par struct retrieval now also applies for statically linked binaries; there might however be cases, where it is necessary to specify the malloc_par struct offset and the used glibc version number

While Glibc version 2.27 does not really change much for the heap from a forensics point of view, version 2.26 introduced a new feature called tcache (per thread cache). Those caches are essentially just a new type of bin, holding freed chunks, but seem to improve the performance.
Sadly, only two days after its first commit, there was already a blog post explaining the functionality (not so sad 😉 ) and also some discovered vulnerabilities (here we go with the sad part).
Besides introducing a new landscape for attackers, this feature added two new heap management structs and one additional chunk per thread-heap (each thread has its own heap, up until an upper limit). This chunk is located at the beginning of each heap and hence, part of a raw heap dump (so be aware of that, if you are doing a raw analysis without the plugins).
The chunk holds the content of the tcache_perthread_struct, which has to members:

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In various scenarios it might be helpful or even required to have a statically compiled version of Nmap available. This applies to e.g. scenarios where only limited user privileges are available and installing anything to the system might not be desirable.

For such cases I’ve started to create recipes to build such binaries. Similar projects are already available on GitHub, but there are several reasons why I chose to create my own tools:

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Following my work with the FreeBSD implementation of RFC 6980 I was happy to present my work at last week’s DENOG 9 meeting.
To make it available to anyone who did not meet me there and go into some more detail that would have exceeded the boundaries of the talk, I will cover the topic here.

After the preceding work on Windows Server 2016 and the FreeBSD testing, as a Linux user, lover and administrator, I of course wanted to take a look at how different Linux systems complied with the RFC 6980 standard.

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As mentioned in my last blogpost, I had the pleasure to participate in this years DFRWS USA and present our paper. The paper and presentation can be freely viewed and downloaded here or here. Note that there is also an extended version of the paper, which can be downloaded here.

The keepassx, zsh and heap analysis plugins are now also part of the Rekall release candidate 1.7.0RC1, so it’s easier to get started.

The conference had some great talks and workshops, which I’m going to briefly sum up.

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