Analyzing the primary UEFI bootkit for Linux

UPDATE (December 2^nd, 2024): The bootkit described on this report appears to be a part of a mission created by cybersecurity college students taking part in Korea’s Better of the Finest (BoB) coaching program. As they knowledgeable us: “The first goal of this mission is to boost consciousness throughout the safety group about potential dangers and to encourage proactive measures to stop related threats. Sadly, few bootkit samples have been disclosed previous to the deliberate convention presentation.” This helps our perception that it was an preliminary proof of idea quite than production-ready malware utilized by actual menace actors. Nonetheless, the weblog publish stays correct – it’s a purposeful bootkit with restricted help and represents the primary UEFI bootkit proof of idea for Linux OS.

Over the previous few years, the UEFI menace panorama, notably that of UEFI bootkits, has developed considerably. It began with the primary UEFI bootkit proof of idea (PoC) described by Andrea Allievi in 2012, which served as an indication of deploying bootkits on fashionable UEFI-based Home windows programs, and was adopted with many different PoCs (EfiGuard, Boot Backdoor, UEFI-bootkit). It took a number of years till the primary two actual UEFI bootkits have been found within the wild (ESPecter, 2021 ESET; FinSpy bootkit, 2021 Kaspersky), and it took two extra years till the notorious BlackLotus – the primary UEFI bootkit able to bypassing UEFI Safe Boot on up-to-date programs – appeared (2023, ESET).

A typical thread amongst these publicly recognized bootkits was their unique focusing on of Home windows programs. In the present day, we unveil our newest discovery: the primary UEFI bootkit designed for Linux programs, named Bootkitty by its creators. We consider this bootkit is merely an preliminary proof of idea, and primarily based on our telemetry, it has not been deployed within the wild. That mentioned, its existence underscores an necessary message: UEFI bootkits are not confined to Home windows programs alone.

The bootkit’s primary aim is to disable the kernel’s signature verification function and to preload two as but unknown ELF binaries through the Linux init course of (which is the primary course of executed by the Linux kernel throughout system startup). Throughout our evaluation, we found a probably associated unsigned kernel module – with indicators suggesting that it might have been developed by the identical creator(s) because the bootkit – that deploys an ELF binary answerable for loading one more kernel module unknown throughout our evaluation.

Key factors of this blogpost:

• In November 2024, a beforehand unknown UEFI utility, named bootkit.efi, was uploaded to VirusTotal.
• Our preliminary evaluation confirmed it’s a UEFI bootkit, named Bootkitty by its creators and surprisingly the primary UEFI bootkit focusing on Linux, particularly, just a few Ubuntu variations.
• Bootkitty is signed by a self-signed certificates, thus isn’t able to working on programs with UEFI Safe Boot enabled except the attackers certificates have been put in.
• Bootkitty is designed besides the Linux kernel seamlessly, whether or not UEFI Safe Boot is enabled or not, because it patches, in reminiscence, the required capabilities answerable for integrity verification earlier than GRUB is executed.
• bootkit.efi accommodates many artifacts suggesting that is extra like a proof of idea than the work of an lively menace actor.
• We found a probably associated kernel module, which we named BCDropper, that deploys an ELF program answerable for loading one other kernel module.

Bootkitty overview

As talked about within the introduction, Bootkitty accommodates many artifacts suggesting that we is likely to be coping with a proof of idea as an alternative of actively used malware. On this part, we glance extra intently at these artifacts, plus different fundamental details about the bootkit.

Bootkitty accommodates two unused capabilities, able to printing particular strings to the display screen throughout its execution. The primary perform, whose output is depicted in Determine 1, can print ASCII artwork that we consider represents a potential identify of the bootkit: Bootkitty.

The second perform, can print textual content, proven in Determine 2, containing the record of potential bootkit authors and different individuals that maybe someway participated in its improvement. One of many names talked about within the picture could be discovered on GitHub, however the profile doesn’t have any public repository that may comprise or point out a UEFI bootkit mission; subsequently, we are able to neither verify nor deny authenticity of the names talked about within the bootkit.

Throughout each boot, Bootkitty prints on display screen the strings proven in Determine 3.

Observe that the BlackCat identify is referenced additionally within the loadable kernel module described later. Regardless of the identify, we consider there is no such thing as a connection to the ALPHV/BlackCat ransomware group. It’s because BlackCat is a reputation utilized by researchers and Bootkitty was developed in C, whereas the group calls itself ALPHV and develops its malware solely in Rust.

As talked about earlier, Bootkitty presently helps solely a restricted variety of programs. The reason being that to search out the capabilities it desires to change in reminiscence, it makes use of hardcoded byte patterns. Whereas byte-pattern matching is a standard approach with regards to bootkits, the authors didn’t use the very best patterns for overlaying a number of kernel or GRUB variations; subsequently, the bootkit is absolutely purposeful just for a restricted variety of configurations. What limits the usage of the bootkit much more is the best way it patches the decompressed Linux kernel: as proven in Determine 4, as soon as the kernel picture is decompressed, Bootkitty merely copies the malicious patches to the hardcoded offsets throughout the kernel picture.

We clarify how the bootkit will get to the precise kernel patching later within the Linux kernel image decompression hook part; for now, simply notice that as a result of lack of kernel-version checks within the perform proven in Determine 4, Bootkitty can get to the purpose the place it patches utterly random code or information at these hardcoded offsets, thus crashing the system as an alternative of compromising it. This is without doubt one of the info that helps proof of idea. However, it is likely to be an preliminary not-production-ready model of malware created by malicious menace actors.

Final however not least, the bootkit binary is signed by the self-signed certificates proven in Determine 5.

Techical evaluation

We begin with an outline of Bootkitty’s execution, as depicted in Determine 6. First, we briefly describe the principle performance after which in subsequent sections we go into extra particulars.

There are three primary components we deal with:

• Execution of the bootkit and patching of the professional GRUB bootloader (factors 4 and 5 in Determine 6).
• Patching of the Linux kernel’s EFI stub loader (factors 6 and seven in Determine 6).
• Patching of the decompressed Linux kernel picture (factors 8 and 9 in Determine 6).

Initialization and GRUB hooking

After Bootkitty is executed by the shim, it checks to see whether or not UEFI Safe Boot is enabled by inspecting the worth of the SecureBoot UEFI variable, and proceeds to hook two capabilities from the UEFI authentication protocols if that’s the case (this course of is proven in Determine 7):

• EFI_SECURITY2_ARCH_PROTOCOL.FileAuthentication: this function is utilized by the firmware to measure and confirm the integrity of UEFI PE pictures. Bootkitty’s hook perform modifies the output of this perform in order that it at all times returns EFI_SUCCESS, that means that the verification succeeded.
• EFI_SECURITY_ARCH_PROTOCOL.FileAuthenticationState: this function is utilized by the firmware to execute a platform-specific coverage in response to totally different authentication standing values. Once more, the bootkit’s hook modifies it in a manner that it at all times returns EFI_SUCCESS, that means that the firmware can use the file no matter its precise authentication standing.

After checking the standing of UEFI Safe Boot, Bootkitty proceeds to load the professional GRUB from the hardcoded path on the EFI system partition: /EFI/ubuntu/grubx64-real.efi. This file needs to be a backup, created by the attacker, of a professional GRUB. As soon as GRUB is loaded (not but executed), the bootkit begins patching and hooking the next code in GRUB’s reminiscence:

• The start_image perform throughout the peimage GRUB module (a module embedded inside GRUB). This perform is answerable for beginning an already loaded PE picture, and it’s invoked by GRUB to start out the Linux kernel’s EFI stub binary (recognized on the whole as vmlinuz.efi or vmlinuz). The hook perform takes benefit of the truth that for the time being the hook is executed, vmlinuz is already loaded into reminiscence (however hasn’t been executed but), and patches the perform answerable for decompressing the precise Linux kernel picture inside vmlinuz (notice that in some circumstances, as a result of manner the Linux kernel is compiled, it may be fairly difficult to search out the precise identify of the perform being patched; nevertheless, we consider that this time it needs to be the zstd_decompress_dctx perform). Extra particulars concerning the decompression hook are within the Linux kernel image decompression hook part.
• The shim_lock_verifier_init perform, which is a part of the shim_lock verifier mechanism inside GRUB – this needs to be activated mechanically if UEFI Safe Boot is enabled. It’s answerable for deciding whether or not the information offered (e.g., GRUB modules, Linux kernel, configurations…) needs to be verified or not through the boot. The put in hook, nevertheless, is someway complicated and the creator’s intentions are unclear as a result of it modifies shim_lock_verifier_init’s output in a manner that it units the output flag to GRUB_VERIFY_FLAGS_SINGLE_CHUNK (worth 2) for any file sort offered, which ought to, in line with the GRUB manual, strengthen the safety much more. Curiously, as a result of hook described within the subsequent level, this shim_lock_verifier_init perform isn’t even known as through the boot, thus turning into irrelevant.
• The grub_verifiers_open perform. This perform is invoked by GRUB anytime it opens a file, and is answerable for checking whether or not the put in GRUB file verifiers (this contains the shim_lock verifier described above) require integrity verification for the file being loaded. The perform is hooked by the bootkit in a manner that it returns instantly with out continuing to any signature checks (notice that because of this it doesn’t even execute the beforehand hooked shim_lock_verifier_init perform).

Linux kernel picture decompression hook

• Rewrites the kernel model and Linux banner strings with the textual content BoB13 (this has no important impression on the system).
• Hooks the module_sig_check perform.
• Patches pointer/handle to the primary surroundings variable of the init course of.

The perform module_sig_check is patched to at all times return 0. This perform is answerable for checking whether or not the module is validly signed. By patching the perform to return 0, the kernel will load any module with out verifying the signature. On Linux programs with UEFI Safe Boot enabled, kernel modules want to be signed if they’re meant to be loaded. That is additionally the case when the kernel is constructed with CONFIG_MODULE_SIG_FORCE enabled or when module.sig_enforce=1 is handed as a kernel command line argument, as described within the Linux kernel documentation. The probably situation is that no less than one malicious kernel module is loaded at a later section, such because the dropper analyzed under.

The primary course of that the Linux kernel executes is init from the primary hardcoded path that works (beginning with /init from initramfs), together with command line arguments and surroundings variables. The hook code replaces the primary surroundings variable with LD_PRELOAD=/choose/injector.so /init. LD_PRELOAD is an surroundings variable that’s used to load ELF shared objects earlier than others and can be utilized to override capabilities. It’s a widespread technique utilized by attackers to load malicious binaries. On this case, the /choose/injector.so and /init ELF shared objects are loaded when the init course of begins. That is the place the intention turns into much less clear, primarily why the second string /init is a part of LD_PRELOAD.

We’ve got not found any of those probably malicious ELF shared objects, though simply as this blogpost was being finalized for publication, a write-up describing the lacking elements talked about in our report has been published. Now it’s clear they’re used simply to load one other stage.

Influence and remediation

Other than loading unknown ELF shared objects, Bootkitty leaves footprints within the system. The primary is the supposed, albeit not needed, modification of kernel model and Linux banner strings. The previous could be seen by working uname -v (Determine 9) and the latter by working dmesg (Determine 10).

Throughout our evaluation, the output of the command dmesg additionally included particulars about how the init course of was run. As depicted in Determine 11, the method was run with the LD_PRELOAD surroundings variable (it was initially HOME=/ and was changed with LD_PRELOAD=/choose/injector.so /init by the bootkit).

Observe in Determine 11 that the phrase /init within the first line corresponds to the professional program in initramfs that finally passes management to systemd on default Ubuntu installations. The presence of the LD_PRELOAD surroundings variable may also be verified by inspecting the file /proc/1/environ.

After booting up a system with Bootkitty in our testing surroundings, we observed that the kernel was marked as tainted (command from Determine 12 can be utilized to verify the contaminated worth), which was not the case when the bootkit was absent. One other solution to inform whether or not the bootkit is current on the system with UEFI Safe Boot enabled is by trying to load an unsigned dummy kernel module throughout runtime. If it’s current, the module can be loaded; if not – the kernel refuses to load it.

A easy treatment tip to do away with the bootkit is to maneuver the professional /EFI/ubuntu/grubx64-real.efi file again to its authentic location, which is /EFI/ubuntu/grubx64.efi. It will make shim execute the professional GRUB and thus the system will boot up with out the bootkit (notice that this covers solely the situation when the bootkit is deployed as /EFI/ubuntu/grubx64.efi).

BCDropper and BCObserver

Along with the bootkit, we found a probably associated unsigned kernel module we named BCDropper, uploaded to VirusTotal across the similar time and by the identical submitter’s ID because the bootkit, containing hints that it might need been developed by the identical creator because the bootkit, resembling:

• a BlackCat string within the output of the modinfo command’s output, proven in Determine 13,
• one other presence of the blackcat string within the debug paths within the module’s binary, proven in Determine 14, and
• it accommodates an unused file-hiding perform that hides particular entries from listing listings. As proven in Determine 15, one of many hardcoded filename string prefixes used to filter-out these entries is injector (notice that Bootkitty tries to preload a shared-library from the trail /choose/injector.so)

Nonetheless, even with the proof introduced, we can not say for certain whether or not or not the kernel module is expounded to Bootkitty (or was created by the identical developer). Additionally, the kernel model talked about in Determine 13 (6.8.0-48-generic) isn’t supported by the bootkit.

As its identify suggests, the kernel module drops an embedded ELF file we named BCObserver, particularly to /choose/observer, and executes it through /bin/bash (Determine 17). On prime of that, the module hides itself by removing its entry from the module list. The kernel module additionally implements different rootkit-related functionalities like hiding information (these in Determine 15), processes, and open ports, however they aren’t instantly utilized by the dropper.

BCObserver is a quite easy utility that waits till the show supervisor gdm3 is working, after which hundreds an unknown kernel module from /choose/rootkit_loader.ko through the finit_module system name. By ready for the show supervisor to start out, the code ensures that the kernel module is loaded after the system is absolutely booted up.

Whereas we can not verify whether or not the dropper is someway associated to the bootkit, and if that’s the case, how it’s meant to be executed, we’re fairly certain that the bootkit patches the module_sig_check perform for a motive, and loading an unsigned kernel module (such because the dropper described right here) would undoubtedly make sense.

Conclusion

Whether or not a proof of idea or not, Bootkitty marks an attention-grabbing transfer ahead within the UEFI menace panorama, breaking the idea about fashionable UEFI bootkits being Home windows-exclusive threats. Despite the fact that the present model from VirusTotal doesn’t, for the time being, characterize an actual menace to the vast majority of Linux programs, it emphasizes the need of being ready for potential future threats.

To maintain your Linux programs secure from such threats, guarantee that UEFI Safe Boot is enabled, your system firmware and OS are up-to-date, and so is your UEFI revocations record.

For any inquiries about our analysis revealed on WeLiveSecurity, please contact us at [email protected]. 

ESET Analysis gives non-public APT intelligence reviews and information feeds. For any inquiries about this service, go to the ESET Threat Intelligence web page.

IoCs

A complete record of indicators of compromise (IoCs) and samples could be present in our GitHub repository.

Recordsdata

MITRE ATT&CK methods

This desk was constructed utilizing version 16 of the MITRE ATT&CK framework.