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 Greatest (BoB) coaching program. As they knowledgeable us: “The first intention of this mission is to lift consciousness throughout the safety group about potential dangers and to encourage proactive measures to stop comparable 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 moderately than production-ready malware utilized by actual menace actors. Nonetheless, the weblog submit stays correct – it’s a practical bootkit with restricted assist and represents the primary UEFI bootkit proof of idea for Linux OS.

Over the previous few years, the UEFI menace panorama, significantly that of UEFI bootkits, has advanced considerably. It began with the primary UEFI bootkit proof of idea (PoC) described by Andrea Allievi in 2012, which served as an illustration of deploying bootkits on fashionable UEFI-based Home windows methods, 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 methods – appeared (2023, ESET).

A typical thread amongst these publicly recognized bootkits was their unique focusing on of Home windows methods. At this time, we unveil our newest discovery: the primary UEFI bootkit designed for Linux methods, named Bootkitty by its creators. We imagine 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 essential message: UEFI bootkits are not confined to Home windows methods alone.

The bootkit’s foremost purpose is to disable the kernel’s signature verification characteristic 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 may have been developed by the identical writer(s) because the bootkit – that deploys an ELF binary accountable for loading yet one more kernel module unknown throughout our evaluation.

Key factors of this blogpost:

• In November 2024, a beforehand unknown UEFI software, 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, a number of Ubuntu variations.
• Bootkitty is signed by a self-signed certificates, thus is just not able to operating on methods with UEFI Safe Boot enabled until the attackers certificates have been put in.
• Bootkitty is designed as well the Linux kernel seamlessly, whether or not UEFI Safe Boot is enabled or not, because it patches, in reminiscence, the mandatory capabilities accountable for integrity verification earlier than GRUB is executed.
• bootkit.efi comprises many artifacts suggesting that is extra like a proof of idea than the work of an energetic menace actor.
• We found a probably associated kernel module, which we named BCDropper, that deploys an ELF program accountable for loading one other kernel module.

Bootkitty overview

As talked about within the introduction, Bootkitty comprises many artifacts suggesting that we may be coping with a proof of idea as a substitute of actively used malware. On this part, we glance extra intently at these artifacts, plus different primary details about the bootkit.

Bootkitty comprises two unused capabilities, able to printing particular strings to the display throughout its execution. The primary operate, whose output is depicted in Determine 1, can print ASCII artwork that we imagine represents a doable title of the bootkit: Bootkitty.

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

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

Word that the BlackCat title is referenced additionally within the loadable kernel module described later. Regardless of the title, we imagine there isn’t any 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 at present helps solely a restricted variety of methods. The reason being that to seek out the capabilities it desires to switch in reminiscence, it makes use of hardcoded byte patterns. Whereas byte-pattern matching is a standard method in the case of bootkits, the authors didn’t use the very best patterns for overlaying a number of kernel or GRUB variations; due to this fact, the bootkit is totally practical just for a restricted variety of configurations. What limits using the bootkit much more is the way in which 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 picture decompression hook part; for now, simply word that as a result of lack of kernel-version checks within the operate proven in Determine 4, Bootkitty can get to the purpose the place it patches utterly random code or knowledge at these hardcoded offsets, thus crashing the system as a substitute of compromising it. This is without doubt one of the details that helps proof of idea. Alternatively, it may 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 summary 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 foremost elements we deal with:

• Execution of the bootkit and patching of the respectable 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 analyzing the worth of the SecureBoot UEFI variable, and proceeds to hook two capabilities from the UEFI authentication protocols in that case (this course of is proven in Determine 7):

• EFI_SECURITY2_ARCH_PROTOCOL.FileAuthentication: this operate is utilized by the firmware to measure and confirm the integrity of UEFI PE photographs. Bootkitty’s hook operate modifies the output of this operate in order that it all the time returns EFI_SUCCESS, which means that the verification succeeded.
• EFI_SECURITY_ARCH_PROTOCOL.FileAuthenticationState: this operate is utilized by the firmware to execute a platform-specific coverage in response to completely different authentication standing values. Once more, the bootkit’s hook modifies it in a approach that it all the time returns EFI_SUCCESS, which 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 respectable GRUB from the hardcoded path on the EFI system partition: /EFI/ubuntu/grubx64-real.efi. This file must be a backup, created by the attacker, of a respectable 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 operate throughout the peimage GRUB module (a module embedded inside GRUB). This operate is accountable for beginning an already loaded PE picture, and it’s invoked by GRUB to begin the Linux kernel’s EFI stub binary (recognized on the whole as vmlinuz.efi or vmlinuz). The hook operate takes benefit of the truth that in the mean time the hook is executed, vmlinuz is already loaded into reminiscence (however hasn’t been executed but), and patches the operate accountable for decompressing the precise Linux kernel picture inside vmlinuz (word that in some instances, as a result of approach the Linux kernel is compiled, it may be fairly difficult to seek out the precise title of the operate being patched; nevertheless, we imagine that this time it must be the zstd_decompress_dctx operate). Extra particulars concerning the decompression hook are within the Linux kernel picture decompression hook part.
• The shim_lock_verifier_init operate, which is a part of the shim_lock verifier mechanism inside GRUB – this must be activated robotically if UEFI Safe Boot is enabled. It’s accountable for deciding whether or not the information supplied (e.g., GRUB modules, Linux kernel, configurations…) must be verified or not in the course of the boot. The put in hook, nevertheless, is in some way complicated and the writer’s intentions are unclear as a result of it modifies shim_lock_verifier_init’s output in a approach that it units the output flag to GRUB_VERIFY_FLAGS_SINGLE_CHUNK (worth 2) for any file kind supplied, which ought to, in response to the GRUB handbook, strengthen the safety much more. Curiously, as a result of hook described within the subsequent level, this shim_lock_verifier_init operate is just not even referred to as in the course of the boot, thus changing into irrelevant.
• The grub_verifiers_open operate. This operate is invoked by GRUB anytime it opens a file, and is accountable 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 operate is hooked by the bootkit in a approach that it returns instantly with out continuing to any signature checks (word that which means that it doesn’t even execute the beforehand hooked shim_lock_verifier_init operate).

Linux kernel picture decompression hook

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

The operate module_sig_check is patched to all the time return 0. This operate is accountable for checking whether or not the module is validly signed. By patching the operate to return 0, the kernel will load any module with out verifying the signature. On Linux methods 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 possible situation is that at the least 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 atmosphere variables. The hook code replaces the primary atmosphere variable with LD_PRELOAD=/decide/injector.so /init. LD_PRELOAD is an atmosphere variable that’s used to load ELF shared objects earlier than others and can be utilized to override capabilities. It’s a widespread method utilized by attackers to load malicious binaries. On this case, the /decide/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.

Now we have 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 revealed. Now it’s clear they’re used simply to load one other stage.

Impression and remediation

Aside from 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 may be seen by operating uname -v (Determine 9) and the latter by operating 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 atmosphere variable (it was initially HOME=/ and was changed with LD_PRELOAD=/decide/injector.so /init by the bootkit).

Word in Determine 11 that the phrase /init within the first line corresponds to the respectable program in initramfs that ultimately passes management to systemd on default Ubuntu installations. The presence of the LD_PRELOAD atmosphere variable will also be verified by inspecting the file /proc/1/environ.

After booting up a system with Bootkitty in our testing atmosphere, we seen that the kernel was marked as tainted (command from Determine 12 can be utilized to test the contaminated worth), which was not the case when the bootkit was absent. One other strategy to inform whether or not the bootkit is current on the system with UEFI Safe Boot enabled is by making an attempt 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 eliminate the bootkit is to maneuver the respectable /EFI/ubuntu/grubx64-real.efi file again to its authentic location, which is /EFI/ubuntu/grubx64.efi. This can make shim execute the respectable GRUB and thus the system will boot up with out the bootkit (word 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 identical time and by the identical submitter’s ID because the bootkit, containing hints that it might need been developed by the identical writer because the bootkit, comparable to:

• 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 comprises an unused file-hiding operate 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 (word that Bootkitty tries to preload a shared-library from the trail /decide/injector.so)

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

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

BCObserver is a moderately easy software that waits till the show supervisor gdm3 is operating, after which masses an unknown kernel module from /decide/rootkit_loader.ko through the finit_module system name. By ready for the show supervisor to begin, the code ensures that the kernel module is loaded after the system is totally booted up.

Whereas we can’t verify whether or not the dropper is in some way associated to the bootkit, and in that case, how it’s meant to be executed, we’re fairly certain that the bootkit patches the module_sig_check operate for a cause, and loading an unsigned kernel module (such because the dropper described right here) would positively 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 assumption about fashionable UEFI bootkits being Home windows-exclusive threats. Though the present model from VirusTotal doesn’t, in the mean time, symbolize an actual menace to nearly all of Linux methods, it emphasizes the need of being ready for potential future threats.

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

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

ESET Analysis gives personal APT intelligence experiences and knowledge feeds. For any inquiries about this service, go to the ESET Risk Intelligence web page.

IoCs

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

Information

MITRE ATT&CK strategies

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