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

Over the previous few years, the UEFI risk 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 had 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 concentrating on of Home windows methods. At present, 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 vital message: UEFI bootkits are not confined to Home windows methods alone.

The bootkit’s foremost aim 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 presumably 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 liable 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 concentrating on Linux, particularly, just a few Ubuntu variations.
• Bootkitty is signed by a self-signed certificates, thus just isn’t able to working on methods 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 features liable for integrity verification earlier than GRUB is executed.
• bootkit.efi incorporates many artifacts suggesting that is extra like a proof of idea than the work of an lively risk actor.
• We found a presumably associated kernel module, which we named BCDropper, that deploys an ELF program liable for loading one other kernel module.

Bootkitty overview

As talked about within the introduction, Bootkitty incorporates 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 carefully at these artifacts, plus different primary details about the bootkit.

Bootkitty incorporates two unused features, 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 potential title of the bootkit: Bootkitty.

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

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

Be aware that the BlackCat title is referenced additionally within the loadable kernel module described later. Regardless of the title, we imagine there isn’t a connection to the ALPHV/BlackCat ransomware group. It is 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 features it needs to change in reminiscence, it makes use of hardcoded byte patterns. Whereas byte-pattern matching is a standard method on the subject of bootkits, the authors didn’t use one of the best patterns for overlaying a number of kernel or GRUB variations; due to this fact, 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 inside 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 observe that because of the 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 information at these hardcoded offsets, thus crashing the system as a substitute of compromising it. This is likely one of the info that helps proof of idea. Alternatively, it may be an preliminary not-production-ready model of malware created by malicious risk 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 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 analyzing the worth of the SecureBoot UEFI variable, and proceeds to hook two features 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, that 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, 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 ought 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 operate inside the peimage GRUB module (a module embedded inside GRUB). This operate is liable for beginning an already loaded PE picture, and it’s invoked by GRUB to start out the Linux kernel’s EFI stub binary (recognized typically 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 liable for decompressing the precise Linux kernel picture inside vmlinuz (observe that in some circumstances, because of the 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 ought to be the zstd_decompress_dctx operate). Extra particulars in regards to 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 ought to be activated routinely if UEFI Safe Boot is enabled. It’s liable for deciding whether or not the recordsdata offered (e.g., GRUB modules, Linux kernel, configurations…) ought to be verified or not throughout the boot. The put in hook, nevertheless, is by some means 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 sort offered, which ought to, in response to the GRUB guide, strengthen the safety much more. Curiously, because of the hook described within the subsequent level, this shim_lock_verifier_init operate just isn’t even known as throughout the boot, thus turning into irrelevant.
• The grub_verifiers_open operate. This operate is invoked by GRUB anytime it opens a file, and is liable for checking whether or not the put in GRUB file verifiers (this consists of 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 (observe that because of this it doesn’t even execute the beforehand hooked shim_lock_verifier_init operate).

Linux kernel picture decompression hook

This hook is liable for patching the decompressed Linux kernel picture. The hook is named proper earlier than the kernel picture is decompressed, so the hook restores the unique decompression operate’s bytes and executes the unique operate to decompress the kernel picture earlier than continuing to the kernel patching.

Now, because the kernel is decompressed and lies within the reminiscence untouched (nonetheless hasn’t been executed), the hook code patches it at hardcoded offsets (in reminiscence solely). Particularly, as proven in Determine 8, it:

• 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 operate.
• Patches pointer/tackle to the primary setting variable of the init course of.

The operate module_sig_check is patched to all the time return 0. This operate is liable 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 doubtless situation is that at the least one malicious kernel module is loaded at a later part, 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 setting variables. The hook code replaces the primary setting variable with LD_PRELOAD=/decide/injector.so /init. LD_PRELOAD is an setting variable that’s used to load ELF shared objects earlier than others and can be utilized to override features. It’s a frequent 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.

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

Affect 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 might 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 setting variable (it was initially HOME=/ and was changed with LD_PRELOAD=/decide/injector.so /init by the bootkit).

Be aware 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 setting variable can be verified by inspecting the file /proc/1/environ.

After booting up a system with Bootkitty in our testing setting, we seen that the kernel was marked as tainted (command from Determine 12 can be utilized to examine 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 shall be loaded; if not – the kernel refuses to load it.

A easy treatment tip to eliminate the bootkit is to maneuver the professional /EFI/ubuntu/grubx64-real.efi file again to its authentic location, which is /EFI/ubuntu/grubx64.efi. This can make shim execute the professional GRUB and thus the system will boot up with out the bootkit (observe 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 presumably 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, reminiscent of:

• 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 incorporates 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 (observe that Bootkitty tries to preload a shared-library from the trail /decide/injector.so)

Nevertheless, even with the proof introduced, we can’t say for positive 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) just isn’t 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 record. The kernel module additionally implements different rootkit-related functionalities like hiding recordsdata (these in Determine 15), processes, and open ports, however they aren’t instantly utilized by the dropper.

BCObserver is a reasonably easy software that waits till the show supervisor gdm3 is working, 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 start out, the code ensures that the kernel module is loaded after the system is absolutely booted up.

Whereas we can’t affirm whether or not the dropper is by some means associated to the bootkit, and in that case, how it’s meant to be executed, we’re fairly positive that the bootkit patches the module_sig_check operate for a motive, 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 fascinating transfer ahead within the UEFI risk panorama, breaking the assumption about fashionable UEFI bootkits being Home windows-exclusive threats. Although the present model from VirusTotal doesn’t, in the mean time, signify an actual risk to the vast majority of Linux methods, it emphasizes the need of being ready for potential future threats.

To maintain your Linux methods 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 threatintel@eset.com. 

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

IoCs

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

Information

MITRE ATT&CK methods

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