Analyzing the primary UEFI bootkit for Linux

Over the previous few years, the UEFI risk panorama, significantly 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 techniques, 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 techniques – appeared (2023, ESET).

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

The bootkit’s principal purpose is to disable the kernel’s signature verification function and to preload two as but unknown ELF binaries by way of 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 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, a couple of Ubuntu variations.
• Bootkitty is signed by a self-signed certificates, thus just isn’t able to operating on techniques with UEFI Safe Boot enabled except 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 required capabilities accountable 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 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 incorporates many artifacts suggesting that we is perhaps 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 primary details about the bootkit.

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

The second perform, can print textual content, proven in Determine 2, containing the listing of attainable 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 might include or point out a UEFI bootkit venture; 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.

Observe that the BlackCat title is referenced additionally within the loadable kernel module described later. Regardless of the title, 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 techniques. The reason being that to seek out the capabilities it needs to change in reminiscence, it makes use of hardcoded byte patterns. Whereas byte-pattern matching is a standard approach on the subject of bootkits, the authors didn’t use one of the best patterns for protecting a number of kernel or GRUB variations; due to this fact, the bootkit is absolutely 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 be aware 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 knowledge at these hardcoded offsets, thus crashing the system as an alternative of compromising it. This is among the information that helps proof of idea. However, it is perhaps 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 principal components we deal with:

• Execution of the bootkit and patching of the reliable 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 if that’s the case (this course of is proven in Determine 7):

• EFI_SECURITY2_ARCH_PROTOCOL.FileAuthentication: this perform is utilized by the firmware to measure and confirm the integrity of UEFI PE photos. Bootkitty’s hook perform modifies the output of this perform in order that it all the time returns EFI_SUCCESS, that means that the verification succeeded.
• EFI_SECURITY_ARCH_PROTOCOL.FileAuthenticationState: this perform 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 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 reliable 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 reliable 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 accountable for beginning an already loaded PE picture, and it’s invoked by GRUB to begin the Linux kernel’s EFI stub binary (recognized basically 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 accountable for decompressing the precise Linux kernel picture inside vmlinuz (be aware that in some circumstances, as a result of manner the Linux kernel is compiled, it may be fairly difficult to seek out the precise title of the perform being patched; nevertheless, we consider that this time it ought to be the zstd_decompress_dctx perform). Extra particulars in regards to the decompression hook are within the Linux kernel picture decompression hook part.
• The shim_lock_verifier_init perform, which is a part of the shim_lock verifier mechanism inside GRUB – this ought to be activated robotically if UEFI Safe Boot is enabled. It’s accountable 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 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 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 guide, strengthen the safety much more. Apparently, as a result of hook described within the subsequent level, this shim_lock_verifier_init perform just isn’t even referred to as throughout the boot, thus turning into irrelevant.
• The grub_verifiers_open perform. This perform 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 perform is hooked by the bootkit in a manner that it returns instantly with out continuing to any signature checks (be aware 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 affect 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 all the time return 0. This perform is accountable 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 techniques 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 state of affairs is that at the very 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 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 approach 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 parts talked about in our report has been revealed. Now it’s clear they’re used simply to load one other stage.

Influence and remediation

Aside from loading unknown ELF shared objects, Bootkitty leaves footprints within the system. The primary is the supposed, albeit not mandatory, 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 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 reliable program in initramfs that ultimately passes management to systemd on default Ubuntu installations. The presence of the LD_PRELOAD surroundings variable can be verified by inspecting the file /proc/1/environ.

After booting up a system with Bootkitty in our testing surroundings, 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 method 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 shall be loaded; if not – the kernel refuses to load it.

A easy treatment tip to do away with the bootkit is to maneuver the reliable /EFI/ubuntu/grubx64-real.efi file again to its authentic location, which is /EFI/ubuntu/grubx64.efi. This may make shim execute the reliable GRUB and thus the system will boot up with out the bootkit (be aware that this covers solely the state of affairs 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, corresponding 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 incorporates 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 (be aware that Bootkitty tries to preload a shared-library from the trail /choose/injector.so)

Nonetheless, even with the proof introduced, we can’t 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) just isn’t supported by the bootkit.

As its title suggests, the kernel module drops an embedded ELF file we named BCObserver, particularly to /choose/observer, and executes it by way of /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 recordsdata (these in Determine 15), processes, and open ports, however they aren’t immediately utilized by the dropper.

BCObserver is a fairly easy utility that waits till the show supervisor gdm3 is operating, after which masses an unknown kernel module from /choose/rootkit_loader.ko by way of 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 absolutely booted up.

Whereas we can’t verify whether or not the dropper is in some way 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 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 fascinating transfer ahead within the UEFI risk panorama, breaking the idea about fashionable UEFI bootkits being Home windows-exclusive threats. Though the present model from VirusTotal doesn’t, for the time being, characterize an actual risk to nearly all of Linux techniques, it emphasizes the need of being ready for potential future threats.

To maintain your Linux techniques protected 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 threatintel@eset.com. 

ESET Analysis presents non-public APT intelligence reviews 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 methods

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