How to remove the RustyBuer malware?

What is RustyBuer malware?

RustyBuer is the name of a new variant of the Buer loader. The main difference between the new and the original variants is that one is written in Rust multi-paradigm programming language, and another one is written in C programming language. Both RustyBuer and Buer function as loaders - they infect computers with other malware.

RustyBuer in detail

It is likely that RustyBuer and the original version are written in different languages because by developing the new variant in Rust language, cybercriminals give it the ability to evade anti-malware detections that are based on features of the malware written in the C language.

RustyBuer has various options on how to download and execute a payload (install malicious software). Some of RustyBuer's samples were (or still are) used to distribute Cobalt Strike Beacon as a second-stage payload, which means cybercriminals may be selling access to infected machines on the dark web, hacker forums.

It is known that RustyBuer was already used to proliferate more than one ransomware strain. Ransomware is a type of malware that encrypts data (locks files) and generates a ransom demanding message with instructions on how to pay for data decryption.

It is worth mentioning that RustyBuer can check for installed virtual machines to avoid being examined by malware analysts. It is very likely that cybercriminals behind RustyBuer are continuing to improve their malware, so it could be more difficult to detect by security vendors.

To summarize, it is known that the new Buer loader variant was already used to distribute Cobalt Strike Beacon and multiple ransomware strains. Sine RustyBuer has numerous options on how to download and execute a payload, this malware could be used to distribute other malicious software as well.

More examples of malware designed to distribute other software of this kind are BazarLoader, HimeraLoader, and AbSent-Loader. It is common that loaders are capable of evading detection, which allows them to distribute malware without being detected and removed by installed anti-virus solutions.

RustyBuer in general

To summarize, it is known that the new Buer loader variant was already used to distribute Cobalt Strike Beacon and multiple ransomware strains. Sine RustyBuer has numerous options on how to download and execute a payload, this malware could be used to distribute other malicious software as well.

More examples of malware designed to distribute other software of this kind are BazarLoader, HimeraLoader, and AbSent-Loader. It is common that loaders are capable of evading detection, which allows them to distribute malware without being detected and removed by installed anti-virus solutions.

How did RustyBuer infiltrate my computer?

Research shows that RustyBuer is distributed via emails disguised as letters regarding COVID-19 vaccination information. It is not the first time when cybercriminals are taking advantage of the coronavirus disease to extract personal information, money or distribute malicious software.

As written in this particular email, CDC and FDA have compiled a complete summary containing information regarding contraindications, complications that may be caused by vaccines available in the United States. The main purpose of this email is to trick recipients into opening the attached Microsoft Excel document and enabling macros commands in it (enabling editing/content).

When users enable macros in malicious MS Office documents, they allow those documents to start the infection chain. It is important to mention that malicious documents opened with older MS Office versions (released prior to the year 2010) do not have the "Protected View" mode - they infect computers automatically.

It is worth mentioning that cybercriminals can use PDF documents, JavaScript files, executable files like EXE, archive files like ZIP, RAR, and other files to deliver malware via email as well. Typically, they disguise their emails as important, official letters from legitimate companies, organizations (for example, CDF, FDA).

How to avoid installation of malware?

Software and files should be downloaded from legitimate websites and via direct links. It is recommended to avoid using Peer-to-Peer networks, unofficial websites, third-party downloaders, unofficial sites, third-party installers, and other sources of this kind to download or install programs.

The operating system and installed on it have to be updated and activated with implemented functions or tools provided by their official developers. It is common that unofficial activation ('cracking') tools and third-party updaters to have malware hidden inside them. Besides, it is not legal to activate licensed programs with various third-party tools.

Attachments and website links in irrelevant emails received from suspicious, unknown senders should not be opened without carefully examining them first. It is common that cybercriminals use such emails as tools to deliver their malware. It is worth mentioning that they often disguise their emails as important, official, and so on.

Additionally, is is recommended to have a reputable antivirus or anti-spyware software installed on the operating system and run virus scans with it regularly. If you believe that your computer is already infected, we recommend running a scan with Combo Cleaner Antivirus for Windows to automatically eliminate infiltrated malware.

Email containing file designed to install RustyBuer:

Text in this email:

Subject: COVID-19 Vaccination information

URGENT INFORMATION ON COVID-19 VACCINATION

This email contains urgent information on latest researches from CDC and FDA in the field of vaccines against COVID-19.

Internal statistics from CDC and FDA on lethal cases and illness after vaccination (read more in attached file), we have not fully studied their real danger. Wide selection of vaccines offer has created illusion of security and feeling that we have a choice.

Studies have shown that possibility of getting infected COVID-10 after a vaccine significantly increases, and also leads to complications in the form of infertility, development of diseases of lungs and central nervous system, and even blindness.

Some prisons have unofficially introduced vaccines to prisoners, those sentenced to life sentences or death penalty. Outbreaks of COVID-19 diseases and even deaths account for areas where people were massively vaccinated. You can avoid risk of getting infected by untested vaccines.

CDC and FDA together have compiled a complete summary of contraindications, chances of death outcome, and complications from use of available vaccines in the US (such as Pfizer-BioNTech, Moderna, Johnson & Johnson / Janssen, etc.) and other countries, as well as the price you will pay for them. Compare conditions under which certain vaccines are not suitable for you, as well as those which are contraindicated for children, pregnant women, elderly people, people with chronic diseases and allergy sufferers. Information valid as of May 2021.

You were assigned personal access to the attached document, your pass code: ohpfdpfjlqysasyl

Sent by American Medicine Publicity, 250 Willow Drive San Pablo, CA 94806,
info@cov19inf.com Unsubscribe

Malicious attachment distributing RustyBuer:

How to remove malware manually?

Most trojan infections can be removed by running a full system scan with Combo Cleaner. The full manual procedure below is for cases where automatic removal failed, where you want to verify a scan caught everything, or where the threat has multiple persistence points.

Tools you'll need

Combo Cleaner Antivirus 7-days free trial

Recommended primary scanner. VB100 certified. The 7-day free trial is available.

Microsoft Defender Offline Free · built-in

Built into Windows. Boots to a clean environment to catch rootkits.

Process Explorer Free

From Microsoft Sysinternals. Better than Task Manager - shows full paths and signatures.

Autoruns Free

From Microsoft Sysinternals. Lists every place malware can persist on your system.

Microsoft Safety Scanner (MSERT) Free

Official Microsoft on-demand scanner. Always-current signatures - expires every 10 days.

Phase 1~ 5 min

Contain - limit the damage

---

1

Disconnect from the network

30 secNo tool needed

Unplug your Ethernet cable and turn off Wi-Fi. If you're on a laptop, the airplane-mode key works fastest.

Why this matters

Many trojans exfiltrate data - saved browser passwords, banking sessions, screenshots - and accept remote commands. Cutting the network stops both immediately. Keep it disconnected until Phase 4.

2

Boot into Safe Mode with Networking

3 min

Safe Mode loads a minimal set of drivers and stops most malware from auto-running. "With Networking" lets you download tools - re-enable Wi-Fi only after you've booted in.

Windows 11

Open Settings → System → Recovery. Click Restart now next to Advanced startup.

Your computer will restart into the "Choose an option" menu. Click Troubleshoot → Advanced options → Startup Settings → Restart. After the second reboot, press 5 or F5.

Windows 10

Click the Windows logo, hold Shift, click the Power icon and choose Restart. In the Choose an option screen, click Troubleshoot → Advanced options → Startup Settings → Restart. Press F5.

Windows 8

Go to the Start Screen, type Advanced, select Settings → Advanced startup options → Restart now → Troubleshoot → Advanced options → Startup settings → Restart → F5.

Note: Windows 8 reached end-of-life in January 2023 - running on it puts you at much higher risk of reinfection.

Windows 7

Click Start → Shut Down → Restart → OK. As the system boots, press F8 repeatedly until the Windows Advanced Options menu appears, then choose Safe Mode with Networking.

Note: Windows 7 reached end-of-life in January 2020 - running on it puts you at much higher risk of reinfection.

Phase 2~ 15 min

Identify - find what's hiding

---

3

Inspect running processes with Process Explorer

Tool: Process Explorer5 min

Download Process Explorer from Microsoft Sysinternals. Run it as Administrator. Click Options → Verify Image Signatures and View → Lower Pane View → DLLs.

Sort by "Verified Signer." Anything unsigned running from %AppData%, %LocalAppData%, %Temp%, or %ProgramData% is suspicious. Right-click → Properties to see the full image path. Right-click → "Search Online" to look up unfamiliar process names.

Why not just Task Manager

Task Manager hides full file paths, doesn't show digital signatures, and groups processes in ways that make malware easy to miss. Process Explorer is the same tool Microsoft engineers use.

4

Map every persistence point with Autoruns

Tool: Autoruns 14.x10 min

Download Autoruns. Right-click Autoruns.exe and choose Run as administrator - without elevation, Autoruns can't inspect protected registry hives.

In the menu, open Options → Hide Microsoft Entries and Options → Hide Empty Locations, then click Refresh.

Modern trojans don't just live in startup. Walk through these tabs and look for unsigned or unfamiliar entries:

• Logon - classic Run/RunOnce keys
• Scheduled Tasks - frequent hiding spot, especially tasks with PowerShell, MSHTA, or rundll32 actions
• Services - unsigned services in non-Microsoft folders
• WMI - Event Subscriptions are a stealthy persistence mechanism (MITRE T1546.003)
• Image Hijacks - Image File Execution Options "Debugger" abuse
• AppInit - DLLs loaded into every GUI process
• Browser plug-ins - unsigned add-ons across all installed browsers

Pro tip

Right-click any suspicious entry → "Jump to Image" to see the actual file. Right-click → "Search Online" opens VirusTotal for the file hash. Don't delete yet - just write down the file paths and entry names. We'll remove them after the scans.

5

Check the hosts file and DNS settings

3 minBuilt into Windows

Trojans often redirect bank, antivirus, and update domains to attacker-controlled servers by editing the hosts file or DNS settings.

Hosts file: Open Notepad as Administrator and open C:\Windows\System32\drivers\etc\hosts. Legitimate entries are commented with #. Any uncommented IP-to-domain mapping is suspicious - especially redirects for microsoft.com, windowsupdate, or your bank.

DNS settings: Open Command Prompt as Administrator and run ipconfig /all. Confirm your DNS servers point to your ISP, your router, or a known public resolver (Cloudflare 1.1.1.1, Google 8.8.8.8). Then open Settings → Network & internet → [your adapter] → DNS server assignment and verify both DNS and DNS over HTTPS (DoH) settings haven't been hijacked.

Why this matters

A clean PC with hijacked DNS is still a compromised PC. You'll think you're at chase.com and you'll be at the attacker's clone.

Phase 3~ 60-90 min

Remove - three scans, one cleanup

---

No single antivirus catches everything. The order below is intentional: offline scan first (catches rootkits before Windows fully loads), then on-demand scanners on the running system, then manual cleanup of anything they missed.

6

Run Microsoft Defender Offline

Tool: Built into Windows15 min

Reboot to normal Windows. Open Windows Security → Virus & threat protection → Scan options. Select Microsoft Defender Antivirus (offline scan) and click Scan now. Your computer will restart into a stripped-down environment and scan before the full operating system loads.

Why this comes first

Rootkits and bootkits hide from any scanner running on the same Windows session that's already infected. A pre-boot offline scan sees the disk before the malware can hide itself.

Recommended antivirus

Combo Cleaner

VB100 certified. Includes anti-trojan, registry/persistence scanning, anti-spyware in one pass. The 7-day free trial is available.

Download Combo Cleaner

7

Run Combo Cleaner full system scan

Tool: Combo Cleaner20-40 min

Install Combo Cleaner and run a full system scan (not the quick scan). Let it complete fully, review the detected threats, and apply the recommended actions. Combo Cleaner will quarantine known trojans and registry persistence.

8

Run Microsoft Safety Scanner as a second-opinion scan

Tool: Microsoft Safety Scanner15 min

Download Microsoft Safety Scanner (MSERT.exe), Microsoft's free on-demand tool. The binary expires every 10 days, which means every download has the latest signatures. Run a full scan after Combo Cleaner to catch anything one engine alone might miss.

Why a second scan

Different scanning engines catch different threats. Running an additional Microsoft scan after Combo Cleaner is the single most underused verification step in malware removal.

9

Manually remove anything still listed in Autoruns

Tool: Autoruns10 min

Reopen Autoruns. Anything you flagged in Step 4 that's still present hasn't been caught by the automated scans. For each remaining suspicious entry: right-click → Delete in Autoruns (this removes the persistence registration), then navigate to the file path and delete the actual file. Empty the Recycle Bin.

Important

Some malware files reappear after deletion because something else on the system recreates them. If you delete a file and it returns within minutes, you have a remaining persistence point that Autoruns didn't show - usually a service or scheduled task running under SYSTEM. Re-run the scans from Steps 6-8 or proceed to a clean install (see end of guide).

Phase 4~ 15 min

Recover - restore browsers and security

---

10

Reset browsers and clear notification permissions

5 min

Trojans and adware frequently hijack browsers - change the default search engine, install rogue extensions, or grant themselves notification permissions to keep displaying scam pop-ups even after the malware is gone.

• Chrome: visit chrome://settings/reset and click "Restore settings to their original defaults." Then visit chrome://settings/content/notifications and remove any unfamiliar sites.
• Edge: visit edge://settings/reset. Then check edge://settings/content/notifications.
• Firefox: open about:support and click Refresh Firefox (Firefox uses "Refresh," not "Reset"). Then check Settings → Privacy & Security → Permissions → Notifications.

Review installed extensions in each browser and remove anything you don't recognize, especially extensions you didn't install yourself.

11

Re-enable Defender, Tamper Protection, and update everything

10 min

Many trojans disable security software. Open Windows Security → Virus & threat protection → Manage settings and confirm Real-time protection, Cloud-delivered protection, and Tamper Protection are all turned on. Tamper Protection prevents malware from disabling Defender silently.

Then run Windows Update (Settings → Windows Update → Check for updates) and update your browsers and applications. The infection vector that let the trojan in originally usually involves outdated software.

Phase 5~ 15 min + 7 days

Verify & secure

---

12

Change passwords from a clean device

20 min⚠ Critical

If the trojan was an info-stealer (Lumma, RedLine, Vidar, and similar are extremely common), assume every password saved in your browser is compromised. Change them from a different, known-clean device - your phone is fine if it's not infected.

Recommended order:

1. Primary email first - it's the recovery hub for everything else
2. Banking and financial accounts
3. Password manager master password (if you use one)
4. Social media, shopping, and remaining accounts

Where available, enable phishing-resistant multi-factor authentication (passkeys / FIDO2 hardware keys / authenticator apps) - SMS-based MFA is bypassable by sophisticated attackers.

13

Verify the clean and monitor for 7 days

5 min + 1 week monitoring

Reconnect to the network and run one final full scan with Combo Cleaner. Open Task Manager (Ctrl + Shift + Esc) and switch to the Startup apps tab - confirm nothing suspicious has reappeared.

Over the next 7 days, watch for symptom recurrence: unexpected high CPU usage, unknown processes returning, browser redirects, or pop-ups. If anything returns, you missed a persistence point and the procedure needs to be repeated - or you may have a deeper compromise that requires a clean install.

When to clean install Windows instead

Manual removal isn't always the right answer. Reinstalling Windows from scratch is the only way to be 100% certain when:

• The malware reappears after every removal attempt
• A bootkit or UEFI rootkit was detected (this class of threat sits below the operating system and survives a normal Windows reinstall)
• A confirmed credential-theft trojan ran with administrator privileges (Lumma, RedLine, Vidar) - the system likely has multiple unknown payloads
• The PC is used for online banking, business email, or anything where partial doubt isn't acceptable

A clean install is a 30-60 minute process: Windows 11 reinstall via Settings → System → Recovery → Reset this PC → Remove everything → Cloud download. Back up documents to external storage first, scan the backup with Combo Cleaner before restoring it, and reinstall applications fresh - never restore them from a backup of the infected system.

Important: Manual removal requires advanced computer skills. If you're not confident identifying system files from malicious files, run an automated scan with Combo Cleaner and Windows Defender and stop there - that path catches the vast majority of consumer trojans without the risk of breaking Windows by deleting the wrong file.