Ostap Downloader

What is Ostap?

Ostap is a JavaScript downloader used by cyber criminals to spread TrickBot, a trojan-type malicious programs that steals various personal, sensitive information. Cyber criminals use the details generate revenue in various ways. If you believe that your computer is infected with Ostap downloader (dropper) or Trickbot, remove these malicious programs immediately.

Ostap malware

Research shows that cyber criminals proliferate Ostap via spam campaigns. They send emails that contain a malicious Microsoft Office Word document that, when opened, asks permission to enable content and editing. If permitted, it then injects Ostap, which downloads and runs/installs TrickBot.

This is a malicious program designed to steal personal, confidential information by hijacking browsers and modifying websites displayed by them. TrickBot then collects entered logins and passwords and autofill data. It can also be used to steal credentials of cryptocurrency wallets, PayPal, banking, emails and other accounts.

The stolen details can be used to make transactions and purchases, trick other people into making money transactions, proliferate malware via spam campaigns, etc. TrickBot is also capable of recording information including browsing data (e.g., history, cookies) and system information (e.g., CPU, operating system, running processes, etc.).

This malicious program is also capable of updating itself. Therefore, all victims with an internet connection will have the latest version of TrickBot installed on their operating systems.

Research shows that newer TrickBot versions can be used to steal PIN codes from users of Verizon Wireless, T-Mobile, and Sprint. Furthermore, TrickBot can proliferate itself further and infect other computers.

Threat Summary:
Name	Ostap dropper
Threat Type	Malware dropper/downloader.
Detection Names	Arcabit (JS:Trojan.Cryxos.DB98), BitDefender (JS:Trojan.Cryxos.2968), Emsisoft (JS:Trojan.Cryxos.2968 (B)), Kaspersky (HEUR:Trojan.MSOffice.SAgent.gen), Full List (VirusTotal)
Payload	TrickBot
Symptoms	Trojans are designed to stealthily infiltrate the victim's computer and remain silent, and thus no particular symptoms are clearly visible on an infected machine.
Distribution methods	Infected email attachments, malicious online advertisements, social engineering, software 'cracks'.
Damage	Stolen passwords and banking information, identity theft, the victim's computer added to a botnet.
Malware Removal (Windows)	To eliminate possible malware infections, scan your computer with legitimate antivirus software. Our security researchers recommend using Combo Cleaner. Download Combo Cleaner To use full-featured product, you have to purchase a license for Combo Cleaner. 7 days free trial available. Combo Cleaner is owned and operated by RCS LT, the parent company of PCRisk.com.

Computers infected with Ostap lead to installation of TrickBot, which can cause financial loss, identity theft, problems with safety, browsing safety, and other serious problems. Therefore, if you believe that your computer is infected with Ostap or TrickBot, these malicious programs should be removed immediately.

More examples of malware droppers/downloaders are ZLoader, Legion Loader and Buer Loader.

How did Ostap infiltrate my computer?

Cyber criminals proliferate Opstap through spam campaigns. They send emails that contain a malicious Microsoft Word attachment, which, if opened, asks permission to enable content/editing. If allowed, it then infects systems with Ostap, which installs TrickBot.

More examples of files that cyber criminals attach to their emails are PDF documents, executable files (.exe), JavaScript files, and archive files (ZIP, RAR). Other ways to proliferate malicious software include the use of untrusted software download channels, Trojans, fake updaters and unofficial activation tools.

Examples of untrusted download channels are Peer-to-Peer networks (e.g., torrent clients, eMule), unofficial websites, third party downloaders (and installers) free file hosting pages, and freeware download sites. Cyber criminals use these to host malicious files that, if downloaded and opened, install malware.

Trojans are malicious programs that often infect systems with other programs of this kind - they cause chain infections by installing malicious programs.

Fake updaters infect operating systems by exploiting bugs/flaws of installed outdated software, or simply by installing malicious software rather than updates and fixes. Unofficial activation tools supposedly bypass activation of licensed software, however, they often distribute/install malicious programs.

How to avoid installation of malware

Do not trust or open links or attachments received in irrelevant emails, especially if the messages are received from unknown, suspicious addresses. Software and files should be downloaded from official websites and via direct download links. None of the other sources mentioned above should be trusted.

Installed programs must be activated and updated through implemented functions/tools that are designed by official software developers. Various third party activation ('cracking') tools should not be used to activate software - this is illegal and can cause installation of high-risk malware.

Keep systems safe by scanning them regularly for viruses with reputable antivirus or anti-spyware software. 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.

Malicious attachment distributing Ostap:

Malicious attachment distributing Ostap malware

Malicious Microsoft Excel document ("request_contr-802079359.xlsm") used to inject Ostap downloader into the system:

Malicious MS Excel file - request_contr-802079359.xlsm - used to inject Ostap downloader

Another malicious MS Excel document used to spread Ostap Downloader:

Malicious MS Excel document designed to inject Ostap Downloader into the system

Example of a spam email used to spread Ostap downloader:

Spam email used to spread Ostap downloader

Text presented within:

Subject: Copy of: Case 924663: invoice #866119 is suspended

Hello Dear valued partner,

Statement approval #74369

Best regards,
Matthies Crump

Screenshot of the attached malicious MS Excel document:

Malicious MS Excel document spreading Ostap downloader (2020-10-15)

Yet another malicious MS Excel document used to inject Ostap downloader:

Ostap downloader-injecting malicious MS Excel document

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

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.

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.

Run Windows 11 in Safe Mode with Networking

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

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.

Process Explorer showing a suspicious unsigned process running from %AppData%

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.

Autoruns 14.x with Hide Microsoft Entries enabled showing the Logon and Scheduled Tasks tabs

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.

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

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.

Combo Cleaner full scan results showing detected threats

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.

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

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.

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

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:

Primary email first - it's the recovery hub for everything else
Banking and financial accounts
Password manager master password (if you use one)
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.

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.

Verifying malware removal in the Windows 11 Task Manager Startup apps tab

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.