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mosquito.exe is not automatically safe or malicious. The filename is used by the Mosquito Win32 backdoor associated with Turla, and Microsoft has documented a TrojanDownloader:Win32/Murlo.S infection that drops a file with this name in a temporary folder. A Siemens SIMATIC eaSie manual also tells users to start an MQTT broker with mosquito.exe from C:Program Filesmosquitto. Treat any unexpected copy as suspicious until you verify its complete path, publisher signature, origin and security-scan result. Do not open it, restore it from quarantine or add an antivirus exclusion while those checks are incomplete.
What is mosquito.exe?
mosquito.exe is simply a Windows executable filename. A name by itself does not identify the publisher, the program that installed it or whether it is safe. Two documented, very different uses explain the confusion.
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A known malware family uses the name
MITRE ATT&CK lists Mosquito as a Win32 backdoor used by Turla (software ID S0256). The MITRE record was created on 17 October 2018 and last modified on 11 April 2024. Its documented capabilities include registry run-key persistence, PowerShell and command-shell execution, encrypted communications, COM hijacking, process discovery and discovery of security software. Those behaviors are characteristics of the malware family, not proof that every file named mosquito.exe is that backdoor.
Microsoft has documented a separate malicious dropper pattern
Microsoft describes a TrojanDownloader:Win32/Murlo.S infection that creates mosquito.exe in a temporary directory and silently installs additional programs. A copy in a user-writable temporary folder is therefore a high-priority warning sign, although path alone is not conclusive.
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A legitimate industrial-software reference also exists
A Siemens SIMATIC eaSie manual (V20.07 SP2, December 2020) describes an MQTT broker installed under C:Program Filesmosquitto and instructs the operator to run mosquito.exe. This documents a legitimate software context; it is not a blanket approval for every file with that spelling. Verify the installed Siemens application, its installer source, the file’s signer and its hash before allowing execution.
| Context | What the source documents | What you should conclude |
|---|---|---|
| MITRE Mosquito (S0256) | Win32 backdoor used by Turla; persistence, command execution, encrypted communications and discovery behaviors are listed. | A matching detection or unexplained copy requires malware triage. |
| Microsoft Murlo.S case | A TrojanDownloader creates mosquito.exe in a temporary directory and installs additional programs. |
An unexpected temporary-folder copy is especially suspicious. |
| Siemens SIMATIC eaSie V20.07 SP2 | An MQTT broker can be started with mosquito.exe from C:Program Filesmosquitto. |
A known industrial installation may be legitimate, but still verify signature and origin. |
Is mosquito.exe safe or a virus?
You cannot answer from the filename alone. Safety depends on the file’s location, digital signature, how it arrived, what security product detected and whether its hash matches a trusted vendor release.
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- The file is in
%TEMP%, a download folder, a user profile or another location where ordinary programs can write without administrator approval. - Windows Security reports the Mosquito family, Murlo.S or another malware detection.
- The executable has no valid digital signature, an unknown publisher or a signature that does not match the application you installed.
- You did not install an MQTT, Siemens or other program that would reasonably contain it.
- A new startup entry, scheduled task, PowerShell command or command-shell process appears at the same time as the file.
- The file repeatedly returns after deletion, or security software reports encrypted outbound communications or attempts to discover security products.
Evidence that can support a legitimate explanation
- The file is inside the expected installation directory of a software package you knowingly installed.
- Windows shows a valid signature from the expected publisher and the signature is current and intact.
- The application’s own documentation identifies that executable, and the file hash matches the vendor’s supplied value.
- The file was installed by an official installer rather than a crack, email attachment, unsolicited download or file-hosting site.
These are triage indicators, not guarantees. A signed file can be misused and a legitimate program can be repackaged. Keep a detection quarantined until the publisher and origin are trusted.
How do I investigate a mosquito.exe file without running it?
Use Windows Security and file properties to collect evidence while preventing execution. Do not double-click the file, choose “Allow on device,” create an exclusion or download a replacement executable from an unofficial site.
- Open the alert details. In Windows Security, select Virus & threat protection, then Protection history. Record the detection name, action taken and timestamp.
- Record the complete path. Expand the event details and copy the full location, including the drive letter and every folder. A temporary-folder path and an expected program directory have very different implications.
- Check publisher information. In File Explorer, right-click the file, choose Properties and inspect the Digital Signatures tab when present. Select the signature and use Details to confirm that Windows reports it as valid and that the signer matches the software you installed.
- Capture a hash if available. A hash lets a vendor or qualified responder compare the exact binary without you distributing or launching it. Do not treat a hash found in an untrusted forum as proof of safety.
- Identify the parent application. Check Settings > Apps > Installed apps for software installed at the same time. If you find a Siemens SIMATIC eaSie installation, confirm that its documentation and installation directory account for the executable.
- Leave the item quarantined. Microsoft explains that quarantine moves a file to a safe location and blocks it from running. Restore it only after the publisher, source and hash are trusted; otherwise keep it blocked while scanning.
Antivirus product names and signatures change. A label that differs from “Mosquito” or “Murlo.S” does not automatically make the file safe or malicious, so evaluate the underlying path and evidence as well as the label.
How do I remove a malicious mosquito.exe infection?
Use the built-in Microsoft workflow first. It is safer than manually deleting an executable that may have persistence mechanisms or related files.
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1. Update Windows and Defender
Install pending Windows updates, then open Windows Security > Virus & threat protection > Protection updates and select Check for updates. Current security intelligence improves the chance of recognizing changed malware samples.
2. Run a full Microsoft Defender scan
In Windows Security > Virus & threat protection > Scan options, select Full scan and start it. Microsoft recommends a full scan when malware is suspected because it checks all locations rather than only the most common ones. Save work first; the scan can take substantially longer than a quick scan.
3. Use Microsoft Defender Offline if the alert returns
If the file reappears, cannot be removed or is detected again after a restart, choose Microsoft Defender Offline scan in the same Scan options screen. Windows restarts into a separate scanning environment before the normal desktop loads, which can make it harder for persistent malware to interfere. Close applications and keep the computer connected to power before starting.
4. Run Microsoft Safety Scanner for a difficult or partial removal
Download the current Microsoft Safety Scanner only from Microsoft, run it on demand and choose a full scan. Microsoft says the downloaded scanner expires 10 days after download, so obtain a fresh copy whenever you need it rather than relying on an old installer. It complements real-time protection; it is not a replacement for an up-to-date antivirus product.
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5. Consider the Malicious Software Removal Tool
Microsoft’s Malicious Software Removal Tool (MSRT) is another official option for prevalent malware. Microsoft says it is generally delivered monthly and does not replace full antimalware protection. The Microsoft Download Center listed MSRT version 5.146, published 13 October 2026; check the current release before running it because versions change.
6. Restart, update and rescan
Restart after removals, apply any remaining updates and run another Defender scan. Review Protection history to confirm whether the same path is still being detected. Do not restore a quarantined copy merely because an application now reports that a file is missing.
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7. Protect accounts if compromise is possible
If the computer handled sensitive work while the suspicious file was active, disconnect it from business networks and avoid signing in to important accounts on that machine. From a known-clean device, change passwords and review account activity. For a system that remains compromised after Defender and offline scans, seek qualified malware-removal assistance; the appropriate escalation depends on the evidence, the accounts used and the organization’s incident-response requirements.
How do I remove mosquito.exe from Windows startup?
First quarantine and scan the file. Disabling an autostart entry without removing the underlying malware can hide symptoms while leaving the infection in place.
Review ordinary startup entries
- Open Task Manager with
Ctrl+Shift+Esc, select Startup apps and sort by name or startup impact. - In Settings > Apps > Startup, compare entries with the path and publisher you recorded. Disable only an entry that clearly points to the suspicious file or to software you do not recognize.
- Restart and run another full scan. If the entry returns, do not keep toggling it; investigate scheduled tasks, services or registry persistence with professional help.
Handle registry entries cautiously
MITRE lists registry run-key persistence for the Mosquito backdoor. Do not delete random values from the registry. If a trusted responder asks you to inspect it, export a backup first and examine the standard Run and RunOnce locations only under that guidance. A wrong edit can stop legitimate software or Windows components from starting.
What should I do when “mosquito.exe not found” appears?
The message can mean that a legitimate application was incompletely uninstalled, a quarantine action removed a file, or a malicious startup reference is left behind. The message alone does not identify which case applies.
- Note which application displays the message and when it appears: at sign-in, during a specific program launch or after a scheduled action.
- Check Windows Security > Protection history for a corresponding quarantine event. If Defender removed the file, do not restore it until its publisher and origin are verified.
- In Settings > Apps > Installed apps, repair or reinstall the associated legitimate application using its official installer. Do not download a stand-alone
mosquito.exefrom a file-hosting site. - Review Task Manager > Startup apps and any vendor-provided startup settings for a stale reference. Disable the reference only after scanning the computer.
- Run a full Defender scan and, if the alert or popup persists, Defender Offline.
How can I fix a mosquito.exe application error?
An application error is a symptom, not a diagnosis. First determine whether the executable belongs to a known application; repairing a legitimate installation is appropriate, while attempting to make a suspicious binary run is unsafe.
If the file belongs to known software
- Confirm the exact installation directory and publisher signature.
- Use the application’s documented repair or reinstall process and obtain the installer from the official vendor.
- Install pending Windows updates and restart before testing again.
- If the error began after a software update, use the vendor’s supported rollback or repair option rather than copying an executable from another computer.
If Windows Security flags it
Do not troubleshoot by granting an exclusion or restoring the file. Keep it quarantined, run the full and offline scans described above and investigate the parent application only after the system is clean.
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If the error appears with no visible file
A stale startup, scheduled-task or service reference may be calling a file that has already been removed. Record the exact message, scan results and triggering application. Removing the reference safely may require an administrator or malware-removal specialist, particularly when persistence or COM hijacking is suspected.
When should I stop troubleshooting and get professional help?
- Defender or Defender Offline reports that removal failed, or the same file returns after reboot.
- Multiple unknown programs, browser changes, new administrator accounts or unexplained network activity appear.
- The computer contains business, financial, medical or other sensitive information.
- You cannot establish the file’s publisher and installation source, or the file is protected by a process you do not recognize.
- Scanning tools disagree and you are considering registry edits, manual deletion of system files or restoring the quarantine item.
Preserve detection names, paths, timestamps and scan reports for the responder. Avoid repeatedly launching the file or experimenting with unofficial “fix” downloads, which can destroy useful evidence or add another unwanted program.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.FAQ
Can a legitimate Mosquitto MQTT installation trigger this concern?
Yes. The Siemens SIMATIC eaSie documentation uses mosquito.exe for an MQTT broker under C:Program Filesmosquitto. Confirm that the Siemens software is actually installed and that the executable’s signature and hash correspond to that installation before allowing it.
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Does quarantine delete mosquito.exe permanently?
Not immediately. Microsoft describes quarantine as moving the file to a safe location and blocking execution, allowing you to review the detection before choosing a final removal action. Keep it quarantined while you verify the publisher and complete scans.
Why does the detection name differ between security products?
Security vendors use their own naming schemes and update signatures at different times. A different label can still refer to the same behavior or sample, so compare the path, detection details, signer and scan results rather than relying on a single family name.
Can I submit the file to an online scanner?
Only do so under your organization’s security policy and never upload confidential or proprietary binaries. Preserve the file in quarantine and ask a qualified responder or the software publisher for a trusted hash comparison instead of downloading a replacement executable.
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What if the computer is offline and I cannot update Defender?
Do not reconnect it to sensitive networks just to test the executable. Use a clean computer to obtain official Microsoft remediation media or guidance, transfer it according to your organization’s procedures and scan the affected system before normal use.
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Can a legitimate Mosquitto MQTT installation trigger this concern?
Yes. The Siemens SIMATIC eaSie documentation uses mosquito.exe for an MQTT broker under C:Program Filesmosquitto. Confirm that the Siemens software is actually installed and that the executable’s signature and hash correspond to that installation before allowing it.
Does quarantine delete mosquito.exe permanently?
Not immediately. Microsoft describes quarantine as moving the file to a safe location and blocking execution, allowing you to review the detection before choosing a final removal action. Keep it quarantined while you verify the publisher and complete scans.
Why does the detection name differ between security products?
Security vendors use their own naming schemes and update signatures at different times. A different label can still refer to the same behavior or sample, so compare the path, detection details, signer and scan results rather than relying on a single family name.
Can I submit the file to an online scanner?
Only do so under your organization’s security policy and never upload confidential or proprietary binaries. Preserve the file in quarantine and ask a qualified responder or the software publisher for a trusted hash comparison instead of downloading a replacement executable.
What if the computer is offline and I cannot update Defender?
Do not reconnect it to sensitive networks just to test the executable. Use a clean computer to obtain official Microsoft remediation media or guidance, transfer it according to your organization’s procedures and scan the affected system before normal use.
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