Trojan-Downloader:JS/Downloader.IOJ is a generic detection for malicious JavaScript code that silently downloads and executes malware on the affected machine.

Security programs use generic detections that look for broad patterns of code or behavior to identify similar programs or files. If you suspect the file was incorrectly detected, go to: Removal: Suspect a file is incorrectly detected (a False Positive)?.

Automatic action

Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.

Suspect a file is incorrectly detected (a False Positive)?

A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:

  • Check for the latest database updates

    First check if your F-Secure security program is using the latest detection database updates, then try scanning the file again.

  • Submit a sample

    After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.

    NOTE If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.

  • Exclude a file from further scanning

    If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.

    Note You need administrative rights to change the settings.

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Submit a sample

Submit a file or URL for further analysis.

Technical Details

The JavaScript code is commonly sent embedded in a file attached to email messages. The file is specially crafted to look like a legitimate resume, with the filename "My Resume" or "My Resume – [random name]".

The JavaScript code itself is obfuscated, making it impossible for the normal user to read the contents. If the user opens the attachment, the JavaScript code is launched.

Obfuscated Trojan-Downloader:JS/Downloader.IOJ JavaScript file

In the sample analyzed (SHA1: 158711c9decb320ce4089fbbb5624f8bd2b9268a), the JavaScript code first runs a preliminary check with WScript; if the check is passed, the malware attempts to contact a remote server (its command and control or C&C). If successful, the script will then silently download additional files to the %TEMP% folder.

Files downloaded by Trojan-Downloader:JS/Downloader.IOK

The files served by the C&C server for download are disguised using the JPG file extension associated with image files. They are however executable files, and once downloaded, the JavaScript executes them using the EXE file extension via the WScript.Shell command.

At the time of writing, the C&C URL is no longer accessible. Further analysis shows that the downloaded files are variants from multiple ransomware families. The downloaded files are separately identified by Generic Detections.