5lo

Classification

Malware

Virus

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5lo

Summary

The first discovery of this virus was made in late October, 1992.

Removal

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.

Find out more

Knowledge Base

Find the latest advice in our Community Knowledge Base.

User Guide

See the user guide for your product on the Help Center.

Contact Support

Chat with or call an expert for help.

Submit a sample

Submit a file or URL for further analysis.

Technical Details

The virus increases file size with approx. 1000-1100 bytes. A typical value for this is 1032 bytes. Directly at the end of a infected file is found the string "92.05.24.5lo.2.23MZ". This is the reason for the odd name of this virus. Also, the virus carries the name of the infected file within itself. Strings "????????.EXE" and "*.EXE" are visible inside the virus code as well.

The virus stays resident and infects only EXE files. Whenever a EXE file is ran the virus infects it and also one other EXE file. The virus leaves its infection date and time into the directory entry, which makes it easier to trace the virus path of infections.

The virus increases a counter within itself after every infection. The counter is never checked, so the virus never activates. It is well written and contains code which makes it impossible to disassemble automatically with programs such as Sourcer and DIS-DOC. This slows down the examination of the code a bit.

The virus appends its code at the end of all infected files. It recognises infected files by changing the field 0Ch in the EXE header (maximum paragraphs to allocate in addition to executable's size) to value of FFAAh. Also, the virus identifies itself from memory by using an interrupt call of INT 21, AX=3521h which it has hooked. All the checks work correctly and the virus won't infect files multiple times and it installs itself to memory only once.

When the virus is active in the memory it is not discoverable by using the command MEM /C. This is because the virus installs itself seemingly to be part of the operating system. Free memory decreases by approx. two kilobytes, though.

5lo does not do anything else than spreads.