Threat description


Category: Malware
Type: Trojan
Platform: W32
Aliases: Rootkit:W32/ZAccess.[variant], Trojan:W32/ZAccess.[variant], Trojan:W64/ZAccess.[variant], Virus:W32/ZAccess.[variant], Virus:W64/ZAccess.[variant], Trojan.Sirefef.[variant], Gen:Variant.Sirefef.[variant]


Rootkit:W32/ZAccess constantly displays advertisements on the infected machine and may silently contact remote servers to retrieve additional advertising information.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

Manual action

Perform a full scan of the computer system with the F-Secure security product to find and disinfect the relevant files. Before scanning, enable the product to scan system files by following these steps:

  • In the F-Secure product, go to "Settings"
  • Go to "Manual scanning"
  • Uncheck "Scan only known file types"

Then perform a full computer scan.

More scanning & removal options

More information on scanning and removal options available in your F-Secure product can be found in the Help Center.

Contact Support

For further assistance, F-Secure customers can request support online via the Request support or the Chat forms on our Home - Global site.

You may also refer to the Knowledge Base on the F-Secure Community site for more assistance.

Suspect a file is malicious?

If in doubt, or in cases where a legitimate file is suspected to contain malicious code, please send a sample to F-Secure Security Labs via the Submit A Sample (SAS) page for analysis. You may want to refer to the following Support articles for more details:

Or Contact Support for further assistance.

Suspect a False Alarm (FA)?

Occassionally, a legitimate program or file containing code sufficiently similar to a known malware signature will inadvertently trigger a False Alarm or False Positive.

For example, 'tmp.edb' and other '.edb' files stored at the location 'C:\WINDOWS\SoftwareDistribution\DataStore\Logs\' may be unintentionally detected as malicious by various security programs.

If the suspect file is known to be legitimate, it may be excluded from scanning with the following instructions:

Microsoft also provides enterprise-level instructions for excluding the file in question from scanning by antivirus software:

Technical Details

Rootkit:W32/ZAccess is distributed in programs disguised as a cracked copies of legitimate commercial software (a distribution method more commonly seen with trojans). It may be distributed via pornography websites. It has also been reported as being distributed via compromised legitimate websites.

Once installed, ZAccess will display unsolicited advertisments on the machine and redirects the web browser to unsolicited advertisement websites. In order to do the latter, the malware may contact remote servers to retrieve additional advertising information; the specific URLs contacted are varied, but the top level domain is always .CN.


On installation, the malware will replace a system driver (in the location %WINDIR%\system32\drivers) with a copy of its rootkit driver. Selection of a system driver to replace is done using an internal algorithm. The rootkit will display the contents of the original system driver, presumably to camouflage its presence on the system.

The malware then creates a hidden, encrypted volume which is used to store the original system driver file that was replaced, as well as other component files used by the malware. The hidden volume has the following format:

  • %WINDIR%\$NtUninstallKB{5_digits}$

It also creates a zero byte Alternate Data Stream file in %WINDIR%\{numbers_only_file} which will be silently executed and run in the background:

Some variants of this malware will drop other component files in the following location:

  • %USERPROFILE%\Local Settings\Application Data\number_only_folder


Suspect a file or URL was wrongly detected? Submit a sample to our Labs for analysis

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