SymbOS/Locknut.A, Trojan:SymbOS/Locknut.A


Trojan:SymbOS/Locknut.A drops binaries that will crash a critical system component, preventing application from being launched in the phone and effectively locking the phone.

Disinfection with two Series 60 phones

Download F-Locknut tool from or directly with phone

  • 1. Install F-Locknut.sis into infected phones memory card with a clean phone
  • 2. Put the memory card with F-Locknut into infected phone
  • 3. Start up the infected phone, the application menu should work now
  • 4. Go to application manager and uninstall the SIS file in which you installed the locknut variant
  • 5. Download and install F-Secure Mobile Anti-Virus ( to remove any possibly Cabirs dropped by the locknut variant, or with mobile itself (
  • 6. Remove the F-Skulls with application manager as the phone is now cleaned

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.

For more Support


Find the latest advice in our Community.

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

Trojan:SymbOS/Locknut.A is a malicous SIS file trojan that pretends to be patch for Symbian Series 60 mobile phones. It is distributed in files named patch_v1.sis and patch_v2.sis.

Locknut.A will only work on devices running Symbian OS 7.0S or newer; devices using Symbian OS 6.0 or 6.1 are unaffected.

Locknut is targeted against Symbian Series 60 devices, but also series 70 devices, such as Nokia 7710 are vulnerable to Locknut. However when trying to install Skulls trojan on Nokia 7710, user will get a warning that the SIS file is not intended for the device, so risk of accidental infection is low.


When Locknut.A sis file is installed the files will be installed into following locations:

  • c:\system\apps\gavno\
  • c:\system\apps\gavno\gavno.rsc
  • c:\system\apps\gavno\gavno_caption.rsc

The Locknut.SIS will will also contain copy of itself that is copied into C:\ directory

When installed Locknut.A, drops binaries that will crash a critical system component, preventing application from being launched in the phone and effectively locking the phone.

There are also claims that Locknut would disable calling functionality, so that user couldn't make calls with infected phone. But we could not reproduce this effect with any phones we have.


Both versions of Locknut.A replace a critical system binary; the patch_v2.sis file will also drop Cabir.B, which will not be able to start on the phone.


There are also versions of Locknut that include Cabir.B in same SIS file (some AV vendors name this variant Gavno.B), but since the actual trojan functionality is totally identical to Locknut.A we call both samples Locknut.A

The Cabir.B included in the Locknut.A samples is harmless as the Locknut kills all applications on the infected phone, including Cabir.B that is installed from the same SIS file. Even if Locknut.B is disinfected the Cabir.B still won't start, as it is installed into wrong directory in the infected phone.

If user starts Cabir.B manually, after disinfecting the Locknut program, Cabir.B will spread independently according to its program - i.e., it will not transfer Locknut.A into other devices.


This trojan was originally named Gavno, but since this word is also a rather vulgar term in Russian, the AV community has decided to rename it as Locknut.