Didier Stevens

Monday 19 February 2007

Restoring Safe Mode with a .REG file

Filed under: Malware — Didier Stevens @ 13:57

I posted about a virus that disables Safe Mode by deleting the SafeBoot registry keys, and later I talked about tricks to restore the SafeBoot keys. Now I’m posting another way to restore the SafeBoot keys: merging a .reg file with the missing SafeBoot entries.

A comment by Mirco made me take a closer look at the SafeBoot registry key. I thought that they would contain settings and drivers that
are hardware dependent, but this turned out to be false. In fact, it just contains a list of references to devices, drivers and services that have to be started when booting into Safe Mode.

The registry keys to boot into Safe Mode are under the SafeBoot key:


You can boot into Safe Mode without or with networking, there is a subkey for each mode: Minimal (no networking) and Network (with networking).

Each device, driver or service that has to be started has a subkey under the Minimal or Network key.
In this screenshot, you see the Cryptographic Services service:


BTW, if you want to disable a device, driver or service in Safe Mode, just delete the corresponding subkey (make a backup first).
I tested this with key {4D36E965-E325-11CE-BFC1-08002BE10318} (resulted in a disabled CD-ROM drive) and PlugPlay (resulted in a disabled Plug and Play service).

I compared several SafeBoot registry keys for Windows XP SP2 on different hardware platforms, and they were all identical. However, there were some small differences when comparing different operatings systems (Windows XP SP1, SP2 and Windows 2003 SP1). Remember that Safe Mode was introduced with Windows 2000.
These are minor differences, just listing devices, drivers or services that are only present on one version of Windows. For example, I found Volume shadow copy on a Windows 2003 and not on Windows XP. And Windows 2003 also had less network services than Windows XP, this is probably a result of the default hardening of Windows 2003: more services and applications are disabled by default on Windows 2003 than on Windows XP.

I’m now publishing a registry export file (.reg) with the SafeBoot keys from a clean Windows XP SP2 install and a clean Windows 2000 SP4 Professional install. You can use it to repair your PC when the SafeBoot keys have been deleted and System Restore cannot help you. I would not be surprised if you can use this REG file with other versions of Windows as well.

Download the ZIP file, extract the SafeBoot-for-Windows-XP-SP2.reg or SafeBoot-for-Windows-2000-SP4-Professional.reg file on the crippled PC and merge it into the registry by double-clicking it:



SafeBoot.zip (https)

MD5: 5C1E3698877F79DD1C35F3107D4DC459

SHA256: 876D1C85E7556A334664C96F263781F5A9DBC9AB4DA26EDC6070AD947D09641D

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