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Introduction to Windows System Internals part I

Introduction to Windows System Internals part I. by Tim Shelton Black Security redsand@blacksecurity.org. Outline. Brief History of the Windows Operating Systems Unicode Explained Registry Basics Windows Services (services.exe) Startup & Shutdown Procedures Q&A.

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Introduction to Windows System Internals part I

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  1. Introduction to Windows System Internals part I by Tim Shelton Black Security redsand@blacksecurity.org

  2. Outline Brief History of the Windows Operating Systems Unicode Explained Registry Basics Windows Services (services.exe) Startup & Shutdown Procedures Q&A

  3. Windows History Overview Microsoft formed its team of 20 developers in November 1988 4 core developers wrote key components Began a dream to write an Advanced Operating System Designed for Desktops and Servers Secure, scalable Multi-Processor design All new code base

  4. Windows History Overview Cont. Microsoft announced its commitment to rigorous discipline Developers are required detailed documentation Developers are required peer code review Developers are required to unit test their code..

  5. Windows History Overview Cont. Past – Personal Computing, 16-32 bits, Windows 9x code base, objective: bringing computers to the consumer.Features – usability and compatibility Present – Enterprise Computing, 32/64 bits, NT code base, solid architectural foundation, objective: reliability, performance, and to meed the demands for Server Processing. Future – Managed Code (.NET Framework), objective: World Domination (go figure!) Longhorn.

  6. Commitment of the NT Architecture Reliability – Crash proof Operating System Security – Built into design from day one. Portability – Multi-processor support, avoiding non-portable solutions, flexible hardware abstraction Layer Modularity – Space to grow and needs to be fulfilled. Performance – Microsoft is willing to sacrifice performance for all of the above.

  7. Common Windows Internal Tools File Monitor – [filemon] www.sysinternals.com List DLLs loaded within specific process' virtual address space – [listdlls] www.sysinternals.com Kernel Debuggers [windbg, kd] Platform SDK and Windows SDK Live Kernel Debugging [livekd] www.sysinternals.com Object Viewer – [winobj] www.sysinternals.com Process Explorer – [procexp] replacement for taskmgr and much more! www.sysinternals.com And More! Visit the Platform SDK or www.sysinternals.com

  8. Add text output/ screen shots here and next few slides

  9. Unicode and Language Independence Most internal text strings are stored and processed as 16-bit Unicode characters. Unicode is an international character set standard that defines unique 2byte values (maximum 65536 characters) for most of the world's known character sets. References: www.unicode.org Or MSDN Documentation

  10. Unicode and Language Independence Cont. Because most applications use 8-bit ANSI character sets, Windows functions that accept string parameters have two entry points: a Unicode and an ANSI version. Ex: CreateProcessA and CreateProcessW (found in kernel32.dll) Lesson learned: Use Unicode for multi-lingual support.

  11. Windows Registry Hives located in %SystemRoot%\Config\ A Hive is a logical file system within a flat file. Keys == directories Values == files Registry is a collection of “Hives”. A Hive contains a collection of “Bins”. A Bin contains a collection of “Cells”. Each Cell is a unit of allocation containing raw data

  12. Windows Registry Cont. Below are a few Example NT API’s available for managing the Windows Registry: NtEnumerateValueKey(KHANDLE, int); NtQueryValueKey(KHANDLE, VarName); NtLoadKey(KHANDLE, HiveFileName); … more found in Advapi32.dll

  13. Windows Registry Cont. A Hive is a file (two if you count the .LOG) - Primary: holds the actual hive data - .LOG: used when flushing the hive (crash recovery) Storage Mapping Types: - Stable: maps to the hive file - Volatile: mapped into paged pool of memory, lost after reboot. Primary file grows in 256k increments to prevent fragmentation. First page (4k) is the registry header, followed by chained Bins

  14. Windows Registry Cont. A Cell is the unit of storage allocation within a Hive. Always 8-byte aligned. Always reuse free cells if one with the same or greater exists. If size is bigger, then split it and re-enlist in free cell table.

  15. Windows Registry Cont. Keys, Values, Security Descriptors, Indexes, etc. are all made up of Cells. Retrieving a value within a Key might involve several faults spread across the Hive file. - Solution: Registry Hive Caching (Win2k), locality enforcement (XP/.NET) to help with performance.

  16. Registry: Hive Flush Most “expensive” operation, called externally by NtFlushKey/RegFlushKey, or anytime a value is written to the Hive. (SetValue, DeleteValue, CreateKey, DeleteKey, etc). Automatic Flush at Shutdown/Reboot “Lazy Flush” waits 5 seconds after write then walks the list of Hives looking for Cells marked as ‘Dirty’. Ignores Hives marked as NO_LAZY_FLUSH. During Flush, registry is marked as read-only No data is written to the Hive File until the Flush is completed. This may lead to a possible loss of data.

  17. Registry: Loading the Hive Loaded at boot time by Boot Loader (NTLDR) and the kernel (ntoskrnl.exe) Explicitly loaded by calling NtLoadKey/RegLoad Key - This requires ‘Restore’ security privileges. Files are opened in “exclusive” mode; and kept open by the kernel. Read Primary header and verify checksums, if failed: - Physical integrity check, walk entire Hive and check each individual cell - Logical integrity check, walk the tree check every key/value.

  18. Registry: Hives Locations Two distinct User hives per account. Located in %USERPROFILE% - NTUSER.DAT: Mounted under HKEY_USERS\SID roaming enabled (if roaming profiles are used) - UsrClass.DAT: local (no roaming) Special hives similar to above; always loaded: - S-1-5-18: SYSTEM account - S-1-5-19: Local Service - S-1-5-20: Network Service

  19. Registry: Review Registry is intended to maintain configuration data. Stored in a special, highly tuned flat file. Native APIs can be found within Advapi32 Used by the kernel, drivers, internal system, applications, security, policies, and more…

  20. Services Explained What are services? Processes that run without the need for an interactive logon. This is the Windows equivalent of the UNIX daemon.

  21. NT Services Started early during boot process by winlogon.exe Responsible for enforcing service load order and dependencies. Starts all service processes marked for load on boot. Manages all service processes - Only allows access to service via API - Access guarded by use of access checks. Can be configured to run under any account (such as LocalSystem).

  22. NT Services Examples of common services: - spoolsv.exe (Print Spooler running as LocalSystem only) - svchost.exe (Generic host, any account) - services.exe (Eventlog, Plug ‘n Play running as NT Authority\SYSTEM privileges.) Services register with both a Service Name and a Service Description. - ex: ALG vs. Application Layer Gateway Service

  23. NT Services Configuration: HKLM\SYSTEM\CurrentControlSet\Services Follows the Service Programming Model: - Requires ServiceMain and Handler (Ex) - Multiple services within each process must implement its own ServiceMain If service is its own executable, it must call StartServiceCtrlDispatcherin WinMain which will in turn call ServiceMain.

  24. svchost.exe Individual services can be configured to run within svchost.exe - Initialized within configuration during Service Creation - %SystemRoot%\system32\svchost.exe –k <service name>- svchost Service list is static, instance must be added to: HKLM\Software\Microsoft\Windows\Svchost When svchost begins, it will read the list of services and setup a generic ServiceMain routine.

  25. Startup Procedure Files Required for Successful Boot:

  26. Startup Procedure Initially the Boot Sector will find and load Ntldr. Below are the steps of Ntldr: When NTLDR runs, it switches the processor into 32-bit flat memory mode (until this point the computer was running in real mode (just like your old 8086 or 8088 CPU). It then starts the appropriate mini-file system (e.g.:FAT, NTFS), so that it can read the files from the disk. It will then read the Boot.ini file, and display the boot menu on the screen.

  27. Startup Procedure Ntldr Continued: If an OS other than Windows 2000 is selected, NTLDR then loads the bootsect.dos file and passes control to it, which then boots the other OS. If a Windows OS is selected, then NTLDR runs Ntdetect.com to gather information about the computer's hardware. Hardware includes: Computer ID, Video Adapter, Keyboard, etc. Computer ID It is also in this step when you can choose to press F8 for troubleshooting and advanced startup options.

  28. Startup Procedure Ntoskrnl begins the first of two phases: Disable Interrupts Calls KiSystemStartup -> HalInitializeProcessor -> KiInitializeKernel (per cpu) Proceeds to call ExpInitializeExecutive which loads critical resource management interfaces. (Plug ‘n Play, Security Monitor, Memory Manger) Phase two begins: Ntoskrnl re-enables Interrupts and displays the Windows Boot Status Screen Ntoskrnl loads the HARDWARE Registry hive Ntoskrnl proceeds to initialize the necessary drivers

  29. Startup Procedure This step begins with the starting of the Session Manager (Smss.exe) Smss, being a native application, can perform unique actions. Creation of Security Tokens Uses its own native API, unavailable to the rest of Windows Smss’s first task is initializing the rest of the Registry Hive Smss then runs any programs defined in HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\BootExecute Smss loads the Windows Subsystem (Win32k.sys) Smss then loads Csrss and Winlogon

  30. Startup Procedure Winlogin and Csrss Winlogon then performs its startup steps such as creating the initial window station and desktop objects. Winlogon then loads Msgina.dll (or replacement) to handle WlxLoggedOutSAS, displaying the standard Windows logon dialog box. Winlogon creates the Service Control Manager (SCM) or services.exe Loads all the necessary services marked for auto-start Loads the Local Security Authentication Subsystem (Lsass)

  31. Startup Procedure Winlogin and Csrss SCM deems boot success and updates “Last Known Good Configuration” located atHKLM\SYSTEM\Select\LastKnownGood to match \CurrentControlset

  32. Startup Procedure Post Authentication Winlogon: maps HKCU and sets the User Environment stored in HKCU\Environment Msgina: loads executables within HKLM\Software\Windows NT\CurrentVersion\WinLogon\Userinit Userinit.exe: processes user scripts and machine logon scripts If group policy specified, user quota loaded (%SystemDirectory%\Proquota.exe) Launches comma-seperated shell(s) specified in HKCU\Software\Microsoft\Windows NT\CurrentVersion\Winlogon\Shell (default is Explorer.exe)

  33. Shutdown Procedure ExitWindowsEx() Csrss impersonates the caller and sends a Windows Message to a hidden window owned by Winlogon, telling it to shutdown Csrss traverses through each user process “informing” it of its intentions. Csrss calls ExitWindowsEx() once again within System space, informing csrss to kill any processes owned by SYSTEM. Winlogon calls NtSetSystemPowerState Sends shutdown I/O packets to all device drivers that have requested shutdown notification. Winlogon then sets the power status to the required request. (Shutdown, Reboot)

  34. Questions? Now is the time to hit me with all you got!

  35. Kill() Time() Windows Shattr Attacks Windows CreateRemoteThread Injection DLL Detach Injection

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