Windows Vista Security

1. Windows Vista Security

2. User Mode Security • User Account Protection (UAP) • Mandatory Integrity Control(MIC) • UI Privlilege Isolation (UIPI) • Restricted Process • Unrestricted Process (Elevation) • Standard methods • The Legacy Shell Trick • Consent Prompts and Admin Brokers • Service Isolation • File and Registry Virtualization • Registry Virtualization • File Virtualization • Low Rights IE Virtualization • Possible Attacks

3. User Account Protection (UAP) • Limited User Accounts • Standard user accounts preferred • Problem: software isn’t always written for “Standard” user accounts • Administrators start as “Protected” • Runs programs with minimal privileges • Must authenticate protected actions • Can run programs unrestricted “Unprotected”

4. Mandatory Integrity Control(MIC) • Every “securable object” has an Integrity • Children inherit integrity parents • Interactions exist at equal or lesser integrity • Higher integrity can act on lower through certain functions • Any interaction allowed through IPC (BAD) • Lower Integrity server can impersonate higher integrity. (ImpersonateNamedPipeClient)

5. Mandatory Integrity Control Levels

6. UI Privilege Isolation (UIPI) • Added to prevent Shatter attacks • LI process can’t send messages to a HI Process • SendMessage • PostMessage • LI process can’t hook into a HI process • SetWindowsHookEx • SetWinEventHook

7. Restricted Process • How is it restricted • Security token normally has all privileges • Some are disabled (Ignored during permission checks) • Process can re-enable them • Security token created with less privileges (CreateRestrictedToken) • Some privileges removed • Some privileges marked deny only • “Group used for deny only” • Explicit denials for group propagate • Explicit allows do not

8. Unrestricted Process (Elevation) • Process are run elevated when • Process is a .msi or .exe and a registered installer • Process exists in app compatibility database • Proper registry with entry value RUNASADMIN • <application_name>.sbd created by CompatAdmin.exe • Aplication Manifest (<appname>.exe.manifest) contains requestedExecutionLevel of requireAdministrator • User right clicks executable and clicks “Run Elevated…” from explorer • Executed by an already privileged process

9. The Legacy Shell Trick • Kill explorer from taskmanager.exe and restart it with file->new task • New shell running with highest integrity • Why does this work? • WinLogon.exe handles Secure Attention Sequence (ctrl+alt+delete and ctrl+shift+esc) • taskmanager started this way is created with high integrity • File->new task creates a process with CreateProcess instead of CreateRestrictedProcess • Fixed in later builds of Vista

10. Consent Prompts and Admin Brokers • Windows Explorer can’t launch unrestricted apps on its own • Restricted Token • Medium Integrity • AppInfo Admin Broker service (runs as LocalSystem) • RunAsAdminProcess • consent.exe run by AppInfo • Creates process • ImpersonateLoggedOnUser • CreateProcessAsUser (not CreateProcess)

11. Security Token Standard User Token Full Administrator Token User In Administrators Group Login Local Security Authority Full Access Consent Standard User Token User In Users Group Login Administrator Credentials

12. Service Isolation • Services use to exist in the same session • Vista Services run in “Isolated Session 0” • Services can’t open dialogs on desktop • Neither can services marked interactive • Dialogs from interactive services are actually a Terminal Service Context • Consent Prompts? • AppInfo runs consent in the user’s desktop session with CreateProcessAsUser

13. File and Registry Virtualization • Why? • Developers don’t code applications properly • Assume the need for admin privileges • Need to provide backwards compatibility • Need to provide separation and safety

14. Registry Virtualization • Implemented by kernel • Write attempts to HKEY_LOCAL_MACHINE\Software redirected to HKEY_CURRENT_USER\Software\Classes\VirtualStore\MACHINE\Software • Provides per-user settings in apps that used registry for storage. • Provides isolation between users.

15. File Virtualization • Implemented as a FS filter driver (luafv.sys) • Example: Program files • Foo writes to c:\Program Files\foo\foo.ini • Foo is running as unprivileged and fails • Filter driver maps c:\Program Files\foo\foo.ini to per-user virtualized area. • %UserProfile%\AppData\Local\VirtualStore\C\Progra~1\foo contains user-specific copy of foo.ini • Certain executable types not virtualized (cmd, bat, exe, dll, etc..) • Provides isolation • Provides per-user settings (in certain cases)

16. Low Rights IE Virtualization • Virtualization not done by Filter Driver, done by AppCompat shim dll • Why? • Low integrity process can’t even write to the virtualized areas • Uses special broker applications for tasks

17. Low Rights IE Virtualization Components • User runs IEUser.exe (Med integrity) • IEUser.exe spawns IExplorer.exe (Low Integrity) • Any admin level requests handled by IEInstall.exe

18. Ex-Possible Attacks • Low Integrity – IE Approach • Medium Integrity • Method 1 – Slight of Hand/Bait and switch • Method 2 – Slight of Hand/Bait and switch

19. Low integrity – IE Approach • Unknown IE Exploit allows injection of arbitrary code • Code is run at low integrity • Low integrity code can loopback on localhost (gains default med integrity) • Code can now insert files into the filesystem eg. Virtualized start menu startup folder • No longer valid as of Beta 2

20. Medium Integrity - Method 1 • User expects consent prompt • User is slow • User clicks through • Malicious app checks for all instances of consent.exe • If called on behalf of spoof target copy our bad version over the good one

21. Medium Integrity - Method 2 • Global COM Objects • HKEY_LOCAL_MACHINE\Software\Classes\CLSID • User Specific COM Objects • HKEY_CURRENT_USER\Software\Classes\CLSID • User objects have prescient over system • Enumerate system COM objects • Create paths to malicious versions in current_user • No longer valid, only local_machine keys are referred to for elevation

22. Kernel Mode Security • Booting Vista • Driver Signing • Patch Guard • Secure Bootup • Restricted user-mode access to \Device\PhysicalMemory

23. Booting Vista (Stage 1) • Locates and runs bootmgr for legacy PC/AT Bios and bootmgr.efi for an efi system • “The Vista Boot Manager calls InitializeLibrary, which in turn calls BlpArchInitialize (GDT, IDT, etc.), … BlpTpmInitialize (TPM), BlpIoInitialize (file systems), … BlBdInitialize (debugging), BlDisplayInitialize, …“ • Boot.init replaced with BCD file • Selects boot description and runs BlImageLoadBootApplication • Calls BlFveSecureBootUnlockBootDevice and BlFveSecureBootCheckpointBootApp if Full Volume Encryption is enabled.

24. Booting Vista (Stage 2) • WINLOAD.EXE replaces NTLDR.EXE as the os loader • Performs many of the same tasks as bootmgr • Discovers disks and loads the hive • Loads OS Signed catalog

25. Booting Vista (Stage 2) cont. • Verifies its own integrity and that of other system files • Does not boot if they don’t match • Will however boot if a debugger is attached except on certain key files • Loads appropriate driver for debugging • Usb • Firewire • Serial • Loads remaining drivers in order from the hive

26. Booting Vista (Stage 3) • Loads NTOSKRNL.EXE • Responsible for code verification of system drivers • Runtime checks (PatchGuard and CI.DLL)

27. Driver Signing • Windows Vista 64-bit edition only • All Kernel mode drivers must have a class 3 cert • Justification: • Stability – less hackish code in kernel • Security – Prevents root kits • Ulterior Motives: • DRM protection

28. Driver Signing (Implementation) • WINLOAD.EXE - Boot driver checks • NTOSKRNL.EXE – All other driver (uses CI.DLL) • Functions • MinCrypL_CheckSignedFile • MinCrypL_CheckImageHash • MinCryptK_FindPageHashesInCatalog

29. Driver Signing (Implementation) • MinCrypL_CheckSignedFile • Used by WINLOAD.EXE and CI.DLL • Parses certificate to check validity • Checks certificate against a root certificate • Hard coded list of 8 certificates in binary • Adding certificates to system certificates doesn’t add to this list. • If certificate is signed by a root authority validate it • Parse public key info/RSA Public Key • Convert the key to a “Safe” public key • Verify signing according to PKCS1

30. Driver Signing (Implementation) • MinCrypL_CheckImageHash • Used by WINLOAD.exe • Verifies driver matches images in the signed catalog • Walks linked list of catalogs pointed to by g_CatalogList calling I_CheckImageHashInCatalog on each • MinCryptK_FindPageHashesInCatalog • Used by CI.DLL • Checks code pages of process or driver at runtime. • Binary searches for matching page hash in ntpe.cat nt5.cat

31. Patch Guard • Can not be disabled • Polls at 5-10 minute intervals to verify kernel structures are intact • SSDT (System Service Descriptor Table) • GDT (Global Descriptor Table) • IDT (Interrupt Descriptor Table) • System images (ntoskrnl.exe, ndis.sys, hal.dll) • Processor MSRs (syscall)

32. Patch Guard (Implementation) • Uses Obfuscation and Misdirection “raise the bar” • Example: • Initialization • nt!KiDivide6432 (What does it do?) • Throws divide processor exception • Patch Guard Initialization called in exception handler

33. Patch Guard (Implementation) • Initialization • Creates random key • Creates random rotate number • Picks a fake memory pool tag • Initializes memory • Zeroes it • Fills it with structures • Encrypts structures in memory

34. Patch Guard (Attacks) • Exception Handler Hooking – Verification relies on exceptions, hook the exception and turn it into a nop • KeBugCheckEX Hook – When called check if bug check code is 0x109 if so reset stack pointer and instruction pointer to the thread and carry on • Finding the timer – Find the timer event and remove it. Not reliable and not portable since it uses an unexported address • Simulating Hotpatching – Use the Hotpatch api to trick windows

35. Secure Bootup • TPM Holds key used for full drive encryption • Takes measurments of boot items such as ROM images and firmware images • Special boot code in TPM decrypts the boot loader • Boot loader asks for full drive encryption key from TPM • Boots the same as detailed in Booting Vista

36. Disabled user-mode access to \Device\PhysicalMemory • Started with Windows Server 2003 SP1 • Crazylord (p59-0x10) – showed a method for detecting bios root kits using \Device\PhysicalMemory

37. The End

38. Frame-Based Exception Handlers • Every thread in a Win32 Process has at least one frame-based exception handler. • A list of EXCEPTION_REGISTRATION structures can be found in the process’s Thread Environment Block at FS: [0] • Overwrite the exception handler with an address which will pop reg pop reg ret

39. Determining a valid handler • Handler can not exist on the stack (determined by TEB FS:[4] FS[8]) • Checked against loaded modules • If the address exists outside of the bounds of these addresses it is ok to call? • If the address exists inside these it is checked against registered handlers. • Checks a value in the PE header if it is set to 0x04 then the module is not allowed. • Finally checks for a Load Configuration Directory if missing function returns 0 and no other checks are done and handler is executed

40. Exploiting Frame-Based Exception Handling (Window 2003 Server) • Methods • Exploit an existing handler that we can manipulate to get us back into our buffer • Find a block of code in an address not associated with a module that will get us back to our buffer • Find a block of code in the address space of a module that does not have a Load Configuration Directory

41. Exploiting an Existing Handler • NTDLL contains several registered exception handlers • Only works the first time since sensitive data is in predictable places 77F45A3F mov ebx,dword ptr [ebp+0Ch] .. 77F45A61 mov esi, dword ptr [ebx+0Ch] 77F45A64 mov edi, dword ptr [ebx+8] .. 77F45A75 lea ecx, [esi+esi*2] 77F45A78 mov eax, dword ptr [edi+ecx*4+4] .. 77F45A64 call eax

42. Finding and exploiting a block of code not associated with a module • Windows 2003 Server Enterprise edition contains such an address at 0x7FFC0AC5. (pop pop ret) • Not usable since Standard addition does not have the same issue • However we can use the address of our EXCEPTION_REGISTRATION struct in the form of a call or jump esp+somevalue

43. Stack Protection and Windows 2003 Server • Security Cookies • Authoritative copy stored in the .data segment • /GS Compiler Flag • Reorders parameters • Places overflowable buffers close to canary values

44. Heap Based Buffer Overflows • Handle to Win32 Heap through GetProcessHeap() and through the PEB • HeapAllocate – Win32 version of brk and brk. • Every heap starts with a struct and contains pointers to the previous and next blocks (similar to malloc). • Use Exception Handlers to overwrite functions such as RtlAccquitePebLock() and RtlReleasePebLock() (Not Usable in Win2k3Server)

45. Heap Overflow Fun • The PEB in a process is fixed across all WinNT Versions. • Step1: Overflow heap to overwrite the PEB + 4 (Return address). • Step2: Allow Program to segfault and terminate. • Step3: Sit back and watch ExitProcess run your code for you. • Make sure to set the pointer back or something else could kill your process if its used elsewhere in the code

46. Vectored Handlers • Similar in structure to Frame based exception handlers. • Stored on the heap instead of stack • Executed before frame based handlers.

47. Overwritting Exception Filters • Overwrite pointer to Unhandled Exception Filter. • Windows exposes a function to do this SetUnhandledExceptionFilter(). • This function shows us where this Handler is stored. • By replacing the address of the function this points to when an unhandled exception happens we gain control.

48. Other Aspects of Heap-Based Overflows • COM Objects and the Heap • COM Objects when instantiated are placed on the heap • A vtable is created to store function pointers for an object and the object is stored above it in the address space • If you overflow an object you can possibly overwrite the vtable of the object above you and redirect code execution. • Overflowing Program Control Data • We don’t always want to execute arbitrary code • Some times we just want to change data on the heap that controls the execution flow. • Ex. Making a directory exposed by a web server writable so anyone can write to it.