windows vista security. user mode security user account protection (uap) mandatory integrity...
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Windows Vista Security
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
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”
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)
Mandatory Integrity Control Levels
Integrity Access Level System Privileges
High Administrative (can install files to the Program Files folder and write to sensitive registry areas like HKEY_LOCAL_MACHINE)
Medium User (can create and modify files in the user's Documents folder and write to user-specific areas of the registry, such as HKEY_CURRENT_USER)
Low Untrusted (can only write to low integrity locations, such as the Temporary Internet Files\Low folder or the HKEY_CURRENT_USER\Software\LowRegistry key)
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
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
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
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
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)
Security Token
User In Administrators
GroupLocal
SecurityAuthority
StandardUser
Token
Full Access Consent
AdministratorCredentials
User In Users Group
Login
Login StandardUser
Token
FullAdministrator
Token
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
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
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.
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)
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
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
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
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
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
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
Kernel Mode Security
Booting Vista Driver Signing Patch Guard Secure Bootup Restricted user-mode access to \
Device\PhysicalMemory
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.
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
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
Booting Vista (Stage 3)
Loads NTOSKRNL.EXE Responsible for code verification of
system drivers Runtime checks (PatchGuard and
CI.DLL)
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
Driver Signing (Implementation)
WINLOAD.EXE - Boot driver checks NTOSKRNL.EXE – All other driver (uses
CI.DLL) Functions
MinCrypL_CheckSignedFile MinCrypL_CheckImageHash MinCryptK_FindPageHashesInCatalog
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
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
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)
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
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
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
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
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
The End
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 regpop regret
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
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
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
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
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
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)
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
Vectored Handlers
Similar in structure to Frame based exception handlers.
Stored on the heap instead of stack Executed before frame based
handlers.
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.
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.