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uninformed 03 06
FUTo
Peter Silberman & C.H.A.O.S.
1) Foreword
Abstract:
Since the introduction of FU, the rootkit world has moved away from
implementing system hooks to hide their presence. Because of this change
in offense, a new defense had to be developed. The new algorithms used
by rootkit detectors, such as BlackLight, attempt to find what the
rootkit is hiding instead of simply detecting the presence of the
rootkit's hooks. This paper will discuss an algorithm that is used by
both Blacklight and IceSword to detect hidden processes. This paper will
also document current weaknesses in the rootkit detection field and
introduce a more complete stealth technique implemented as a prototype
in FUTo.
Thanks:
Peter would like to thank bugcheck, skape, thief, pedram, F-Secure for
doing great research, and all the nologin/research'ers who encourage
mind growth.
C.H.A.O.S. would like to thank Amy, Santa (this work was three hours on
Christmas day), lonerancher, Pedram, valerino, and HBG Unit.
2) Introduction
In the past year or two, there have been several major developments in
the rootkit world. Recent milestones include the introduction of the FU
rootkit, which uses Direct Kernel Object Manipulation (DKOM); the
introduction of VICE, one of the first rootkit detection programs; the
birth of Sysinternals' Rootkit Revealer and F-Secure's Blacklight, the
first mainstream Windows rootkit detection tools; and most recently the
introduction of Shadow Walker, a rootkit that hooks the memory manager
to hide in plain sight.
Enter Blacklight and IceSword. The authors chose to investigate the
algorithms used by both Blacklight and IceSword because they are
considered by many in the field to be the best detection tools.
Blacklight, developed by the Finnish security company F-Secure, is
primarily concerned with detecting hidden processes. It does not attempt
to detect system hooks; it is only concerned with hidden processes.
IceSword uses a very similar method to Blacklight. IceSword
differentiates itself from Blacklight in that it is a more robust tool
allowing the user to see what system calls are hooked, what drivers are
hidden, and what TCP/UDP ports are open that programs, such as netstat,
do not.
3) Blacklight
This paper will focus primarily on Blacklight due to its algorithm being
the research focus for this paper. Also, it became apparent after
researching Blacklight that IceSword used a very similiar algorithm.
Therefore, if a weakness was found in Blacklight, it would most likely
exist in IceSword as well.
Blacklight takes a userland approach to detecting processes. Although
simplistic, its algorithm is amazingly effective. Blacklight uses some
very strong anti-debugging features that begin by creating a Thread
Local Storage (TLS) callback table. Blacklight's TLS callback attempts
to befuddle debuggers by forking the main process before the process
object is fully created. This can occur because the TLS callback routine
is called before the process is completely initialized. Blacklight also
has anti-debugging measures that detect the presence of debuggers
attaching to it. Rather than attempting to beat the anti-debugging
measures by circumventing the TLS callback and making other program
modifications, the authors decided to just disable the TLS routine. To
do this, the authors used a tool called LordPE. LordPE allows users to
edit PE files. The authors used this tool to zero out the TLS callback
table. This disabled the forking routine and gave the authors the
ability to use an API Monitor. It should be noted that disabling the
callback routine would allow you to attach a debugger, but when the user
clicked "scan" in the Blacklight GUI Blacklight would detect the
debugger and exit. Instead of working up a second measure to circumvent
the anti-debugging routines, the authors decided to analyze the calls
occuring within Blacklight. To this end, the authors used Rohitabs API
Monitor.
In testing, one can see failed calls to the API OpenProcess (tls zero is
Blacklight without a TLS table). Blacklight tries opening a process with
process id (PID) of 0x1CC, 0x1D0, 0x1D4, 0x1D8 and so on. The authors
dubbed the method Blacklight uses as PID Bruteforce (PIDB). Blacklight
loops through all possible PIDS calling OpenProcess on the PIDs in the
range of 0x0 to 0x4E1C. Blacklight keeps a list of all processes it is
able to open, using the PIDB method. Blacklight then calls
CreateToolhelp32Snapshot, which gives Blacklight a second list of
processes. Blacklight then compares the two lists, to see if there are
any processes in the PIDB list that are not in the list returned by the
CreateToolhelp32Snapshot function. If there is any discrepancy, these
processes are considered hidden and reported to the user.
3.1) Windows OpenProcess
In Windows, the OpenProcess function is a wrapper to the NtOpenProcess
routine. NtOpenProcess is implemented in the kernel by NTOSKRNL.EXE. The
function prototype for NtOpenProcess is:
NTSTATUS NtOpenProcess (
OUT PHANDLE ProcessHandle,
IN ACCESS_MASK DesiredAccess,
IN POBJECT_ATTRIBUTES ObjectAttributes,
IN PCLIENT_ID ClientId OPTIONAL);
The ClientId parameter is the actual PID that is passed by OpenProcess.
This parameter is optional, but during our observation the OpenProcess
function always specified a ClientId when calling NtOpenProcess.
NtOpenProcess performs three primary functions:
1. It verifies the process exists by calling PsLookupProcessByProcessId.
2. It attempts to open a handle to the process by calling
ObOpenObjectByPointer.
3. If it was successful opening a handle to the process, it passes the
handle back to the caller.
PsLookupProcessByProcessId was the next obvious place for research. One
of the outstanding questions was how does PsLookupProcessByProcessId
know that a given PID is part of a valid process? The answer becomes
clear in the first few lines of the disassembly:
PsLookupProcessByProcessId:
mov edi, edi
push ebp
mov ebp, esp
push ebx
push esi
mov eax, large fs:124h
push [ebp+arg_4]
mov esi, eax
dec dword ptr [esi+0D4h]
push PspCidTable
call ExMapHandleToPointer
From the above disassembly, it is clear that ExMapHandleToPointer
queries the PspCidTable for the process ID.
Now we have a complete picture of how Blacklight detects hidden processes:
1. Blacklight starts looping through the range of valid process IDs, 0
through 0x41DC.
2. Blacklight calls OpenProcess on every possible PID.
3. OpenProcess calls NtOpenProcess.
4. NtOpenProcess calls PsLookupProcessByProcessId to verify the
process exists.
5. PsLookupProcessByProcessId uses the PspCidTable to verify the
processes exists.
6. NtOpenProcess calls ObOpenObjectByPointer to get the handle to the
process.
7. If OpenProcess was successful, Blacklight stores the information
about the process and continues to loop.
8. Once the process list has been created by exhausting all possible
PIDs. Blacklight compares the PIDB list with the list it creates by
calling CreateToolhelp32Snapshot. CreateToolhelp32Snapshot is a Win32
API that takes a snapshot of all running processes on the system. A
discrepancy between the two lists implies that there is a hidden
process. This case is reported by Blacklight.
3.2) The PspCidTable
The PspCidTable is a "handle table for process and thread client IDs".
Every process' PID corresponds to its location in the PspCidTable. The
PspCidTable is a pointer to a HANDLE_TABLE structure.
typedef struct _HANDLE_TABLE {
PVOID p_hTable;
PEPROCESS QuotaProcess;
PVOID UniqueProcessId;
EX_PUSH_LOCK HandleTableLock [4];
LIST_ENTRY HandleTableList;
EX_PUSH_LOCK HandleContentionEvent;
PHANDLE_TRACE_DEBUG_INFO DebugInfo;
DWORD ExtraInfoPages;
DWORD FirstFree;
DWORD LastFree;
DWORD NextHandleNeedingPool;
DWORD HandleCount;
DWORD Flags;
};
Windows offers a variety of non-exported functions to manipulate and retrieve
information from the PspCidTable. These include:
- [ExCreateHandleTable] creates non-process handle tables. The
objects within all handle tables except the PspCidTable are pointers
to object headers and not the address of the objects themselves.
- [ExDupHandleTable] is called when spawning a process.
- [ExSweepHandleTable] is used for process rundown.
- [ExDestroyHandleTable] is called when a process is exiting.
- [ExCreateHandle] creates new handle table entries.
- [ExChangeHandle] is used to change the access mask on a handle.
- [ExDestroyHandle] implements the functionality of CloseHandle.
- [ExMapHandleToPointer] returns the address of the object corresponding to the handle.
- [ExReferenceHandleDebugIn] tracing handles.
- [ExSnapShotHandleTables] is used for handle searchers (for example in oh.exe).
Below is code that uses non-exported functions to remove a process
object from the PspCidTable. It uses hardcoded addresses for the
non-exported functions necessary; however, a rootkit could find these
function addresses dynamically.
typedef PHANDLE_TABLE_ENTRY (*ExMapHandleToPointerFUNC)
( IN PHANDLE_TABLE HandleTable,
IN HANDLE ProcessId);
void HideFromBlacklight(DWORD eproc)
{
PHANDLE_TABLE_ENTRY CidEntry;
ExMapHandleToPointerFUNC map;
ExUnlockHandleTableEntryFUNC umap;
PEPROCESS p;
CLIENT_ID ClientId;
map = (ExMapHandleToPointerFUNC)0x80493285;
CidEntry = map((PHANDLE_TABLE)0x8188d7c8,
LongToHandle( *((DWORD*)(eproc+PIDOFFSET)) ) );
if(CidEntry != NULL)
{
CidEntry->Object = 0;
}
return;
}
Since the job of the PspCidTable is to keep track of all the processes
and threads, it is logical that a rootkit detector could use the
PspCidTable to find hidden processes. However, relying on a single data
structure is not a very robust algorithm. If a rootkit alters this one
data structure, the operating system and other programs will have no
idea that the hidden process exists. New rootkit detection algorithms
should be devised that have overlapping dependencies so that a single
change will not go undetected.
4) FUTo
To demonstrate the weaknesses in the algorithms currently used by
rootkit detection software such as Blacklight and Icesword, the authors
have created FUTo. FUTo is a new version of the FU rootkit. FUTo has
the added ability to manipulate the PspCidTable without using any
function calls. It uses DKOM techniques to hide particular objects
within the PspCidTable.
There were some design considerations when implementing the new features
in FUTo. The first was that, like the ExMapHandleXXX functions, the
PspCidTable is not exported by the kernel. In order to overcome this,
FUTo automatically detects the PspCidTable by finding the
PsLookupProcessByProcessId function and disassembling it looking for the
first function call. At the time of this writing, the first function
call is always to ExMapHandleToPointer. ExMapHandleToPointer takes the
PspCidTable as its first parameter. Using this knowledge, it is fairly
straightforward to find the PspCidTable.
PsLookupProcessByProcessId:
mov edi, edi
push ebp
mov ebp, esp
push ebx
push esi
mov eax, large fs:124h
push [ebp+arg_4]
mov esi, eax
dec dword ptr [esi+0D4h]
push PspCidTable
call ExMapHandleToPointer
A more robust method to find the PspCidTable could be written as this
algorithm will fail if even simple compiler optimizations are made on
the kernel. Opc0de wrote a more robust method to detect non-exported
variables like PspCidTable, PspActiveProcessHead, PspLoadedModuleList,
etc. Opc0des method does not requires memory scanning like the method
currently used in FUTo. Instead Opc0de found that the KdVersionBlock
field in the Process Control Region structure pointed to a structure
KDDEBUGGER_DATA32. The structure looks like this:
typedef struct _KDDEBUGGER_DATA32 {
DBGKD_DEBUG_DATA_HEADER32 Header;
ULONG KernBase;
ULONG BreakpointWithStatus; // address of breakpoint
ULONG SavedContext;
USHORT ThCallbackStack; // offset in thread data
USHORT NextCallback; // saved pointer to next callback frame
USHORT FramePointer; // saved frame pointer
USHORT PaeEnabled:1;
ULONG KiCallUserMode; // kernel routine
ULONG KeUserCallbackDispatcher; // address in ntdll
ULONG PsLoadedModuleList;
ULONG PsActiveProcessHead;
ULONG PspCidTable;
ULONG ExpSystemResourcesList;
ULONG ExpPagedPoolDescriptor;
ULONG ExpNumberOfPagedPools;
[...]
ULONG KdPrintCircularBuffer;
ULONG KdPrintCircularBufferEnd;
ULONG KdPrintWritePointer;
ULONG KdPrintRolloverCount;
ULONG MmLoadedUserImageList;
} KDDEBUGGER_DATA32, *PKDDEBUGGER_DATA32;
As the reader can see the structure contains pointers to many of the
commonly needed/used non-exported variables. This is one more robust
method to finding the PspCidTable and other variables like it.
The second design consideration was a little more troubling. When FUTo
removes an object from the PspCidTable, the HANDLE_ENTRY is replaced with
NULLs representing the fact that the process "does not exist." The
problem then occurs when the process that is hidden (and has no
PspCidTable entries) is closed. When the system tries to close the
process, it will index into the PspCidTable and dereference a null
object causing a blue screen. The solution to this problem is simple but
not elegant. First, FUTo sets up a process notify routine by calling
PsSetCreateProcessNotifyRoutine. The callback function will be invoked
whenever a process is created, but more importantly it will be called
whenever a process is deleted. The callback executes before the hidden
process is terminated; therefore, it gets called before the system
crashes. When FUTo deletes the indexes that contain objects that point
to the rogue process, FUTo will save the value of the HANDLE_ENTRYs and
the index for later use. When the process is closed, FUTo will restore
the objects before the process is closed allowing the system to
dereference valid objects.
5) Conclusion
The catch phrase in 2005 was, ``We are raising the bar [again] for
rootkit detection''. Hopefully the reader has walked away with a better
understanding of how the top rootkit detection programs are detecting
hidden processes and how they can be improved. Some readers may ask
"What can I do?" Well, the simple solution is not to connect to the
Internet, but a combination of using both Blacklight, IceSword and
Rootkit Revealer will greatly help your chances of staying rootkit free.
A new tool called RAIDE (Rootkit Analysis Identification Elimination)
will be unveiled in the coming months at Blackhat Amsterdam. This new
tool does not suffer from the problems brought forth here.
Bibliography
Blacklight Homepage. F-Secure Blacklight
http://www.f-secure.com/blacklight/
FU Project Page. FU
http://www.rootkit.com/project.php?id=12
IceSword Homepage. IceSword
http://www.xfocus.net/tools/200505/1032.html
LordPE Homepage. LordPE Info
http://mitglied.lycos.de/yoda2k/LordPE/info.htm
Opc0de. 2005. How to get some hidden kernel variables without scanning
http://www.rootkit.com/newsread.php?newsid=101
Rohitabs API Monitor. API Monitor - Spy on API calls
http://www.rohitab.com/apimonitor/
Russinovich, Solomon. Microsoft Windows Internals Fourth Edition.
Silberman. RAIDE:Rootkit Analysis Identification Elimination
http://www.blackhat.com/html/bh-europe-06/bh-eu-06-speakers.htmlSilberman
