SafeArrayGetVartype


Author: David Zimmer
Date: 09.12.19 - 8:22pm



Tonight's tip comes from Mark Johnstone who was doing research into how safe arrays can store type information. The safe array structure itself does not have an internal type field.

In my vbdec data viewer form I was giving the user the choice on how to decode the data based on element size, essentially brute forcing it and going on intution base on if displayed values made sense, Not the best approach! lol

Mark found an undocumented pre-structure data blob that can contain type information.
Awesome find and thanks for sharing the hard fought research Mark!

Its pretty safe to say I would have never stumbled across this. (Had never even heard of SafeArrayGetVartype yet)

Here is Marks code: ManagedCharSafeArray.cls along with some links that turned up in the course of his research: ReactOS Safearray Source, and Where is SAFEARRAY var type stored?.

I will mirror the key bits of the stack overflow info here:

Where is SAFEARRAY var type stored?

I'd like to know about SAFEARRAY implementation.

It seems to me that there's no field in SAFEARRAY structure
 that is used for storing element type information, such as
 VT_I4(3) or VT_R4(4), but SafeArrayGetVartype function 
returns the correct type.

Somebody commented on the MSDN page below saying that the 
high word of the cLocks holds the type info: SAFEARRAY 
structure on MSDN

But when I passed Long and Single arrays from VBA to a DLL 
function via a type libray, those arrays' fFeatures are
 both 0x80, cLocks are both 0, and stll SafeArrayGetVartype 
can tell VT_I4(3) and VT_R4(4).

asked Sep 13 '13 at 10:49 - Fumito Hamamura

--------------------------------

Depending on how the safearray was created, the variant type
 may be stored in memory before (at the offset -4 from the 
start of) the SAFEARRAY structure. FADF_HAVEVARTYPE flag 
in fFeatures indicates whether the type is available.

Similarly, FADF_HAVEIID indicates that the GUID (see 
SafeArrayCreateEx) is stored at offset of -16, and 
available via SafeArrayGetIID. FADF_HAVEVARTYPE and 
FADF_HAVEIID can never be present simultaneously 
(because otherwise the VARTYPE and the GUID would overlap
 in memory), but SafeArrayGetVartype is smart enough to 
synthesize one of VT_RECORD, VT_DISPATCH or VT_UNKNOWN 
types when it sees corresponding feature flags.

answered Sep 13 '13 at 12:55 - Igor Tandetnik

--------------------------------------

Thank you for the clear answer. So, to summarize, for 
the fFeatures field, 
A) FADF_HAVEVARTYPE or 
B) FADF_HAVEIID or 
C) FADF_RECORD can be set. 

For A) 4, for B) 16, for C) 4 bytes before SAFEARRAY are 
used to carry supplemental information about the SAFEARRAY. 

Only one of the A, B or C can be set at a time. – 

Fumito Hamamura Sep 13 '13 at 14:08






Comments: (0)

 
Leave Comment:
Name:
Email: (not shown)
Message: (Required)
Math Question: 4 + 70 = ? followed by the letter: D 



About Me
More Blogs
Main Site
Posts:
SafeArrayGetVartype
vbdec dbg updates
vb6 PCode NOP
vb6 API and call backs
how pcode works Pt1
PrintFile
ImpAdCallNonVirt
Reversing PCode Args
VB6 PCode Disassembly
VB6 PCode Debugger
UConnect Disable Cell Modem
IDA python over IPC
dns wildcard blocking
64bit IDA Plugins
anterior lines
misc news/updates
KANAL Mod
Decoders again
CDO.Message Breakpoints
SysAnalyzer Updates
SysAnalyzer and Site Updates
crazy decoder
ida js w/dbg
flash patching #2
JS Graphing
packet reassembly
Delphi IDA Plugin
scdbg IDA integration
API Hash Database
Winmerge plugin
IDACompare Updates
Guest Post @ hexblog
TCP Stream Reassembly
SysAnalyzer Updates
Apilogger Video
Shellcode2Exe trainer
scdbg updates
IDA Javascript w/IDE
Rop Analysis II
scdbg vrs ROP
flash patching
x64 Hooks
micro hook
jmp api+5 *2
SysAnalyzer Updates
InjDll runtime config
C# Asm/Dsm Library
Shellcode Hook Detection
Updates II
findDll
Java Hacking
Windows 8
Win7 x64
Graphing ideas
.Net Hacking
Old iDefense Releases
BootLoaders
hll shellcode
ActionScript Tips
-patch fu
scdbg ordinal lookup
scdbg -api mode
Peb Module Lists
scdbg vrs Process Injection
GetProcAddress Scanner
scdbg fopen mode
scdbg findsc mode
scdbg MemMonitor
demo shellcodes
scdbg download
api hashs redux
Api hash gen
Retro XSS Chat Codes
Exe as DLL
Olly Plugins
Debugging Explorer
Attach to hidden process
JS Refactoring
Asm and Shellcode in CSharp
Fancy Return Address
PDF Stream Dumper
Malcode Call API by Hash
WinDbg Cheat Sheet
GPG Automation