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ANATOMY OF DOC FILES

INTRODUCTION 1. WHICH MACROVIRUSES? 2. WHAT IS A DOCFILE 3. HOW THE WORD DOCUMENT IS STRUCTURED 4. ABOUT ANTI-VIRUS 5. DESCRIPTION OF THE EXAMPLE LITERATURE ATTACHMENT. SOURCE OF DEMONSTRATION UTILITY.

INTRODUCTION

This short article is addressed to those who want to know where specifically, macro viruses live, how to look at them with eyes, and how they touch with handles.

1. WHICH MACROVIRUSES?

Only about macro viruses for MS Word. Macroviruses for MS Access, MS Excel, etc. are not covered here. In addition, we will keep in mind that we are mainly macro viruses written in Visual Basic for Application (VBA) for MS Word versions 97, 2000 and XP. Macro viruses for Word 6.0 / 7.0 are no longer are relevant, but how things are in Word 2003 is not yet very clear.

2. WHAT IS A DOCFILE

For details see / 1,2 /, and here in brief. It is a .DOC (or .DOT) file with a complex internal structure called "structured storage" (structured storage). MS Word can also create documents in "non-native" formats, for example, in .RTF, but we will not consider this situation. The structured storage rules stipulate that: - the file is split into many sectors (the default sector size is 512 bytes); - sectors are numbered and linked in chains;

             + ----------------------- +
             | V

  • --------- + + ---- + ---- + + --------- + + --------- + + ----- ---- + + --------- + | Title | | 0 | | 1 | | 2 | | 3 | -> | 4 |

  • --------- + + --------- + + ---- + ---- + + --------- + + ----- ---- + + --------- + | ^ + ----------------------- +

    • inside the file there is a header describing storage parameters;
    • inside the file there is a FAT table describing which sectors to which the chains belong;
    • inside the file there are directories describing the name of the chain and its address first sector. Chains form streams, which contain all useful information. (Actually, there are also lock bytes in which may store "unnamed" information about which MS Word does not knows).
    1. HOW THE WORD DOCUMENT IS STRUCTURED

    See details in / 3,4 /, and here in short. In general, a document takes up several streams within a file (but not necessarily all!). Here is a fairly typical thread configuration for healthy document:

    1Table <1> CompObj ObjectPool WordDocument <- Document text here <5> SummaryInformation <5> DocumentSummaryInformation

    But for a document containing macros (virus):

    1Table Macros VBA dir __SRP_0 <- + __SRP_1 | Here exe-code of macros __SRP_2 | __SRP_3 <- + ThisDocument <- Macro s-code and p-code here Tralivali <- And here Newmacros <- Here too _VBA_PROJECT <- Here is a description of the various parts of the project PROJECT PROJECTwm [1] CompObj ObjectPool WordDocument <- Document text here [5] SummaryInformation [5] DocumentSummaryInformation

    The same macro is distributed across different threads:

    1. s-code is its text source compressed by the LZSS method;
    2. p-code is a conversion of the source into an intermediate postfix language like

    ; a = 1234

    00A4 Let 1234 1234 0027 -> 0220 number in the list of variable names

    1. exe-code is its code, linked to execute virtual VBA machine, with commands like:

    Push Argument1 Push Argument2 [Stack]: = [Stack] + [Stack + 2]

    S-code and its corresponding p-code lie together in the same stream. All such streams begin with signature 01 16 01, according to this feature their easy to distinguish.

    1. ABOUT ANTI-VIRUS

    Actually, in 95% of cases the source text of the macro is quite enough to recognize or suspect a specific virus infection. The presence of an infection is usually indicated by the presence of methods:

    1. OrganizerCopy;
    2. .Import / .Export;
    3. .InsertLine, .AddFile, etc.

    The remaining 5% falls on cases:

    1. 1% (in fact, even less often) - when the packed source from the file is deleted (for example, by some crooked antivirus);
    2. 4% - when the virus is polymorphic.

    You can try this: do not decompile p-code and even more so ex-code from undocumented Microsoft codes (although so come in "real" antiviruses), but on the contrary, it's good to drive known VBA grammar in some Yacc and get a translator s-code to your own internal code. And according to this code (stripped of spaces, comments, "concatenated" lines, etc.) recognize virae and even try to emulate some of their pieces.

    1. DESCRIPTION OF THE EXAMPLE

    The DOC file structure can be examined using the DocFile plugin Browser to FAR Manager: see the object tree, view the insides of the stream in the form of a hex dump ... But FAR cannot unpack and show the source text of the macros hiding inside, and we can / 3 /. As an example, a console utility that traverses streams in the doc file, finds the signature 01 16 01 in them, finds in these streams the packed source, unpacks it, looks for substrings '.Import' and '.Export' and swears if suddenly found. Utility uses OLE2.DLL library and its methods to work with structured storage / 5 /. In addition, the utility will not work under Windows 95/98 and will quietly end, because these operating systems only have stub of the NTDLL.DLL library. Solutions to the problem: 1) rip out "good" library code for the RtlDecompressBuffer procedure; 2) to unpack the source codes of macros on your own, it's quite simple.

    LITERATURE

    1. Martin Schwartz. Laola file system.
    2. DrMAD. Internal document format MS Word.
    3. DrMAD. Direct access to macros in MS Word documents.
    4. DrMAD. How to Catch Your Ponytail. Chapter 6. Fight against macro viruses.
    5. Artem Kaev. ActiveX step by step.

    ATTACHMENT. SOURCE OF DEMONSTRATION UTILITY.

    Usage: SVIR.EXE docfile image