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\begin{document}
\begin{center}
\huge Creating signatures for ClamAV\\
\vspace{2cm}
\end{center}
\noindent
\section{Introduction}
CVD (ClamAV Virus Database) is a digitally signed container that
includes signature databases in various text formats. The header
of the container is a 512 bytes long string with colon separated fields:
\begin{verbatim}
ClamAV-VDB:build time:version:number of signatures:functionality
level required:MD5 checksum:digital signature:builder name:build
time (sec)
\end{verbatim}
\verb+sigtool --info+ displays detailed information about a given CVD file:
\begin{verbatim}
zolw@localhost:/usr/local/share/clamav$ sigtool -i main.cvd
File: main.cvd
Build time: 09 Dec 2007 15:50 +0000
Version: 45
Signatures: 169676
Functionality level: 21
Builder: sven
MD5: b35429d8d5d60368eea9630062f7c75a
Digital signature: dxsusO/HWP3/GAA7VuZpxYwVsE9b+tCk+tPN6OyjVF/U8
JVh4vYmW8mZ62ZHYMlM903TMZFg5hZIxcjQB3SX0TapdF1SFNzoWjsyH53eXvMDY
eaPVNe2ccXLfEegoda4xU2TezbGfbSEGoU1qolyQYLX674sNA2Ni6l6/CEKYYh
Verification OK.
\end{verbatim}
The ClamAV project distributes a number of CVD files, including
\emph{main.cvd} and \emph{daily.cvd}.
\section{Debug information from libclamav}
In order to create efficient signatures for ClamAV it's important
to understand how the engine handles input files. The best way
to see how it works is having a look at the debug information from
libclamav. You can do it by calling \verb+clamscan+ with the
\verb+--debug+ and \verb+--leave-temps+ flags. The first switch
makes clamscan display all the interesting information from
libclamav and the second one avoids deleting temporary files so
they can be analyzed further. The now important part of the info
is:
\begin{verbatim}
$ clamscan --debug attachment.exe
[...]
LibClamAV debug: Recognized MS-EXE/DLL file
LibClamAV debug: Matched signature for file type PE
LibClamAV debug: File type: Executable
\end{verbatim}
The engine recognized a windows executable.
\begin{verbatim}
LibClamAV debug: Machine type: 80386
LibClamAV debug: NumberOfSections: 3
LibClamAV debug: TimeDateStamp: Fri Jan 10 04:57:55 2003
LibClamAV debug: SizeOfOptionalHeader: e0
LibClamAV debug: File format: PE
LibClamAV debug: MajorLinkerVersion: 6
LibClamAV debug: MinorLinkerVersion: 0
LibClamAV debug: SizeOfCode: 0x9000
LibClamAV debug: SizeOfInitializedData: 0x1000
LibClamAV debug: SizeOfUninitializedData: 0x1e000
LibClamAV debug: AddressOfEntryPoint: 0x27070
LibClamAV debug: BaseOfCode: 0x1f000
LibClamAV debug: SectionAlignment: 0x1000
LibClamAV debug: FileAlignment: 0x200
LibClamAV debug: MajorSubsystemVersion: 4
LibClamAV debug: MinorSubsystemVersion: 0
LibClamAV debug: SizeOfImage: 0x29000
LibClamAV debug: SizeOfHeaders: 0x400
LibClamAV debug: NumberOfRvaAndSizes: 16
LibClamAV debug: Subsystem: Win32 GUI
LibClamAV debug: ------------------------------------
LibClamAV debug: Section 0
LibClamAV debug: Section name: UPX0
LibClamAV debug: Section data (from headers - in memory)
LibClamAV debug: VirtualSize: 0x1e000 0x1e000
LibClamAV debug: VirtualAddress: 0x1000 0x1000
LibClamAV debug: SizeOfRawData: 0x0 0x0
LibClamAV debug: PointerToRawData: 0x400 0x400
LibClamAV debug: Section's memory is executable
LibClamAV debug: Section's memory is writeable
LibClamAV debug: ------------------------------------
LibClamAV debug: Section 1
LibClamAV debug: Section name: UPX1
LibClamAV debug: Section data (from headers - in memory)
LibClamAV debug: VirtualSize: 0x9000 0x9000
LibClamAV debug: VirtualAddress: 0x1f000 0x1f000
LibClamAV debug: SizeOfRawData: 0x8200 0x8200
LibClamAV debug: PointerToRawData: 0x400 0x400
LibClamAV debug: Section's memory is executable
LibClamAV debug: Section's memory is writeable
LibClamAV debug: ------------------------------------
LibClamAV debug: Section 2
LibClamAV debug: Section name: UPX2
LibClamAV debug: Section data (from headers - in memory)
LibClamAV debug: VirtualSize: 0x1000 0x1000
LibClamAV debug: VirtualAddress: 0x28000 0x28000
LibClamAV debug: SizeOfRawData: 0x200 0x1ff
LibClamAV debug: PointerToRawData: 0x8600 0x8600
LibClamAV debug: Section's memory is writeable
LibClamAV debug: ------------------------------------
LibClamAV debug: EntryPoint offset: 0x8470 (33904)
\end{verbatim}
The section structure displayed above suggests the executable is
packed with UPX.
\begin{verbatim}
LibClamAV debug: ------------------------------------
LibClamAV debug: EntryPoint offset: 0x8470 (33904)
LibClamAV debug: UPX/FSG/MEW: empty section found - assuming
compression
LibClamAV debug: UPX: bad magic - scanning for imports
LibClamAV debug: UPX: PE structure rebuilt from compressed file
LibClamAV debug: UPX: Successfully decompressed with NRV2B
LibClamAV debug: UPX/FSG: Decompressed data saved in
/tmp/clamav-90d2d25c9dca42bae6fa9a764a4bcede
LibClamAV debug: ***** Scanning decompressed file *****
LibClamAV debug: Recognized MS-EXE/DLL file
LibClamAV debug: Matched signature for file type PE
\end{verbatim}
Indeed, libclamav recognizes the UPX data and saves the decompressed
(and rebuilt) executable into \verb+/tmp/clamav-90d2d25c9dca42bae6fa9a764a4bcede+.
Then it continues by scanning this new file:
\begin{verbatim}
LibClamAV debug: File type: Executable
LibClamAV debug: Machine type: 80386
LibClamAV debug: NumberOfSections: 3
LibClamAV debug: TimeDateStamp: Thu Jan 27 11:43:15 2011
LibClamAV debug: SizeOfOptionalHeader: e0
LibClamAV debug: File format: PE
LibClamAV debug: MajorLinkerVersion: 6
LibClamAV debug: MinorLinkerVersion: 0
LibClamAV debug: SizeOfCode: 0xc000
LibClamAV debug: SizeOfInitializedData: 0x19000
LibClamAV debug: SizeOfUninitializedData: 0x0
LibClamAV debug: AddressOfEntryPoint: 0x7b9f
LibClamAV debug: BaseOfCode: 0x1000
LibClamAV debug: SectionAlignment: 0x1000
LibClamAV debug: FileAlignment: 0x1000
LibClamAV debug: MajorSubsystemVersion: 4
LibClamAV debug: MinorSubsystemVersion: 0
LibClamAV debug: SizeOfImage: 0x26000
LibClamAV debug: SizeOfHeaders: 0x1000
LibClamAV debug: NumberOfRvaAndSizes: 16
LibClamAV debug: Subsystem: Win32 GUI
LibClamAV debug: ------------------------------------
LibClamAV debug: Section 0
LibClamAV debug: Section name: .text
LibClamAV debug: Section data (from headers - in memory)
LibClamAV debug: VirtualSize: 0xc000 0xc000
LibClamAV debug: VirtualAddress: 0x1000 0x1000
LibClamAV debug: SizeOfRawData: 0xc000 0xc000
LibClamAV debug: PointerToRawData: 0x1000 0x1000
LibClamAV debug: Section contains executable code
LibClamAV debug: Section's memory is executable
LibClamAV debug: ------------------------------------
LibClamAV debug: Section 1
LibClamAV debug: Section name: .rdata
LibClamAV debug: Section data (from headers - in memory)
LibClamAV debug: VirtualSize: 0x2000 0x2000
LibClamAV debug: VirtualAddress: 0xd000 0xd000
LibClamAV debug: SizeOfRawData: 0x2000 0x2000
LibClamAV debug: PointerToRawData: 0xd000 0xd000
LibClamAV debug: ------------------------------------
LibClamAV debug: Section 2
LibClamAV debug: Section name: .data
LibClamAV debug: Section data (from headers - in memory)
LibClamAV debug: VirtualSize: 0x17000 0x17000
LibClamAV debug: VirtualAddress: 0xf000 0xf000
LibClamAV debug: SizeOfRawData: 0x17000 0x17000
LibClamAV debug: PointerToRawData: 0xf000 0xf000
LibClamAV debug: Section's memory is writeable
LibClamAV debug: ------------------------------------
LibClamAV debug: EntryPoint offset: 0x7b9f (31647)
LibClamAV debug: Bytecode executing hook id 257 (0 hooks)
attachment.exe: OK
[...]
\end{verbatim}
No additional files get created by libclamav. By writing
a signature for the decompressed file you have more chances
that the engine will detect the target data when it gets
compressed with another packer.
This method should be applied to all files for which you want
to create signatures. By analyzing the debug information you
can quickly see how the engine recognizes and preprocesses
the data and what additional files get created. Signatures
created for bottom-level temporary files are usually more
generic and should help detecting the same malware in
different forms.
\section{Signature formats}
\subsection{MD5}
The easiest way to create signatures for ClamAV is to use MD5 checksums,
however this method can be only used against static malware. To create
a signature for \verb+test.exe+ use the \verb+--md5+ option of sigtool:
\begin{verbatim}
zolw@localhost:/tmp/test$ sigtool --md5 test.exe > test.hdb
zolw@localhost:/tmp/test$ cat test.hdb
48c4533230e1ae1c118c741c0db19dfb:17387:test.exe
\end{verbatim}
That's it! The signature is ready for use:
\begin{verbatim}
zolw@localhost:/tmp/test$ clamscan -d test.hdb test.exe
test.exe: test.exe FOUND
----------- SCAN SUMMARY -----------
Known viruses: 1
Scanned directories: 0
Engine version: 0.92.1
Scanned files: 1
Infected files: 1
Data scanned: 0.02 MB
Time: 0.024 sec (0 m 0 s)
\end{verbatim}
You can change the name (by default sigtool uses the name of the file)
and place it inside a \verb+*.hdb+ file. A single database file can
include any number of signatures. To get them automatically loaded
each time clamscan/clamd starts just copy the database file(s) into
the local virus database directory (eg. /usr/local/share/clamav).
\emph{The hash-based signatures shall not be used for text files,
HTML and any other data that gets internally preprocessed before
pattern matching. If you really want to use a hash signature in
such a case, run clamscan with --debug and --leave-temps flags
as described above and create a signature for a preprocessed file
left in /tmp. Please keep in mind that a hash signature will stop
matching as soon as a single byte changes in the target file.}
\subsection{MD5, PE section based}
You can create a MD5 signature for a specific section in a PE file.
Such signatures shall be stored inside \verb+.mdb+ files in the
following format:
\begin{verbatim}
PESectionSize:MD5:MalwareName
\end{verbatim}
The easiest way to generate MD5 based section signatures is to extract
target PE sections into separate files and then run sigtool with the
option \verb+--mdb+
\subsection{Body-based signatures}
ClamAV stores all body-based signatures in a hexadecimal format. In this
section by a hex-signature we mean a fragment of malware's body converted
into a hexadecimal string which can be additionally extended using various
wildcards.
\subsubsection{Hexadecimal format}
You can use \verb+sigtool --hex-dump+ to convert any data into a hex-string:
\begin{verbatim}
zolw@localhost:/tmp/test$ sigtool --hex-dump
How do I look in hex?
486f7720646f2049206c6f6f6b20696e206865783f0a
\end{verbatim}
\subsubsection{Wildcards}
ClamAV supports the following extensions for hex-signatures:
\begin{itemize}
\item \verb+??+\\
Match any byte.
\item \verb+a?+\\
Match a high nibble (the four high bits).\\ \textbf{IMPORTANT NOTE:}
The nibble matching is only available in libclamav with the
functionality level 17 and higher therefore please only use it with
.ndb signatures followed by ":17" (MinEngineFunctionalityLevel,
see \ref{ndb}).
\item \verb+?a+\\
Match a low nibble (the four low bits).
\item \verb+*+\\
Match any number of bytes.
\item \verb+{n}+\\
Match $n$ bytes.
\item \verb+{-n}+\\
Match $n$ or less bytes.
\item \verb+{n-}+\\
Match $n$ or more bytes.
\item \verb+{n-m}+\\
Match between $n$ and $m$ bytes ($m > n$).
\item \verb+(aa|bb|cc|..)+\\
Match aa or bb or cc..
\item \verb+!(aa|bb|cc|..)+\\
Match any byte except aa and bb and cc.. (ClamAV$\ge$0.96)
\item \verb+HEXSIG[x-y]aa+ or \verb+aa[x-y]HEXSIG+\\
Match aa anchored to a hex-signature, see
\url{https://wwws.clamav.net/bugzilla/show_bug.cgi?id=776} for
discussion and examples.
\item \verb+(B)+\\
Match word boundary (including file boundaries).
\item \verb+(L)+\\
Match CR, CRLF or file boundaries.
\end{itemize}
The range signatures \verb+*+ and \verb+{}+ virtually separate
a hex-signature into two parts, eg. \verb+aabbcc*bbaacc+ is treated
as two sub-signatures \verb+aabbcc+ and \verb+bbaacc+ with any number
of bytes between them. It's a requirement that each sub-signature
includes a block of two static characters somewhere in its body.
\subsubsection{Basic signature format}
The simplest (and now deprecated) signature format is:
\begin{verbatim}
MalwareName=HexSignature
\end{verbatim}
ClamAV will scan the entire file looking for HexSignature. All
signatures of this type must be placed inside \verb+*.db+ files.
\subsubsection{Extended signature format}\label{ndb}
The extended signature format allows for specification of additional
information such as a target file type, virus offset or engine version,
making the detection more reliable. The format is:
\begin{verbatim}
MalwareName:TargetType:Offset:HexSignature[:MinFL:[MaxFL]]
\end{verbatim}
where \verb+TargetType+ is one of the following numbers specifying
the type of the target file:
\begin{itemize}
\item 0 = any file
\item 1 = Portable Executable, both 32- and 64-bit.
\item 2 = file inside OLE2 container (e.g. image, embedded executable,
VBA script). The OLE2 format is primarily used by MS Office and MSI
installation files.
\item 3 = HTML (normalized: whitespace transformed to spaces, tags/tag
attributes normalized, all lowercase), Javascript is normalized too:
all strings are normalized (hex encoding is decoded), numbers are
parsed and normalized, local variables/function names are normalized
to 'n001' format, argument to eval() is parsed as JS again,
unescape() is handled, some simple JS packers are handled,
output is whitespace normalized.
\item 4 = Mail file
\item 5 = Graphics
\item 6 = ELF
\item 7 = ASCII text file (normalized)
\item 8 = Unused
\item 9 = Mach-O files
\end{itemize}
And \verb+Offset+ is an asterisk or a decimal number \verb+n+ possibly
combined with a special modifier:
\begin{itemize}
\item \verb+*+ = any
\item \verb+n+ = absolute offset
\item \verb+EOF-n+ = end of file minus \verb+n+ bytes
\end{itemize}
Signatures for PE, ELF and Mach-O files additionally support:
\begin{itemize}
\item \verb#EP+n# = entry point plus n bytes (\verb#EP+0# for \verb+EP+)
\item \verb#EP-n# = entry point minus n bytes
\item \verb#Sx+n# = start of section \verb+x+'s (counted from 0)
data plus \verb+n+ bytes
\item \verb#Sx-n# = start of section \verb+x+'s data minus \verb+n+ bytes
\item \verb#SL+n# = start of last section plus \verb+n+ bytes
\item \verb#SL-n# = start of last section minus \verb+n+ bytes
\end{itemize}
All the above offsets except \verb+*+ can be turned into
\textbf{floating offsets} and represented as \verb+Offset,MaxShift+ where
\verb+MaxShift+ is an unsigned integer. A floating offset will match every
offset between \verb+Offset+ and \verb#Offset+MaxShift#, eg. \verb+10,5+
will match all offsets from 10 to 15 and \verb#EP+n,y# will match all
offsets from \verb#EP+n# to \verb#EP+n+y#. Versions of ClamAV older than
0.91 will silently ignore the \verb+MaxShift+ extension and only use
\verb+Offset+.\\
\noindent
Optional \verb+MinFL+ and \verb+MaxFL+ parameters can restrict the signature
to specific engine releases. All signatures in the extended format must be
placed inside \verb+*.ndb+ files.
\subsubsection{Logical signatures}\label{ndb}
Logical signatures allow combining of multiple signatures in extended
format using logical operators. They can provide both more detailed and
flexible pattern matching. The logical sigs are stored inside \verb+*.ldb+
files in the following format:
\begin{verbatim}
SignatureName;TargetDescriptionBlock;LogicalExpression;Subsig0;
Subsig1;Subsig2;...
\end{verbatim}
where:
\begin{itemize}
\item \verb+TargetDescriptionBlock+ provides information about the
engine and target file with comma separated \verb+Arg:Val+ pairs,
currently (as of 0.95.1) only \verb+Target:X+ and \verb+Engine:X-Y+
are supported.
\item \verb+LogicalExpression+ specifies the logical expression
describing the relationship between \verb+Subsig0...SubsigN+.\\
\textbf{Basis clause:} 0,1,...,N decimal indexes are SUB-EXPRESSIONS
representing \verb+Subsig0, Subsig1,...,SubsigN+ respectively.\\
\textbf{Inductive clause:} if \verb+A+ and \verb+B+ are
SUB-EXPRESSIONS and \verb+X, Y+ are decimal numbers then
\verb+(A&B)+, \verb+(A|B)+, \verb+A=X+, \verb+A=X,Y+, \verb+A>X+,
\verb+A>X,Y+, \verb+A<X+ and \verb+A<X,Y+ are SUB-EXPRESSIONS
\item \verb+SubsigN+ is n-th subsignature in extended format possibly
preceded with an offset. There can be specified up to 64 subsigs.
\end{itemize}
Keywords used in \verb+TargetDescriptionBlock+:
\begin{itemize}
\item \verb+Target:X+: Target file type
\item \verb+Engine:X-Y+: Required engine functionality (range; 0.96)
\item \verb+FileSize:X-Y+: Required file size (range in bytes; 0.96)
\item \verb+EntryPoint+: Entry point offset (range in bytes; 0.96)
\item \verb+NumberOfSections+: Required number of sections in executable (range; 0.96)
\item \verb+Container:CL_TYPE_*+: File type of the container which stores the scanned file
\end{itemize}
Modifiers for subexpressions:
\begin{itemize}
\item \verb+A=X+: If the SUB-EXPRESSION A refers to a single signature
then this signature must get matched exactly X times; if it refers to
a (logical) block of signatures then this block must generate exactly
X matches (with any of its sigs).
\item \verb+A=0+ specifies negation (signature or block of signatures
cannot be matched)
\item \verb+A=X,Y+: If the SUB-EXPRESSION A refers to a single signature
then this signature must be matched exactly X times; if it refers to
a (logical) block of signatures then this block must generate X matches
and at least Y different signatures must get matched.
\item \verb+A>X+: If the SUB-EXPRESSION A refers to a single signature
then this signature must get matched more than X times; if it refers to
a (logical) block of signatures then this block must generate more
than X matches (with any of its sigs).
\item \verb+A>X,Y+: If the SUB-EXPRESSION A refers to a single signature
then this signature must get matched more than X times; if it refers to
a (logical) block of signatures then this block must generate more than
X matches and at least Y different signatures must be matched.
\item \verb+A<X+ and \verb+A<X,Y+ as above with the change of "more"
to "less".
\end{itemize}
Examples:
\begin{verbatim}
Sig1;Target:0;(0&1&2&3)&(4|1);6b6f74656b;616c61;7a6f6c77;7374656
6616e;deadbeef
Sig2;Target:0;((0|1|2)>5,2)&(3|1);6b6f74656b;616c61;7a6f6c77;737
46566616e
Sig3;Target:0;((0|1|2|3)=2)&(4|1);6b6f74656b;616c61;7a6f6c77;737
46566616e;deadbeef
Sig4;Target:1,Engine:18-20;((0|1)&(2|3))&4;EP+123:33c06834f04100
f2aef7d14951684cf04100e8110a00;S2+78:22??232c2d252229{-15}6e6573
(63|64)61706528;S+50:68efa311c3b9963cb1ee8e586d32aeb9043e;f9c58d
cf43987e4f519d629b103375;SL+550:6300680065005c0046006900
\end{verbatim}
ClamAV 0.96 introduced support for special macro subsignatures in
the following format: \verb+${min-max}MACROID$+, where \verb+MACROID+
points to a group of signatures and \verb+{min-max}+ specifies the
offset range at which one of the group signatures should match.
The range is calculated against the match offset of the previous
subsignature. The macro subsignature makes its preceding subsignature
considered a match only if both of them get matched. For more
information and examples please see
\url{https://wwws.clamav.net/bugzilla/show_bug.cgi?id=164}.
\subsection{Icon signatures for PE files}
ClamAV 0.96 includes an approximate/fuzzy icon matcher to help
detecting malicious executables disguising themselves as innocent
looking image files, office documents and the like.
Icon matching is only triggered via .ldb signatures using the special
attribute tokens \verb+IconGroup1+ or \verb+IconGroup2+. These identify
two (optional) groups of icons defined in a .idb database file. The
format of the .idb file is:
\begin{verbatim}
ICONNAME:GROUP1:GROUP2:ICON_HASH
\end{verbatim}
where:
\begin{itemize}
\item \verb+ICON_NAME+ is a unique string identifier for a specific
icon,
\item \verb+GROUP1+ is a string identifier for the first group of
icons (\verb+IconGroup1+)
\item \verb+GROUP2+ is a string identifier for the second group of
icons (\verb+IconGroup2+),
\item \verb+ICON_HASH+ is a fuzzy hash of the icon image
\end{itemize}
The \verb+ICON_HASH+ field can be obtained from the debug output of
libclamav. For example:
\begin{verbatim}
LibClamAV debug: ICO SIGNATURE:
ICON_NAME:GROUP1:GROUP2:18e2e0304ce60a0cc3a09053a30000414100057e
000afe0000e 80006e510078b0a08910d11ad04105e0811510f084e01040c080
a1d0b0021000a39002a41
\end{verbatim}
\subsection{Signatures for Version Information metadata in PE files}
Starting with ClamAV 0.96 it is possible to easily match certain
information built into PE files (executables and dynamic link libraries).
Whenever you lookup the properties of a PE executable file in windows,
you are presented with a bunch of details about the file itself.
These info are stored in a special area of the file resources which goes
under the name of \verb+VS_VERSION_INFORMATION+ (or versioninfo for short).
It is divided into 2 parts. The first part (which is rather uninteresting)
is really a bunch of numbers and flags indicating the product and file
version. It was originally intended for use with installers which, after
parsing it, should be able to determine whether a certain executable or
library are to be upgraded/overwritten or are already up to date. Suffice
to say, this approach never really worked and is generally never used.
The second block is much more interesting: it is a simple list of key/value
strings, intended for user information and completely ignored by the OS.
For example, if you look at ping.exe you can see the company being \emph{"Microsoft
Corporation"}, the description \emph{"TCP/IP Ping command"}, the internal name
\emph{"ping.exe"} and so on... Depending on the OS version, some keys may be given
peculiar visibility in the file properties dialog, however they are internally
all the same.
To match a versioninfo key/value pair, the special file offset anchor \verb+VI+ was
introduced. This is similar to the other anchors (like \verb+EP+ and \verb+SL+)
except that, instead of matching the hex pattern against a single offset, it checks
it against each and every key/value pair in the file. The \verb+VI+ token doesn't
need nor accept a \verb#+/-# offset like e.g. \verb#EP+1#. As for the hex signature
itself, it's just the utf16 dump of the key and value. Only the \verb+??+ and
\verb+(aa|bb)+ wildcards are allowed in the signature. Usually, you don't need to
bother figuring it out: each key/value pair together with the corresponding VI-based
signature is printed by \verb+clamscan+ when the \verb+--debug+ option is given.
For example \verb+clamscan --debug freecell.exe+ produces:
\begin{verbatim}
[...]
Recognized MS-EXE/DLL file
in cli_peheader
versioninfo_cb: type: 10, name: 1, lang: 410, rva: 9608
cli_peheader: parsing version info @ rva 9608 (1/1)
VersionInfo (d2de): 'CompanyName'='Microsoft Corporation' -
VI:43006f006d00700061006e0079004e0061006d006500000000004d006900
630072006f0073006f0066007400200043006f00720070006f0072006100740
069006f006e000000
VersionInfo (d32a): 'FileDescription'='Entertainment Pack
FreeCell Game' - VI:460069006c006500440065007300630072006900700
0740069006f006e000000000045006e007400650072007400610069006e006d
0065006e00740020005000610063006b0020004600720065006500430065006
c006c002000470061006d0065000000
VersionInfo (d396): 'FileVersion'='5.1.2600.0 (xpclient.010817
-1148)' - VI:460069006c006500560065007200730069006f006e00000000
0035002e0031002e0032003600300030002e003000200028007800700063006
c00690065006e0074002e003000310030003800310037002d00310031003400
380029000000
VersionInfo (d3fa): 'InternalName'='freecell' - VI:49006e007400
650072006e0061006c004e0061006d006500000066007200650065006300650
06c006c000000
VersionInfo (d4ba): 'OriginalFilename'='freecell' - VI:4f007200
6900670069006e0061006c00460069006c0065006e0061006d0065000000660
0720065006500630065006c006c000000
VersionInfo (d4f6): 'ProductName'='Sistema operativo Microsoft
Windows' - VI:500072006f0064007500630074004e0061006d00650000000
000530069007300740065006d00610020006f00700065007200610074006900
76006f0020004d006900630072006f0073006f0066007400ae0020005700690
06e0064006f0077007300ae000000
VersionInfo (d562): 'ProductVersion'='5.1.2600.0' - VI:50007200
6f006400750063007400560065007200730069006f006e00000035002e00310
02e0032003600300030002e0030000000
[...]
\end{verbatim}
Although VI-based signatures are intended for use in logical signatures you can test them
using ordinary \verb+.ndb+ files. For example:
\begin{verbatim}
my_test_vi_sig:1:VI:paste_your_hex_sig_here
\end{verbatim}
Final note. If you want to decode a VI-based signature into a human readable form you can use:
\begin{verbatim}
echo hex_string | xxd -r -p | strings -el
\end{verbatim}
For example:
\begin{verbatim}
$ echo 460069006c0065004400650073006300720069007000740069006f006e
000000000045006e007400650072007400610069006e006d0065006e007400200
05000610063006b0020004600720065006500430065006c006c00200047006100
6d0065000000 | xxd -r -p | strings -el
FileDescription
Entertainment Pack FreeCell Game
\end{verbatim}
\subsection{Signatures based on container metadata}
ClamAV 0.96 allows creating generic signatures matching files stored
inside different container types which meet specific conditions.
The signature format is
\begin{verbatim}
VirusName:ContainerType:ContainerSize:FileNameREGEX:
FileSizeInContainer:FileSizeReal:IsEncrypted:FilePos:
Res1:Res2[:MinFL[:MaxFL]]
\end{verbatim}
where the corresponding fields are:
\begin{itemize}
\item \verb+VirusName:+ Virus name to be displayed when signature matches
\item \verb+ContainerType:+ one of \verb+CL_TYPE_ZIP+, \verb+CL_TYPE_RAR+,
\verb+CL_TYPE_ARJ+,\\
\verb+CL_TYPE_CAB+, \verb+CL_TYPE_7Z+, \verb+CL_TYPE_MAIL+, \verb+CL_TYPE_(POSIX|OLD)_TAR+,\\
\verb+CL_TYPE_CPIO_(OLD|ODC|NEWC|CRC)+ or \verb+*+ to match
any of the container types listed here
\item \verb+ContainerSize:+ size of the container file itself (eg. size of
the zip archive) specified in bytes as absolute value or range \verb+x-y+
\item \verb+FileNameREGEX:+ regular expression describing name of the target file
\item \verb+FileSizeInContainer:+ usually compressed size; for MAIL, TAR and CPIO ==
\verb+FileSizeReal+; specified in bytes as absolute value or range
\item \verb+FileSizeReal:+ usually uncompressed size; for MAIL, TAR and CPIO ==
\verb+FileSizeInContainer+; absolute value or range
\item \verb+IsEncrypted+: 1 if the target file is encrypted, 0 if it's not and
\verb+*+ to ignore
\item \verb+FilePos+: file position in container (counting from 1); absolute value
or range
\item \verb+Res1+: when \verb+ContainerType+ is \verb+CL_TYPE_ZIP+ or
\verb+CL_TYPE_RAR+ this field is treated as a CRC sum of the target file
specified in hexadecimal format; for other container types it's ignored
\item \verb+Res2+: not used as of ClamAV 0.96
\end{itemize}
The signatures for container files are stored inside \verb+.cdb+ files.
\subsection{Signatures based on ZIP/RAR metadata (obsolete)}
The (now obsolete) archive metadata signatures can be only applied
to ZIP and RAR files and have the following format:
\begin{verbatim}
virname:encrypted:filename:normal size:csize:crc32:cmethod:
fileno:max depth
\end{verbatim}
where the corresponding fields are:
\begin{itemize}
\item Virus name
\item Encryption flag (1 -- encrypted, 0 -- not encrypted)
\item File name (this is a regular expression - * to ignore)
\item Normal (uncompressed) size (* to ignore)
\item Compressed size (* to ignore)
\item CRC32 (* to ignore)
\item Compression method (* to ignore)
\item File position in archive (* to ignore)
\item Maximum number of nested archives (* to ignore)
\end{itemize}
The database file should have the extension of \verb+.zmd+ or
\verb+.rmd+ for zip or rar metadata respectively.
\subsection{Whitelist databases}
To whitelist a specific file use the MD5 signature format and place
it inside a database file with the extension of \verb+.fp+.\\
\noindent
To whitelist a specific signature from the database you just add
its name into a local file called local.ign2 stored inside the
database directory. You can additionally follow the signature name
with the MD5 of the entire database entry for this signature, eg:
\begin{verbatim}
Eicar-Test-Signature:bc356bae4c42f19a3de16e333ba3569c
\end{verbatim}
In such a case, the signature will no longer be whitelisted when
its entry in the database gets modified (eg. the signature gets
updated to avoid false alerts).
\subsection{Signature names}
ClamAV uses the following prefixes for signature names:
\begin{itemize}
\item \emph{Worm} for Internet worms
\item \emph{Trojan} for backdoor programs
\item \emph{Adware} for adware
\item \emph{Flooder} for flooders
\item \emph{HTML} for HTML files
\item \emph{Email} for email messages
\item \emph{IRC} for IRC trojans
\item \emph{JS} for Java Script malware
\item \emph{PHP} for PHP malware
\item \emph{ASP} for ASP malware
\item \emph{VBS} for VBS malware
\item \emph{BAT} for BAT malware
\item \emph{W97M}, \emph{W2000M} for Word macro viruses
\item \emph{X97M}, \emph{X2000M} for Excel macro viruses
\item \emph{O97M}, \emph{O2000M} for generic Office macro viruses
\item \emph{DoS} for Denial of Service attack software
\item \emph{DOS} for old DOS malware
\item \emph{Exploit} for popular exploits
\item \emph{VirTool} for virus construction kits
\item \emph{Dialer} for dialers
\item \emph{Joke} for hoaxes
\end{itemize}
Important rules of the naming convention:
\begin{itemize}
\item always use a -zippwd suffix in the malware name for signatures
of type zmd,
\item always use a -rarpwd suffix in the malware name for signatures
of type rmd,
\item only use alphanumeric characters, dash (-), dot (.), underscores
(\_) in malware names, never use space, apostrophe or quote mark.
\end{itemize}
\section{Special files}
\subsection{HTML}
ClamAV contains a special HTML normalisation code which helps to detect
HTML exploits. Running \verb+sigtool --html-normalise+ on a HTML file
should generate the following files:
\begin{itemize}
\item nocomment.html - the file is normalized, lower-case, with all
comments and superflous white space removed
\item notags.html - as above but with all HTML tags removed
\end{itemize}
The code automatically decodes JScript.encode parts and char ref's (e.g.
\verb+f+). You need to create a signature against one of the created
files. To eliminate potential false positive alerts the target type should
be set to 3.
\subsection{Text files}
Similarly to HTML all ASCII text files get normalized (converted
to lower-case, all superflous white space and control characters removed,
etc.) before scanning. Use \verb+clamscan --leave-temps+ to obtain
a normalized file then create a signature with the target type 7.
\subsection{Compressed Portable Executable files}
If the file is compressed with UPX, FSG, Petite or other PE packer
supported by libclamav, run \verb+clamscan+ with
\verb+--debug --leave-temps+. Example output for a FSG compressed file:
\begin{verbatim}
LibClamAV debug: UPX/FSG/MEW: empty section found - assuming compression
LibClamAV debug: FSG: found old EP @119e0
LibClamAV debug: FSG: Unpacked and rebuilt executable saved in
/tmp/clamav-f592b20f9329ac1c91f0e12137bcce6c
\end{verbatim}
Next create a type 1 signature for \verb+/tmp/clamav-f592b20f9329ac1c91f0e12137bcce6c+
\end{document}