Table Of Contents
In order to detect malware and other file-based threats, ClamAV relies on signatures to differentiate clean and malicious/unwanted files. ClamAV signatures are primarily text-based and conform to one of the ClamAV-specific signature formats associated with a given method of detection. These formats are explained in the Signature formats section below. In addition, ClamAV 0.99 and above support signatures written in the YARA format. More information on this can be found in the Using YARA rules in ClamAV section.
The ClamAV project distributes a collection of signatures in the form of CVD (ClamAV Virus Database) files. The CVD file format provides a digitally-signed container that encapsulates the signatures and ensures that they can't be modified by a malicious third-party. This signature set is actively maintained by Cisco Talos and can be downloaded using the freshclam
application that ships with ClamAV. For more details on this, see the CVD file section.
ClamAV CVD and CLD database archives may be unpacked to the current directory using sigtool -u <database name>
. For more details on inspecting CVD and CLD files, see Inspecting signatures inside a CVD file. Once unpacked, you'll observe a large collection of database files with various extensions described below.
The CVD and CLD database archives may be supplemented with custom database files in the formats described to gain additional detection functionality. This is done simply by adding files of the following formats to the database directory, typically /usr/local/share/clamav
or "C:\Program Files\ClamAV\database"
. Alternatively, clamd
and clamscan
can be instructed to load the database from an alternative database file or database directory manually using the clamd
DatabaseDirectory
config option or the clamscan -d
command line option.
ClamAV provides a handful of configuration related databases along side the signature definitions.
*.cfg
: Dynamic config settings
*.cat
*.crb
: Trusted and revoked PE certs
*.ftm
: File Type Magic (FTM)
Note: Signature databases with an extension ending in u
are only loaded when Potentially Unwanted Application (PUA) signatures are enabled (default: off).
Body-based signature content is a definition that matches not based on a hash but based on the specific sequences of bytes exhibited by the target file.
ClamAV body-based signature content has a special format to allow regex-like matching of data that is not entirely known. This format is used extensively in both Extended Signatures and Logical Signatures.
*.ndb
*.ndu
: Extended signatures
*.ldb
*.ldu
; *.idb
: Logical Signatures
*.cdb
: Container Metadata Signatures
*.cbc
: Bytecode Signatures
*.pdb
*.gdb
*.wdb
: Phishing URL Signatures
*.hdb
*.hsb
*.hdu
*.hsu
: File hash signatures
*.mdb
*.msb
*.mdu
*.msu
: PE section hash signatures
*.yar
*.yara
: Yara rules
*.fp
*.sfp
*.ign
*.ign2
: Whitelisted files, signatures
*.pwdb
: Encrypted archive passwords
*.info
: Database information`
ClamAV uses the following prefixes for signature names:
Important rules of the naming convention:
clamscan
To test a new signature, first create a text file with the extension corresponding to the signature type (Ex: .ldb
for logical signatures). Then, add the signature as it's own line within the file. This file can be passed to clamscan
via the -d
option, which tells ClamAV to load signatures from the file specified. If the signature is not formatted correctly, ClamAV will display an error - run clamscan
with --debug --verbose
to see additional information about the error message. Some common causes of errors include:
If the rule is formatted correctly, clamscan
will load the signature(s) in and scan any files specified via the command line invocation (or all files in the current directory if none are specified). A successful detection will look like the following:
clamscan -d test.ldb text.exe test.exe: Win.Malware.Agent.UNOFFICIAL FOUND ----------- SCAN SUMMARY ----------- Known viruses: 1 Engine version: 0.100.0 Scanned directories: 0 Scanned files: 1 Infected files: 1 Data scanned: 17.45 MB Data read: 17.45 MB (ratio 1.00:1) Time: 0.400 sec (0 m 0 s)
If the rule did not match as intended:
clamscan
with the following options to see if raising the limits addresses the issue: --max-filesize=2000M --max-scansize=2000M --max-files=2000000 --max-recursion=2000000 --max-embeddedpe=2000M --max-htmlnormalize=2000000 --max-htmlnotags=2000000 --max-scriptnormalize=2000000 --max-ziptypercg=2000000 --max-partitions=2000000 --max-iconspe=2000000 --max-rechwp3=2000000 --pcre-match-limit=2000000 --pcre-recmatch-limit=2000000 --pcre-max-filesize=2000M
.NOTE: If you run clamscan
with a -d
flag, ClamAV will not load in the signatures downloaded via freshclam
. This means that:
.crb
rulesIf any of this functionality is needed, load in the CVD files manually with additional -d
flags.
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 clamscan
with the --debug
and --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:
$ clamscan --debug attachment.exe [...] LibClamAV debug: Recognized MS-EXE/DLL file LibClamAV debug: Matched signature for file type PE LibClamAV debug: File type: Executable
The engine recognized a windows executable.
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)
The section structure displayed above suggests the executable is packed
with UPX.
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
Indeed, libclamav recognizes the UPX data and saves the decompressed
(and rebuilt) executable into
/tmp/clamav-90d2d25c9dca42bae6fa9a764a4bcede
. Then it continues by
scanning this new file:
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 [...]
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.
ClamAV contains HTML normalization code which makes it easier to write signatures for HTML data that might differ based on white space, capitalization, and other insignificant differences. Running sigtool --html-normalise
on a HTML file can be used to see what a file's contents will look like after normalization. This command should generate the following files:
nocomment.html - the file is normalized, lower-case, with all comments and superfluous white space removed
notags.html - as above but with all HTML tags removed
javascript - any script contents are normalized and the results appended to this file
The code automatically decodes JScript.encode parts and char ref’s (e.g. f
). To create a successful signature for the input file type, the rule must match on the contents of one of the created files. Signatures matching on normalized HTML should have a target type of 3. For reference, see Target Types.
Similarly to HTML all ASCII text files get normalized (converted to lower-case, all superfluous white space and control characters removed, etc.) before scanning. Running sigtool --ascii-normalise
on a text file will result in a normalized version being written to the file named 'normalised_text'. Rules matching on normalized ASCII text should have a target type of 7. For reference, see Target Types.
If the file is compressed with UPX, FSG, Petite or another PE packer supported by libclamav, ClamAV will attempt to automatically unpack the executable and evaluate signatures against the unpacked executable. To inspect the executable that results from ClamAV's unpacking process, run clamscan
with --debug --leave-temps
. Example output for a FSG compressed file:
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
In the example above, /tmp/clamav-f592b20f9329ac1c91f0e12137bcce6c
is the unpacked executable, and a signature can be written based off of this file.
sigtool
sigtool
pulls in libclamav and provides shortcuts to doing tasks that clamscan
does behind the scenes. These can be really useful when writing a signature or trying to get information about a signature that might be causing FPs or performance problems.
The following sigtool
flags can be especially useful for signature writing:
--md5
/ --sha1
/ --sha256
: Generate the MD5/SHA1/SHA256 hash and calculate the file size, outputting both as a properly-formatted .hdb
/.hsb
signature
--mdb
: Generate section hashes of the specified file. This is useful when generating .mdb
signatures.
--decode
: Given a ClamAV signature from STDIN, show a more user-friendly representation of it. An example usage of this flag is cat test.ldb | sigtool --decode
.
--hex-dump
: Given a sequence of bytes from STDIN, print the hex equivalent. An example usage of this flag is echo -n "Match on this" | sigtool --hex-dump
.
--html-normalise
: Normalize the specified HTML file in the way that clamscan
will before looking for rule matches. Writing signatures off of these files makes it easier to write rules for target type HTML (you'll know what white space, capitalization, etc. to expect). See the HTML section for more details.
--ascii-normalise
: Normalize the specified ASCII text file in the way that clamscan
will before looking for rule matches. Writing signatures off of this normalized file data makes it easier to write rules for target type Txt (you'll know what white space, capitalization, etc. to expect). See the Text files sectino for more details.
--print-certs
: Print the Authenticode signatures of any PE files specified.
This is useful when writing signature-based .crb
rule files.
--vba
: Extract VBA/Word6 macro code
--test-sigs
: Given a signature and a sample, determine whether the signature matches and, if so, display the offset into the file where the match occurred. This can be useful for investigating false positive matches in clean files.
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:
ClamAV-VDB:build time:version:number of signatures:functionality level required:MD5 checksum:digital signature:builder name:build time (sec)
sigtool --info
displays detailed information about a given CVD file:
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.
The ClamAV project distributes a number of CVD files, including main.cvd
and daily.cvd
.
To view the signature associated with a given detection name, the CVD files can be unpacked and the underlying text files searched for a rule definition using a tool like grep
. To do this, use sigtool
's --unpack
flag as follows:
$ mkdir /tmp/clamav-sigs $ cd /tmp/clamav-sigs/ $ sigtool --unpack /var/lib/clamav/main.cvd $ ls COPYING main.fp main.hsb main.mdb main.ndb main.crb main.hdb main.info main.msb main.sfp
Below are tools that can be helpful when writing ClamAV signatures: