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:
The ClamAV project distributes a number of CVD files, including *main.cvd* and *daily.cvd*.
# 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 `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:
```bash
$ 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.
```bash
LibClamAV debug: Machine type: 80386
LibClamAV debug: NumberOfSections: 3
LibClamAV debug: TimeDateStamp: Fri Jan 10 04:57:55 2003
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.
# Signature formats
## Hash-based signatures
The easiest way to create signatures for ClamAV is to use filehash checksums, however this method can be only used against static malware.
### MD5 hash-based signatures
To create a MD5 signature for `test.exe` use the `--md5` option of
You can change the name (by default sigtool uses the name of the file) and place it inside a `*.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).
*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.*
### SHA1 and SHA256 hash-based signatures
ClamAV 0.98 has also added support for SHA1 and SHA256 file checksums. The format is the same as for MD5 file checksum. It can differentiate between them based on the length of the hash string in the signature. For best backwards compatibility, these should be placed inside a `*.hsb` file. The format is:
```
HashString:FileSize:MalwareName
```
### PE section based hash signatures
You can create a hash signature for a specific section in a PE file. Such signatures shall be stored inside `.mdb` files in the following format:
```
PESectionSize:PESectionHash:MalwareName
```
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 `--mdb`
ClamAV 0.98 has also added support for SHA1 and SHA256 section based signatures. The format is the same as for MD5 PE section based signatures. It can differentiate between them based on the length of the hash string in the signature. For best backwards compatibility, these should be placed inside a `*.msb` file.
### Hash signatures with unknown size
ClamAV 0.98 has also added support for hash signatures where the size is not known but the hash is. It is much more performance-efficient to use signatures with specific sizes, so be cautious when using this feature. For these cases, the ’\*’ character can be used in the size field. To ensure proper backwards compatibility with older versions of ClamAV, these signatures must have a minimum functional level of 73 or higher. Signatures that use the wildcard size without this level set will be rejected as malformed.
```
Sample .hsb signature matching any size
HashString:*:MalwareName:73
Sample .msb signature matching any size
*:PESectionHash:MalwareName:73
```
## 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.
### Hexadecimal format
You can use `sigtool --hex-dump` to convert any data into a hex-string:
```bash
zolw@localhost:/tmp/test$ sigtool --hex-dump
How do I look in hex?
486f7720646f2049206c6f6f6b20696e206865783f0a
```
### Wildcards
ClamAV supports the following wildcards for hex-signatures:
-`??`
Match any byte.
-`a?`
Match a high nibble (the four high bits).
**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 [3.2.7](#ndb)).
-`?a`
Match a low nibble (the four low bits).
-`*`
Match any number of bytes.
-`{n}`
Match \(n\) bytes.
-`{-n}`
Match \(n\) or less bytes.
-`{n-}`
Match \(n\) or more bytes.
-`{n-m}`
Match between \(n\) and \(m\) bytes (\(m > n\)).
-`HEXSIG[x-y]aa` or `aa[x-y]HEXSIG`
Match aa anchored to a hex-signature, see
<https://bugzilla.clamav.net/show_bug.cgi?id=776> for discussion and
examples.
The range signatures `*` and `{}` virtually separate a hex-signature into two parts, eg. `aabbcc*bbaacc` is treated as two sub-signatures `aabbcc` and `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. Note that there is one exception to this restriction; that is when the range wildcard is of the form `{n}` with `n<128`. In this case, ClamAV uses an optimization and translates `{n}` to the string consisting of `n ??` character wildcards. Character wildcards do not divide hex signatures into two parts and so the two static character requirement does not apply.
### Character classes
ClamAV supports the following character classes for hex-signatures:
-`(B)`
Match word boundary (including file boundaries).
-`(L)`
Match CR, CRLF or file boundaries.
-`(W)`
Match a non-alphanumeric character.
### Alternate strings
- Single-byte alternates (clamav-0.96) `(aa|bb|cc|...)` or `!(aa|bb|cc|...)` Match a member from a set of bytes \[aa, bb, cc, ...\].
- Negation operation can be applied to match any non-member, assumed to be one-byte in length.
- Signature modifiers and wildcards cannot be applied.
- Multi-byte fixed length alternates `(aaaa|bbbb|cccc|...)` or `!(aaaa|bbbb|cccc|...)` Match a member from a set of multi-byte alternates \[aaaa, bbbb, cccc, ...\] of n-length.
- All set members must be the same length.
- Negation operation can be applied to match any non-member, assumed to be n-bytes in length (clamav-0.98.2).
- Signature modifiers and wildcards cannot be applied.
- Generic alternates (clamav-0.99) `(alt1|alt2|alt3|...)` Match a member from a set of alternates \[alt1, alt2, alt3, ...\] that can be of variable lengths.
- Negation operation cannot be applied.
- Signature modifiers and nibble wildcards \[`??, a?, ?a`\] can be applied.
- Ranged wildcards \[`{n-m}`\] are limited to a fixed range of less than 128 bytes \[`{1} -> {127}`\].
Note that using signature modifiers and wildcards classifies the alternate type to be a generic alternate. Thus single-byte alternates and multi-byte fixed length alternates can use signature modifiers and wildcards but will be classified as generic alternate. This means that negation cannot be applied in this situation and there is a slight performance impact.
### Basic signature format
The simplest (and now deprecated) signature format is:
```
MalwareName=HexSignature
```
ClamAV will scan the entire file looking for HexSignature. All signatures of this type must be placed inside `*.db` files.
### Extended signature format
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:
where `TargetType` is one of the following numbers specifying the type of the target file:
- 0 = any file
- 1 = Portable Executable, both 32- and 64-bit.
- 2 = OLE2 containers, including their specific macros. The OLE2 format is primarily used by MS Office and MSI installation files.
- 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.
- 4 = Mail file
- 5 = Graphics
- 6 = ELF
- 7 = ASCII text file (normalized)
- 8 = Unused
- 9 = Mach-O files
- 10 = PDF files
- 11 = Flash files
- 12 = Java class files
And `Offset` is an asterisk or a decimal number `n` possibly combined with a special modifier:
-`*` = any
-`n` = absolute offset
-`EOF-n` = end of file minus `n` bytes
Signatures for PE, ELF and Mach-O files additionally support:
-`EP+n` = entry point plus n bytes (`EP+0` for `EP`)
-`EP-n` = entry point minus n bytes
-`Sx+n` = start of section `x`’s (counted from 0) data plus `n` bytes
-`SEx` = entire section `x` (offset must lie within section boundaries)
-`SL+n` = start of last section plus `n` bytes
All the above offsets except `*` can be turned into **floating offsets** and represented as `Offset,MaxShift` where `MaxShift` is an unsigned integer. A floating offset will match every offset between `Offset` and `Offset+MaxShift`, eg. `10,5` will match all offsets from 10 to 15 and `EP+n,y` will match all offsets from `EP+n` to `EP+n+y`. Versions of ClamAV older than 0.91 will silently ignore the `MaxShift` extension and only use `Offset`. Optional `MinFL` and `MaxFL` parameters can restrict the signature to specific engine releases. All signatures in the extended format must be placed inside `*.ndb` files.
### Logical signatures
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 `*.ldb` files in the following format:
-`TargetDescriptionBlock` provides information about the engine and target file with comma separated `Arg:Val` pairs. For args where `Val` is a range, the minimum and maximum values should be expressed as `min-max`.
-`LogicalExpression` specifies the logical expression describing the relationship between `Subsig0...SubsigN`. **Basis clause:** 0,1,...,N decimal indexes are SUB-EXPRESSIONS representing `Subsig0, Subsig1,...,SubsigN` respectively. **Inductive clause:** if `A` and `B` are SUB-EXPRESSIONS and `X, Y` are decimal numbers then `(A&B)`, `(A|B)`, `A=X`, `A=X,Y`, `A>X`, `A>X,Y`, `A<X` and `A<X,Y` are SUB-EXPRESSIONS
-`SubsigN` is n-th subsignature in extended format possibly preceded with an offset. There can be specified up to 64 subsigs.
Keywords used in `TargetDescriptionBlock`:
-`Target:X`: Target file type
-`Engine:X-Y`: Required engine functionality (range; 0.96). Note that if the `Engine` keyword is used, it must be the first one in the `TargetDescriptionBlock` for backwards compatibility
-`FileSize:X-Y`: Required file size (range in bytes; 0.96)
-`EntryPoint`: Entry point offset (range in bytes; 0.96)
-`NumberOfSections`: Required number of sections in executable (range; 0.96)
-`Container:CL_TYPE_*`: File type of the container which stores the scanned file. Specifying `CL_TYPE_ANY` matches on root objects only.
-`Intermediates:CL_TYPE_*>CL_TYPE_*`: File types of intermediate containers which stores the scanned file. Specify 1-16 file types separated by ’`>`’ in top-down order (’`>`’ separator not needed for single file type), last type should be the immediate container for the malicious content. `CL_TYPE_ANY` can be used as a wildcard file type. (expr; 0.100.0)
-`IconGroup1`: Icon group name 1 from .idb signature Required engine functionality (range; 0.96)
-`IconGroup2`: Icon group name 2 from .idb signature Required engine functionality (range; 0.96)
Modifiers for subexpressions:
-`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).
-`A=0` specifies negation (signature or block of signatures cannot be matched)
-`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.
-`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).
-`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.
-`A<X` and `A<X,Y` as above with the change of "more" to "less".
ClamAV (clamav-0.99) supports a number of additional subsignature
modifiers for logical signatures. This is done by specifying ’::’
followed by a number of characters representing the desired options.
Signatures using subsignature modifiers require `Engine:81-255` for
backwards-compatibility.
- Case-Insensitive \[`i`\]
Specifying the `i` modifier causes ClamAV to match all alphabetic hex bytes as case-insensitive. All patterns in ClamAV are case-sensitive by default.
- Wide \[`w`\]
Specifying the `w` causes ClamAV to match all hex bytes encoded with two bytes per character. Note this simply interweaves each character with NULL characters and does not truly support UTF-16 characters. Wildcards for ’wide’ subsignatures are not treated as wide (i.e. there can be an odd number of intermittent characters). This can be combined with `a` to search for patterns in both wide and ascii.
- Fullword \[`f`\]
Match subsignature as a fullword (delimited by non-alphanumeric characters).
- Ascii \[`a`\]
Match subsignature as ascii characters. This can be combined with `w` to search for patterns in both ascii and wide.
-`MACROID` points to a group of signatures; there can be at most 32 macro groups.
- In the example, `MACROID` is `12` and both `D1` and `D2` are members of macro group `12`. `D3` is a member of separate macro group `30`.
-`{min-max}` specifies the offset range at which one of the group signatures should match; the offset range is relative to the starting offset of the preceding subsignature. This means a macro subsignature cannot be the first subsignature.
- In the example, `{min-max}` is `{6-7}` and it is relative to the start of a `616161` match.
Byte compare subsignatures can be used to evaluate a numeric value at a given offset from the start of another (matched) subsignature within the same logical signature. These are executed after all other subsignatures within the logical subsignature are fired, with the exception of PCRE subsignatures. They can evaluate offsets only from a single referenced subsignature, and that subsignature must give a valid match for the evaluation to occur.
-`subsigid_trigger` is a required field and may refer to any single non-PCRE, non-Byte Compare subsignature within the lsig. The byte compare subsig will evaluate if `subsigid_trigger` matches. Triggering on multiple subsigs or logic based triggering is not currently supported.
-`offset` is a required field that consists of an `offset_modifier` and a numeric `offset` (hex or decimal offsets are okay).
-`offset_modifier` can be either `>>` or `<<` where the former denotes a positive offset and the latter denotes a negative offset. The offset is calculated from the start of `subsigid_trigger`, which allows for byte extraction before the specified match, after the match, and within the match itself.
-`offset` must be a positive hex or decimal value. This will be the number of bytes from the start of the referenced `subsigid_trigger` match within the file buffer to begin the comparison.
-`byte_options` are used to specify the numeric type and endianess of the extracted byte sequence in that order as well as the number of bytes to be read. By default ClamAV will attempt to matchup up to the number of byte specified, unless the `e` (exact) option is specified or the numeric type is `b` (binary). This field follows the form `[h|d|a|i][l|b][e]num_bytes`
-`h|d|a|i` where `h` specifies the byte sequence will be in hex, `d` decimal, `a` automatic detection of hex or decimal at runtime, and `i` signifies raw binary data.
-`l|b` where `l` specifies the byte sequence will be in little endian order and `b` big endian. If decimal `d` is specified, big-endian is implied and using `l` will result in a malformed database error.
-`e` specifies that ClamAV will only evaluate the comparison if it can extract the exact number of bytes specified. This option is implicitly declared when using the `i` flag.
-`num_bytes` specifies the number of bytes to extract. This can be a hex or decimal value. If `i` is specified only 1, 2, 4, and 8 are valid options.
-`comparisons` are a required field which denotes how to evaluate the extracted byte sequence. Each Byte Compare signature can have one or two `comparison_sets` separated by a comma. Each `comparison_set` consists of a `Comparison_symbol` and a `Comparison_value` and takes the form `Comparison_symbolComparison_value`. Thus, `comparisons` takes the form `comparison_set[,comparison_set]`
-`Comparison_value` is a required field which must be a numeric hex or decimal value. If all other conditions are met, the byte compare subsig will evalutate the extracted byte sequence against this number based on the provided `comparison_symbol`.
PCRE subsignatures are used within a logical signature (`.ldb`) to specify regex matches that execute once triggered by a conditional based on preceding subsignatures. Signatures using PCRE subsignatures require `Engine:81-255` for backwards-compatibility.
-`Trigger` is a required field that is a valid `LogicalExpression` and may refer to any subsignatures that precede this subsignature. Triggers cannot be self-referential and cannot refer to subsequent subsignatures.
-`PCRE` is the expression representing the regex to execute. `PCRE` must be delimited by ’/’ and usage of ’/’ within the expression need to be escaped. For backward compatibility, ’;’ within the expression must be expressed as ’`\x3B`’. `PCRE` cannot be empty and (?UTF\*) control sequence is not allowed. If debug is specified, named capture groups are displayed in a post-execution report.
-`Flags` are a series of characters which affect the compilation and execution of `PCRE` within the PCRE compiler and the ClamAV engine. This field is optional.
-`g [CLAMAV_GLOBAL]` specifies to search for ALL matches of PCRE (default is to search for first match). NOTE: INCREASES the time needed to run the PCRE.
-`r [CLAMAV_ROLLING]` specifies to use the given offset as the starting location to search for a match as opposed to the only location; applies to subsigs without maxshifts. By default, in order to facilatate normal ClamAV offset behavior, PCREs are auto-anchored (only attempt match on first offset); using the rolling option disables the auto-anchoring.
-`e [CLAMAV_ENCOMPASS]` specifies to CONFINE matching between the specified offset and maxshift; applies only when maxshift is specified. Note: DECREASES time needed to run the PCRE.
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 `IconGroup1` or `IconGroup2`. These identify two (optional) groups of icons defined in a .idb database file. The format of the .idb file is:
```
ICONNAME:GROUP1:GROUP2:ICON_HASH
```
where:
-`ICON_NAME` is a unique string identifier for a specific icon,
-`GROUP1` is a string identifier for the first group of icons (`IconGroup1`)
-`GROUP2` is a string identifier for the second group of icons (`IconGroup2`),
-`ICON_HASH` is a fuzzy hash of the icon image
The `ICON_HASH` field can be obtained from the debug output of libclamav. For example:
## 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 `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 *"Microsoft Corporation"*, the description *"TCP/IP Ping command"*, the internal name *"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 `VI` was introduced. This is similar to the other anchors (like `EP` and `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 `VI` token doesn’t need nor accept a `+/-` offset like e.g. `EP+1`. As for the hex signature itself, it’s just the utf16 dump of the key and value. Only the `??` and `(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 `clamscan` when the `--debug` option is given.
For example `clamscan --debug freecell.exe` produces:
Clamav 0.98 checks signed PE files for certificates and verifies each certificate in the chain against a database of trusted and revoked certificates. The signature format is
-`Trusted:` bit field, specifying whether the cert is trusted. 1 for trusted. 0 for revoked
-`Subject:` sha1 of the Subject field in hex
-`Serial:` the serial number as clamscan –debug –verbose reports
-`Pubkey:` the public key in hex
-`Exponent:` the exponent in hex. Currently ignored and hardcoded to 010001 (in hex)
-`CodeSign:` bit field, specifying whether this cert can sign code. 1 for true, 0 for false
-`TimeSign:` bit field. 1 for true, 0 for false
-`CertSign:` bit field, specifying whether this cert can sign other certs. 1 for true, 0 for false
-`NotBefore:` integer, cert should not be added before this variable. Defaults to 0 if left empty
-`Comment:` comments for this entry
The signatures for certs are stored inside `.crb` files.
## 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
-`VirusName:` Virus name to be displayed when signature matches
-`ContainerType:` one of
-`CL_TYPE_ZIP`,
-`CL_TYPE_RAR`,
-`CL_TYPE_ARJ`,
-`CL_TYPE_MSCAB`,
-`CL_TYPE_7Z`,
-`CL_TYPE_MAIL`,
-`CL_TYPE_(POSIX|OLD)_TAR`,
-`CL_TYPE_CPIO_(OLD|ODC|NEWC|CRC)` or
-`*` to match any of the container types listed here
-`ContainerSize:` size of the container file itself (eg. size of the zip archive) specified in bytes as absolute value or range `x-y`
-`FileNameREGEX:` regular expression describing name of the target file
-`FileSizeInContainer:` usually compressed size; for MAIL, TAR and CPIO == `FileSizeReal`; specified in bytes as absolute value or range
-`FileSizeReal:` usually uncompressed size; for MAIL, TAR and CPIO == `FileSizeInContainer`; absolute value or range
-`IsEncrypted`: 1 if the target file is encrypted, 0 if it’s not and `*` to ignore
-`FilePos`: file position in container (counting from 1); absolute value or range
-`Res1`: when `ContainerType` is `CL_TYPE_ZIP` or `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
-`Res2`: not used as of ClamAV 0.96
The signatures for container files are stored inside `.cdb` files.
## Signatures based on ZIP/RAR metadata (obsolete)
The (now obsolete) archive metadata signatures can be only applied to
- Encryption flag (1 – encrypted, 0 – not encrypted)
- File name (this is a regular expression - \* to ignore)
- Normal (uncompressed) size (\* to ignore)
- Compressed size (\* to ignore)
- CRC32 (\* to ignore)
- Compression method (\* to ignore)
- File position in archive (\* to ignore)
- Maximum number of nested archives (\* to ignore)
The database file should have the extension of `.zmd` or `.rmd` for zip or rar metadata respectively.
## Whitelist databases
To whitelist a specific file use the MD5 signature format and place it inside a database file with the extension of `.fp`. To whitelist a specific file with the SHA1 or SHA256 file hash signature format, place the signature inside a database file with the extension of `.sfp`. 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:
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).
## Signature names
ClamAV uses the following prefixes for signature names:
- *Worm* for Internet worms
- *Trojan* for backdoor programs
- *Adware* for adware
- *Flooder* for flooders
- *HTML* for HTML files
- *Email* for email messages
- *IRC* for IRC trojans
- *JS* for Java Script malware
- *PHP* for PHP malware
- *ASP* for ASP malware
- *VBS* for VBS malware
- *BAT* for BAT malware
- *W97M*, *W2000M* for Word macro viruses
- *X97M*, *X2000M* for Excel macro viruses
- *O97M*, *O2000M* for generic Office macro viruses
- *DoS* for Denial of Service attack software
- *DOS* for old DOS malware
- *Exploit* for popular exploits
- *VirTool* for virus construction kits
- *Dialer* for dialers
- *Joke* for hoaxes
Important rules of the naming convention:
- always use a -zippwd suffix in the malware name for signatures of type zmd,
- always use a -rarpwd suffix in the malware name for signatures of type rmd,
- only use alphanumeric characters, dash (-), dot (.), underscores (_) in malware names, never use space, apostrophe or quote mark.
## Using YARA rules in ClamAV
ClamAV version 0.99 and above can process YARA rules. ClamAV virus database file names ending with “.yar” or “.yara” are parsed as yara rule files. The link to the YARA rule grammar documentation may be found at http://plusvic.github.io/yara/. There are currently a few limitations on using YARA rules within ClamAV:
- YARA modules are not yet supported by ClamAV. This includes the “import” keyword and any YARA module-specific keywords.
- Global rules(“global” keyword) are not supported by ClamAV.
- External variables(“contains” and “matches” keywords) are not supported.
- YARA rules pre-compiled with the *yarac* command are not supported.
- As in the ClamAV logical and extended signature formats, YARA strings and segments of strings separated by wild cards must represent at least two octets of data.
- There is a maximum of 64 strings per YARA rule.
- YARA rules in ClamAV must contain at least one literal, hexadecimal, or regular expression string.
In addition, there are a few more ClamAV processing modes that may affect the outcome of YARA rules.
- *File decomposition and decompression* - Since ClamAV uses file decomposition and decompression to find viruses within de-archived and uncompressed inner files, YARA rules executed by ClamAV will match against these files as well.
- *Normalization* - By default, ClamAV normalizes HTML, JavaScript, and ASCII text files. YARA rules in ClamAV will match against the normalized result. The effects of normalization of these file types may be captured using `clamscan --leave-temps --tempdir=mytempdir`. YARA rules may then be written using the normalized file(s) found in `mytempdir`. Alternatively, starting with ClamAV 0.100.0, `clamscan --normalize=no` will prevent normalization and only scan the raw file. To obtain similar behavior prior to 0.99.2, use `clamscan --scan-html=no`. The corresponding parameters for clamd.conf are `Normalize` and `ScanHTML`.
- *YARA conditions driven by string matches* - All YARA conditions are driven by string matches in ClamAV. This saves from executing every YARA rule on every file. Any YARA condition may be augmented with a string match clause which is always true, such as:
```yara
rule CheckFileSize
{
strings:
$abc = "abc"
condition:
($abc or not $abc) and filesize <200KB
}
```
This will ensure that the YARA condition always performs the desired action (checking the file size in this example),
## Passwords for archive files \[experimental\]
ClamAV 0.99 allows for users to specify password attempts for certain password-compatible archives. Passwords will be attempted in order of appearance in the password signature file which use the extension of `.pwdb`. If no passwords apply or none are provided, ClamAV will default to the original behavior of parsing the file. Currently, as of ClamAV 0.99 \[flevel 81\], only `.zip` archives using the traditional PKWARE encryption are supported. The signature format is
-`SignatureName`: name to be displayed during debug when a password is successful
-`TargetDescriptionBlock`: provides information about the engine and target file with comma separated Arg:Val pairs
-`Engine:X-Y`: Required engine functionality
-`Container:CL_TYPE_*`: File type of applicable containers
-`PWStorageType`: determines how the password field is parsed
- 0 = cleartext
- 1 = hex
-`Password`: value used in password attempt
The signatures for password attempts are stored inside `.pwdb` files.
# Special files
## HTML
ClamAV contains a special HTML normalisation code which helps to detect HTML exploits. Running `sigtool --html-normalise` on a HTML file 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
The code automatically decodes JScript.encode parts and char ref’s (e.g. `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.
## Text files
Similarly to HTML all ASCII text files get normalized (converted to lower-case, all superfluous white space and control characters removed, etc.) before scanning. Use `clamscan --leave-temps` to obtain a normalized file then create a signature with the target type 7.
## Compressed Portable Executable files
If the file is compressed with UPX, FSG, Petite or other PE packer supported by libclamav, run `clamscan` with `--debug --leave-temps`. Example output for a FSG compressed file:
```bash
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
```
Next create a type 1 signature for `/tmp/clamav-f592b20f9329ac1c91f0e12137bcce6c`
