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.
You can use sigtool --hex-dump to convert any data into a hex-string:
zolw@localhost:/tmp/test$ sigtool --hex-dump
How do I look in hex?
486f7720646f2049206c6f6f6b20696e206865783f0a
ClamAV supports the following wildcards for hex-signatures:
??Match any byte.
a?Match a high nibble (the four high bits).
?aMatch 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 (where m > n).
HEXSIG[x-y]aa or aa[x-y]HEXSIGMatch aa anchored to a hex-signature, see Bugzilla ticket 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.
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.
(aa|bb|cc|...) or !(aa|bb|cc|...) Match a member from a set of bytes (eg: aa, bb, cc, ...).Signature modifiers and wildcards cannot be applied.
(aaaa|bbbb|cccc|...) or !(aaaa|bbbb|cccc|...) Match a member from a set of multi-byte alternates (eg: aaaa, bbbb, cccc, ...) of n-length.Signature modifiers and wildcards cannot be applied.
(alt1|alt2|alt3|...) Match a member from a set of alternates (eg: alt1, alt2, alt3, ...) that can be of variable lengths.??, a?, ?a) can be applied.Ranged wildcards (eg: {n-m}) are limited to a fixed range of less than 128 bytes (eg: {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.