gmh5225 / yara-x

Experimenting with YARA and Rust

What's YARA-X?

This is an experimental project for evaluating the feasibility of writing a full-fledged implementation of YARA in Rust. For the time being don't take this project very seriously, it may be abandoned at any time if it doesn't prove to be worth the effort.

However, I would like to get something useful out of this, so the intention is at the very least implementing a code formatting tool for YARA in the spirit of rustfmt and gofmt. In the best case scenario this could evolve into becoming a serious replacement for YARA.

Changes with respect to YARA 4.x

This section describes the differences that YARA-X has with respect to YARA 4.x so far. These differences are not set in stone yet and may change in the future.

Negative numbers are not accepted in array indexing

The expression @a[-1] is valid in YARA 4.x, but its value is always undefined. In YARA-X this is an error.

Integers and strings are not promoted automatically to booleans

This rule is valid in YARA 4.x because the integer expression 2 is promoted to true.

rule type_promotion_example {
  condition:
    2
}

This however can lead to subtle errors due to the unintended use of integers or strings as booleans.

Duplicate rule modifiers are not accepted

In YARA 4.x rules can have any number of global or private, for example the following is valid:

global global global rule duplicated_global  {
   ... 
}

In YARA-X you can specify each modifier once. They can still appear in any order, though.

<quantifier> of <tuple> statements accept tuples of boolean expressions

In YARA 4.x the of statement accepts a tuple of string or rule identifiers. In both cases the identifiers can contain wildcards. For example both of these are valid:

1 of ($a, $c, $b*, $*)

1 of (some_rule, another_rule*)

The first case remains the same, but the second one has been generalized to accept arbitrary boolean expressions, like in...

1 of (true, false)

1 of ($a and not $b, $c, false)

Notice however that we have lost the possibility of using wildcards with rule names. So, this is valid...

1 of (some_rule)

But this is not valid...

1 of (some_rule*)