Differences with YARA

One of the main goals of YARA-X is maintaining compatibility with YARA at the rule level as much as possible. Most of your YARA rules will work with YARA-X without changes, however, some differences are inevitable.

Our guiding principles are:

• Incompatibilities are a nuisance for our users and should be minimized.
• When some incompatibility exists it should be either a small one (i.e: unlikely to happen in real-life rules), or it should be for a good reason.

This document covers the differences between YARA-X and YARA. They are ordered by importance, with the most important differences first.

The { character must be escaped in regular expressions#

The { character holds special significance in regular expressions, particularly as part of the repetition operator (e.g., {1,3}). In YARA 4.x, the { character can be used without escaping, with its interpretation depending on the context. For instance, in /abc{/, the { is treated as a literal, while in /abc{1,2}/, it is interpreted as part of the repetition operator {1,2} associated with the c literal.

However, in YARA-X /abc{/ is considered an invalid regular expression because YARA-X mandates that the { character be escaped when used outside a repetition operator. Therefore, /abc{/ must be written as /abc\{/.

At first glance, YARA-X’s stricter requirement might seem inconvenient. However, there is a valid reason for this. Consider the following regular expression from an actual YARA rule:

/http:\/\/[^\/]+:[0-9]{1:5}/

Focus on the [0-9]{1:5} portion of the regular expression. The intention was to repeat a decimal digit between 1 and 5 times, but the user mistakenly wrote {1:5} instead of {1,5}. As {1:5} is not a valid repetition operator, the curly brackets are interpreted by YARA 4.x as literals, matching the literal string "{1:5}". In YARA-X, this error is flagged because the curly brackets must be explicitly escaped.

Here’s another real-life example:

 /(http|https):\/\/(\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}):280\/.{,N}[0-9a-zA-Z].zip/

Notice the .{,N} part of the regular expression. The intended pattern likely was to repeat . (any character) an unbounded number of times, typically expressed as .*. Instead, the user wrote .{,N}, which is not a valid repetition operator and is interpreted by YARA 4.x as the literal string ".{,N}". In YARA-X, such an error would be detected because of the requirement to escape the curly brackets explicitly.

Stricter escaped characters in regular expressions#

YARA 4.x accepts invalid escaped characters in regular expressions, and simply treat them as the character itself. For instance, in /foo\gbar/ the \g sequence is not a valid escaped character and YARA translates \g into g, thus, /foo\gbar/ is equivalent to /foogbar/.

This has proven to be problematic, because it’s rarely the desired behaviour and often hides errors in the regular expression. For example, these are real-life patterns where the relaxed policy around escaped characters is backfiring:

/\\x64\Release\\create.pdb/

In the pattern above notice the \R in \Release. The intention was obviously to match \\x64\\Release\\create.pdb/, but the missing \ goes unnoticed and the resulting regular expression is /\\x64Release\\create.pdb/, which is incorrect. Some other examples are:

/%TEMP%\NewGame/

/(debug|release)\eda2.pdb/

/\\AppData\\Roaming\\[0-9]{9,12}\VMwareCplLauncher\.exe/

/[a-z,A-Z]:\\SAM\\clients\\Sam3\\enc\\SAM\obj\\Release\\samsam\.pdb/

YARA 4.4 introduced the --strict-escape argument that turns on a strict check on escaped characters and returns an error in such cases. This is also the default behaviour in YARA-X.

Differences in base64 patterns#

In YARA 4.x you can use the base64 modifier with strings shorter than 3 characters, but YARA-X requires at least 3 characters. In the other hand, YARA-X won’t produce false positives with base64 patterns as YARA does. This is a well-known YARA 4.x issue described in the documentation:

Because of the way that YARA strips the leading and trailing characters after base64 encoding, one of the base64 encodings of “Dhis program cannow” and " This program cannot" are identical.

YARA-X doesn’t suffer from these false positives, but the price to pay is that patterns must be at least 3 characters long.

Alphabets for base64 modifiers#

In YARA 4.x if you use both base64 and base64wide in the same string they must use the same alphabet. If you specify a custom alphabet for base64, you must do the same for base64wide, this is an error:

$a = "foo" base64 base64wide("./ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")

In YARA-X you can specify different alphabets for base64 and base64wide in the same pattern. In the example above, base64 will use the default alphabet as always, while base64wide will use the custom alphabet.

“of” statement accepts tuples of boolean expressions#

In YARA 4.x the of statement accepts a tuple of pattern 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*)

In YARA-X 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*)

Using xor and fullword together#

In YARA 4.x the combination xor and fullword looks for the bytes before and after the XORed pattern and makes sure that they are not alphanumeric, so the pattern "mississippi" xor(1) fullword matches {lhrrhrrhqqh}, which is the result of XORing mississippi with 1. The pattern matches because the XORed mississippi is delimited by the non-alphanumeric characters { and }.

In YARA-X the bytes before and after the pattern are also XORed before checking if they are alphanumeric, therefore {lhrrhrrhqqh} becomes zmississippiz, which doesn’t match "mississippi" xor(1) fullword. In other words, YARA-X searches for full words contained inside a longer XORed string, which is the intended behavior in most cases.

Negative numbers as array indexes#

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

Jump bounds in hex patterns#

In YARA 4.x the following hex pattern is invalid:

{ 01 02 03 [0x00-0x100] 04 05 06 }

This is because the jump’s upper and lower bounds can be expressed in base 10 only, 0x00 and 0x100 are not valid bounds. In YARA-X hex and octal values are accepted.

Duplicate rule modifiers#

In YARA 4.x rules can have any number of global or private modifiers, for instance, 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. This very unlikely to affect any real-life rule.