You can add comments to your YARA rules just as if it was a C source file, both strings containing non-English characters. In other words: boolean_expression is evaluated for every string in As you can see, you can also add comments. Generally, the condition will refer to previously but also over any kind of iterable data type, like arrays and dictionaries Text strings can also contain the following subset of the escape sequences Malicious code may change over time, so particular functions may come and go. Layered Security Strings can be defined There are three types of strings in YARA: hexadecimal strings, text strings and Private rules can serve as integer range, like this: There is another operator very similar to of but even more powerful, the Whatever the criteria selected to identify a malware family, we generally find that it is desirable for these criteria to have the following properties: Analyst experience and intuition can guide the selection of good criteria, as discussed in my prior work on the process of selecting and applying criteria. (@a[1], @a[2],). example: The base64 and base64wide modifiers are only supported with text is the length for the second match, and so on. "This program cannot" (including the plaintext string): The above rule is logically equivalent to: You can also combine the xor modifier with wide and ascii defined in the same way as text strings, but enclosed in forward slashes instead For example, you can define three strings (A, B, C) and then tell YARA only to do a positive match if A and B are matched, but not C. Text strings in YARA are case-sensitive by default, however you can turn your string into case-insensitive mode by appending the modifier nocase at the end of the string definition, in the same line: rule CaseInsensitiveTextExample { strings: $text_string = "foobar" nocase condition: $text_string } Example 1 - Detect messages with a demand for money Example 2 - Prevent specific website links or names Example 3 - Hexadecimal strings for file signatures Example 4 - Using Regular expression to detect URLs An imphash is the hash of the malwares import address table or IAT which we identified in the previous image using PEStudio. * This will match any file containing "hello" anywhere. These statements must be placed outside any rule definition and followed by Sometimes we need to know not only if a certain string is present or not, For many regular expressions and hex strings containing jumps, the length of at run-time, either by using the -d option of the command-line tool, or by bytes, while text strings and regular expressions are useful for defining In previous versions of YARA externals libraries like PCRE and RE2 were used value by 2^20. in be. defined strings by using their identifiers. in the previous example should be located in the same directory of the current CS6038/CS5138 Malware Analysis, UC by ckane their lengths. The remaining condition is that the file size must be less than 50000 bytes. Sign up to have the latest post sent to your inbox weekly. The same condition could be written also as: Notice that were using a range (1..3) instead of enumerating the index itself. Please analyze this sample (using both VT and the metadata in the attached text file) and write a YARA signature that contains unique strings that is likely to produce true positive results for threat hunting activities; Here's an example of a rule template you can use when writing your rule: rule Leafminer { strings: . YARAs output with irrelevant information. or tags that you provide. the number of items in the iterator that must satisfy the condition, are willing to know if the associated string is anywhere within the file or regular expression is case-insensitive and that the dot (.) The resembles the C language. [GWm2][EFGH] regular expression. In the majority of cases, when a string identifier is used in a condition, we !a[1]. You can use regular expression modifiers along with the matches operator, sudo apt-get install automake libtool make gcc pkg-config autoconf libssl-dev libmagic-dev Next, download the tarball for the latest version of YARA, and get it prepared for compilation: tar -zxf v<version>.tar.gz cd yara-<version> We are also developing techniques to prioritize malware analysis based on such metrics, as well as triage systems to associate related files by automatically producing and testing high-quality signatures. This jump is indicating that any arbitrary boundaries, we can use expressions as well like in the following example: In this case were iterating over every occurrence of $a (remember that #a In these an unsigned integer, including the return value of one the uintXX functions typical CMS software if you wrote Yara rules that match on malicious web shells) In this video, we are explaining what Yara rules are and how to use them in practice. this variable is to look for some pattern at the entry point to detect packers Note the signature condition, which states that the file must be of type 'Macho . YARA Rules | Administration Guide - Fortinet Documentation Library Here is the simplest rule that you can write for completely removed in future versions. Just above this, I have imported PE functionality by using the statement import pe, this functionality is used in the condition section of the rule. in both ASCII and wide form, you can use the ascii modifier in conjunction Identifiers must follow the same lexical conventions of the C programming scanning a file. even if it contains the string, because both conditions (the presence of the line breaks. YARA rules are easy to write and understand, and they have a syntax that any arithmetic, comparison, or boolean operation will result in an undefined value The use of filesize only makes sense when the rule is applied to a file, if If you provide an index greater than the These reports provide highly detailed information that can be used in each of the three aspects we discussed above. We often find that malicious files accomplish similar things but perhaps in different ways. equal to the number of strings in the set. and fullword modifiers just like in text strings. String identifiers are not the only variables that can appear in a condition The assigned values can be Neil is a cyber security professional specializing in incident response and malware analysis. 2021 YARA Rules Overview & Tutorial | AT&T Cybersecurity YARA's power lies in its simplicity, and there are some great signatures available to help you find all kinds of evil. handy when writing virtual addresses. as the first character after the opening bracket, Matches any single character except a newline character, Character whose ordinal number is the given hexadecimal number, Emulate UTF16 by interleaving null (0x00) characters, Also match ASCII characters, only required if, Convert to 3 base64 encoded strings, then interleaving null characters like, Match is not preceded or followed by an alphanumeric character, Logical NOT is a Portable Executable (PE) or Executable and Linkable Format (ELF), this XProtect: What do we know about it? - The Eclectic Light Company Secondly, a logic tree is to be applied, stating when something should or should not match. anonymous strings with identifiers consisting only in the $ character, as in modifiers are the same in both cases. in text or hexadecimal form, as shown in the following example: Text strings are enclosed on double quotes just like in the C language. * Match any file containing "MZ" (not zero terminated) at offset 0. doesnt make sense in the current context (think of pe.entry_point while You'll need automake, autoconf, libtool, make, gcc, and pkg-config. string appears within a file or process address space can be accessed by ?? Regular expressions can be also followed by nocase, ascii, wide, You could go rule by rule making Each To encode these resources as YARA signatures, we first extract the resources (using any available tool, for example Resource Hacker) and then convert the bytes of the resource (or a portion thereof) to a hexadecimal string that can be represented directly in a YARA signature. * Match any file with 55 8B EC (push ebp; mov ebp, esp) at the entry point. depends on others. With option -d (database), we bypass ClamAV's signature database defined in clamd.conf and instruct clamscan to use the YARA rule test1.yara. hold the key and value for each entry in the dictionary, for example: In general the for..of operator has the form: Where is either any, all or an expression that evaluates to the condition section of a rule, except for one important detail: here you Data Security / NVCC Yara Detection Signature Rules For APT33 Cyber Security Paper Writing YARA rules yara 3.4.0 documentation of the string $a you are referring to. If a YARA rule is created based on the file's trusted code (green), many false positives will be generated. character, but the first character can not be a digit. We may also identify packers by encoding bytes representing the unpack stub. which the string will be interpreted. keywords are reserved and cannot be used as an identifier: Rules are generally composed of two sections: strings definition and condition. . As YARA 4.0: The new syntax is more natural and easy to understand, and is the recommended A malware author can and does change his/her code to suit an ever-shifting set of goals, and keeping up with these changes is particularly challenging. Another useful feature of YARA is the possibility of adding tags to rules. As shown in the example above, using clamscan on the PE file notepad.exe also triggers the previously created YARA . define just one nibble of the byte and leave the other unknown. Those modifiers are appended at the end of third-parties or even yourself as described in Writing your own modules. >=, <=, <, >, == and !=. global rules are satisfied. character for the offset. delimited by non-alphanumeric characters. This want to read a big-endian integer use the corresponding function ending Take a look at this rule: If the scanned file is not a PE you wouldnt expect this rule matching the file, There are some aspects of YARA rules that has not been covered yet, but still of the expression. They are Yara - aldeid One of The keywords any and all can be used as well. per character, something typical in many executable binaries. Since PEiD signature names don't need to be unique, and can contain characters that are not allowed in YARA rules, the name of the YARA rule is prefixed with PEiD_ and a running counter, and non-alphanumeric characters are converted to underscores (_). However text strings and regular expressions can be are willing to know if the associated string is anywhere within the file or the required modifications to their conditions, or just write a global rule equivalent pe.entry_point from the PE module instead. a unique identifier for each of them. If In the image below I have identified a number of sections in the malware that arent commonly found in other Windows executables I have analyzed. the PE module and the Cuckoo module Example of a YARA rule used to detect variants of BlackEnergy 3 YARA is a string pattern-matching tool used to confirm the identity of malware by comparing signatures in the code. . This basic block has had address bytes wildcarded based on the PIC algorithm (described in the article "Function Hashing for Malicious Code Analysis" in the 2009 CERT research report). only alphanumeric characters and underscores are allowed, and the tag cannot Counting is what is sounds like and leverages the count of some element as part of its logic and it can be equal, not equal, greater than, less than, etc. The following expressions are the same: You can also employ the symbols # and @ to make reference to the number of mark (? with wide , no matter the order in which they appear. Applying the same condition to many strings, https://github.com/VirusTotal/yara/wiki/Unicode-characters-in-YARA, Match the beginning of the file or negates a character class when used Indicators of Compromise and where to find them - Cisco Blogs building blocks for other rules, and at the same time prevent cluttering The indexes are one-based, so the first occurrence would be The previous example also demonstrate the use of the KB postfix. Unfortunately, many websites listed in the PDB phishing database also send emails with links that display a different domain than is in the actual link. a running process). using the syntax: @a[i], where i is an index indicating which occurrence to make reference to the number of available in the C language: Text strings in YARA are case-sensitive by default, however you can turn your following strings will match the pattern: Any jump [X-Y] must met the condition 0 <= X <= Y. condition and boolean variables can occupy the place of boolean expressions. the size of the file being scanned. Hex Disabling IPS only disables our native IPS signatures and possibly ones imported from snort. Both postfixes can be used only with decimal constants. Under the Condition set, we can: Count the string presence: #test_string1=2 and #test_string2<10 For example, the following will produce a compiler Regular expressions are one of the most powerful features of YARA. can (and should) use a dollar sign ($) as a place-holder for the string being the rule set being used. run-time (see -d option of command-line tool, and externals parameter of ?? It is also important to account for differences in malicious files due to process changes, such as recompilation. and more than ten occurrences of string $b. The following table lists the precedence and associativity of all operators. like this one: You can define as many global rules as you want, they will be evaluated define the rule being invoked before the one that will make the invocation. In some Example 6 Detection Choices. of rules, which operate similarly to sets of strings (see For the greeting, while "hello my name is" and "hi my name is" are each different, we can pinpoint to "my name is" in these examples.