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I am trying to generate clamav signatures for a malware dataset that I have.

Initially I have recognized some strings which are prominent in a class of malware, hence, those are considered and a ldb signature is generated using the below method.

The name of the signature, Engine version, Target as 0. We further have 'x' number of sub-signatures here x is 100 each with logical or. All the strings are converted to hex representation. Below is the example which is generated.

ramnit.Signature;Engine:0-500,Target:0;0|1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|32|33|34|35|36|37|38|39|40|41|42|43|44|45|46|47|48|49|50|51|52|53|54|55|56|57|58|59|60|61|62|63|64|65|66|67|68|69|70|71|72|73|74|75|76|77|78|79|80|81|82|83|84|85|86|87|88|89|90|91|92|93|94|95|96|97|98|99;636f6e6e6;686b65795;363530393;52656c656;633a5c5c7;436f6e766;313937313;6c6f63616;576169744;363337363;686b65795;353238363;736c65657;633a5c5c7;636f6e6e6;686b65795;633a5c5c7;737663686;363030363;633a5c5c7;313935353;633a5c5c7;636f6e6e6;6765746d6;536574437;313933393;686b65795;633a5c5c7;323232363;353537363;686b65795;686b65795;686b65795;686b65795;686b65795;686b65795;686b65795;686b65795;353130363;64656c657;633a5c5c7;633a5c5c7;686b65795;53656e644;6b7975666;6c6f63616;494d41474;686b65795;686b65795;686b65795;696573716;737663686;313237303;363033353;363039383;686b65795;686b65795;633a5c5c7;686b65795;333139313;686b65795;437265617;686b65795;476574546;353631323;633a5c5c7;686b65795;496e74657;686b65795;686b65795;686b65795;686b65795;3f7365745;633a5c5c7;476574537;527063426;686b65795;686b65795;566572517;353630353;686b65795;4f70656e5;353138343;4c6f6f6b7;633a5c5c7;476574546;363139393;633a5c5c7;686b65795;353638333;676574707;6f6c65333;5065656b4;343230353;536574576;5c5c3f3f5;5265674f7;633a5c5c7;686b65795;686b65795

Now, the problem is in case there are <=65 sub-signatures then everything works fine however, if they increase beyond that, it results in the following error.

LibClamAV Error: cli_loadldb: The number of subsignatures (== 65) doesn't match the IDs in the logical expression (== 100)
LibClamAV Error: Problem parsing database at line 1
LibClamAV Error: Can't load ramnit.ldb: Malformed database
ERROR: Malformed database

Is it that the ldb signatures are limited to only 65 conditions? If not what causes this issue and how to solve it?

user1683894
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1 Answers1

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Why on earth do you have so many repeated subsignatures?! Once you drop all of those, you will basically cut your rule in half. You could even think about making multiple versions of this rule and break it down to 10-20 subsigs for each.

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