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Im developing a website on test server and so when debugging some stuff I had to look in access log, this is a small chunk of what I saw:

37.99.49.203 - - [13/May/2019:17:51:34 +0200] "H\x1E\xF3qxZ\xA8[\x13\x87\x94\xB8Yc\x85\xC9\xA0?\x8B\x95\xCE\x9E\x97vR\x151\xFF2\x8ABaM2p\x81PJ\x12r~A\xB8\xFA\x90lq\x8F\x03q\x09M\x8A\xD1\xE0R\xE2m\xF5\xC2\xD8\xE5\xDA\x14\xE0\xA5\xE2:\xD5@K8\xBD/\xF6\x92" 400 150 "-" "-" "-"
[...]
37.99.49.203 - - [13/May/2019:19:11:42 +0200] "W>\xFFM,C\xD2\x828i\xF1\xEF\xEC\x801j\xAD\xDC\x85\xED\xEA\x1B1M\xDF\xA1\xA8\x92\xAC\xD1\x02#\xA9]\x87Gv\x1BV\xE7Q\xCA\xBE\x1CJ\xCD\x11\x1A\x19\xB7\x0B\x16\xEEk5\xD9\x12\xA5\x16\x84\x18m\xD4~i\xD6_JBEs\xA1@\x04\xF2;\xB8\xCDR!\x5C\x09\xB2\xB8\xD3\xB8\x9D\x9D69\xB8\x12\x0C\x14k\x14\xA4\xD4\xDC\xA9\x87\xDB\xAC\x1F>$%l\xF6\xF6t\xE8T\xE8\x87h\xC9\x97* Kh\xD3\x97DV\xB5|\x19\x8Bd\xE2\x8F\xF0\xAB\x09\xB4\x9E\xDDL\x84\xDDT=\xAF\x9C}\x8F\xF2O\xFA0K3F\xF6\xCA1\xF2\x89w\xE5\x9E\x95fV\x06\xF9^\x7F\x9B\x15)b\x80C \xE1\x09\xBB\xAE\xF7[\x8A\xE5\x22\xE3\xA2\x83\xBF\xE0*>[\x81H\xA1\x140{\x18f\xCEV\xB1<\x17\xBE\x12\xD7uh\xB5V\xF1\xCD5\xC0\xF5V%\xB7" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:11:42 +0200] "ap\x1F\xA1h\x84\xE19B)\xA0\x0C\xB8\xAE\xA5o_ip\x98\xCF\xC0\xF8J\xA7\x94\xD86%\xA1\x22\xF9g{\xB9\x85\xFC\x0F\x85RV\xEF\xF9\x82A\xFCv \x81>|\xC8\x13\x22\xF2R%\xE9\xDD2Q\x95n\xCDQ\xDD\x1E\x04\xF1\x1Do\x09\xF4a.\xBB\xD4\xEA@-\xB6\xDB\xAB\xDC+;]T\xE1\xF7f\x8B\xA1\xDD\xAB\xDB&\x94ii]\x04\xF2\xCB\xC8\xAA:\xED\xE2\xB7A:9{\xDASG^*\x08\xD2\x03\x1A\xA8" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:11:42 +0200] "!\xBC\xA0F\xC1\xDA\xF0\xF5mfM\xBA\xCF\x16" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:11:46 +0200] "G\xFA\xE9\x153\x04\xF7Q0\xC0\x91\x83\xBC\xCC\xACgV_\xE2\x84\xE5nAe\x02ZGH\x1A\x8Bn\xF5j\x14\x81nM\xFA\x9C\xE6\xF1\xD7\x1D\x8C\xB9\xDE\xDE\x8FT\x81\x07\xE6\xFC\xD2\x12F\x1DXJu\xEA\xE3\x1D\x9A\xCE\x0FmL\x81\x93IN\xD3\xD8!\x16\xB532\xC6\x91%4\x00\x83\x1F<\x0C\x92\xC2\x22%\x00\xFF[\xA2O\x0E\x9F\xBC\xB8\x87P\xE9Pz\x13\xB2\xF1\xA0\xB8@\x88\xEC\xD3O\xAE\xDE\xC0\xCA\xED\x12_TE\x0BU\xCBp\x22n\xA6\x16\xD0\x89dc\x03\xCEJZ\xF3\xBDnX\xD0h[\x96%\x9C\xB3\xB2n8~\xE6Sn\x99\x983\x0B+K\xF1\x06\x1D\x8B\x9AV" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:11:58 +0200] "8@\x82\xBA\xFC\x1C\xDA$l\xA9p\x9F1\xB5p\x002\xDB\xEF\xF9\xAD\xCCn\xD4\xF2\x9E\xE27\xB6\x1E\x1C\xAC\xF6\x06\xEBQN\xDA\x19\xB4\x81Y\xA6\xA9$\xF1\xDEJ\xED\xB3\xF4A*\x1B1\xC0\xE2\x22p\xAB\x83}j\x9Ed\xFBg\xCBF\x0E\xED\x15(2\x86X\xFE" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:12:02 +0200] "L" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:12:06 +0200] "q\xEE5\xA1\x12\x9B\x80\x85\x19\xB3\xC2\xFF\x19Ej\xE3K\x9B\xCBY\xEE\xF6\x92{\xBE[k\xA3\x8C\xF2\x05\x8C\x11\xCA\x09\x9BU\x8E\xFF\xE2\x8D.\x08\xC2~L\x1E\x9F\x97q\x9CM69\xCB\xA3\x91MOe\x10]\x81\xE4\x83<\xF6" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:12:06 +0200] "\x0F?c\x1B\x032*\xB5\xFB'\xB0\xB5\xF0X\xB3\x1Dc\xBE\x02\x06e4\x82x\x09>F\xC5W\xCB\xF1s\xD5w\xA3\x1B\xB5\xFB\x1D\x8A\xD3\xE4\x0F\xE8\xB0\xFC\xEE\x98\xFBM\xFC\xCB\xB09\xB4\x8B\xA1e\x8D\xA7\xD4\x95\xE5\xF9\x1D\xAE\xD6J\xCFP2Z\xBC\xC8Z4\x90\xDA\xDA\x08V$\x1A$\x8D7L\x1Eo\xCA\x0Fa\x0F\xF3\x94\xF8\x5Cf\xB4\x83\x11\xB7\x1B\x94\x9F\xD3\xD7\xFB\x84C\x0F\xCF'\xF7\xD9\x03\xE5\x8C\x12\x0F\xB7\x15S=\x80\x98\xC4\xC4\x95,\xEEV\xC1\xF1\xBDj\xE6\x8F\x06X\xE7LO[0\x00f\x0C\x0E\xA6C\xEBH\x9C\xFE.\xB8\xDE7\x0B\xBF=\x9BVC\xA3n>;J0\x10\x15>\xA0\x8E\xB1^m\x9E\xE3\xD4Wkf\xB9\xB681\xDF\xD5\xA7~\xD9\xD9%\x84cNX_\xAB!T\xF6`\xB6A\xE5\xB9\x14\x98\xE9\xD1\xD9\xCB\x0E\x0E\xDBb\x1Do\x190\x0B" 400 150 "-" "-" "-"
37.99.49.203 - - [13/May/2019:19:12:08 +0200] "\x1F\xAE\xAD$\xE1\xD1\xBC\xF6\x8D\x04,Y\xBC\x08[Vd\xB3\xC9\xC9\x09<r\xC7\xA9O^\x9Ddq\xA3g\xB7\x095\xCDA\xDB<GsI\x1CZT!\x1FT\xEB\xB43" 400 150 "-" "-" "-"
[...]
37.99.49.203 - - [13/May/2019:19:18:14 +0200] "\xAE2\xDFwn\xC3\x04\xC1\xF1\xCF\xCEN\xB4A\xE6\xFD\xE6\x93\x9D(\x14\x97\xBB\x5C\xAE:bI4\x1AIiaD\xF4\xEA\x16\x86J\xB8A\x8Ea\x10\x9F\x07\x86d>\xDF\xAF\xC5R\xDF\xDA\x87\xF0\xFF\xD7\x927\x9A8\x04\xEF\xA2|\xD4?_m\xCD\xFB\xAB\xC4" 400 150 "-" "-" "-"

During past hour this bot managed to make approximately 1800 requests to my HTTP server. It's obviously malicious.

It also seems really persistent too because when I turned off my nginx server for a few minutes in order to make some configuration changes it came back and continued this behaviour.

I don't think it's denial of service attack of any kind as requests are too slow (for the most part he didnt even trigger rate limiting restrictions).

Request bodies seem like UTF8 literals but I don't think this is it.

Even though I block all HTTP requests that are not GET/POST/HEAD and I also block certain countries like his country of origin, he gets 400 bad request response because his requests are gibberish and don't specify request method.

Can anyone, please, explain to me why is this happening? What is this bot trying to achieve? Im going to block him in my firewall in a minute but I wanted to know what kind of anomaly is this because I've never seen this before.

  • 3
    It may be a protocol mismatch, a client using https to talk to a server listening with http. – Richard Smith May 13 at 19:00
  • Yeah, looks pretty high entropy, so maybe just encrypted data or random bytes? The (natural log) entropy of the strings you provided is about 5.41. This can be compared to highest possible entropy of byte strings, which is ln(256) or about 5.55. Can also be compared to the (natural log) entropy of typical 26 character English [citation: Shannon 1951], which is about 2.87. – hft May 13 at 19:43
  • @RichardSmith interesting, indeed my server doesnt have ssl certificate. It didn't stop yet, over 6000 requests were made until now. Im guessing it has no purpose and it's just really badly written bot because it wasted 100mb of bandwidth and kept my cpu usage at whooping 2% since a few hours now. Nothing else really happened so I am just going to block him in my firewall now. If anyone has any other theory I am eager to hear it – reckoner May 13 at 20:14
  • Looks like normal web fuzzing to me. – multithr3at3d May 13 at 22:33
  • What port(s) are you listening on? – jcaron May 14 at 19:36

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