Threat Level: green Handler on Duty: Didier Stevens

SANS ISC: InfoSec Handlers Diary Blog - Internet Storm Center Diary 2018-06-15 InfoSec Handlers Diary Blog


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SMTP Strangeness - Possible C2

Published: 2018-06-15
Last Updated: 2018-06-15 05:56:22 UTC
by Lorna Hutcheson (Version: 1)
5 comment(s)

We received an email today that provided some interesting information from a reader (Bjorn) about some observed SMTP traffic that was unusal.  From the appearance it could be related to exfil or C2.  The domain in question is donotspamtoday.com whose IP is 185.14.30.147 and there is an DNS TXT entry for SPF.  The domain was registered March 20, 2018.  I have been unable to find any additional examples or information of similar traffic.

Bjorn provided a good analysis of what was observed and I'd like to pass it along to see if anyone is seeing or has seen traffic similar to this. Here is what he sent to us:  

multiple compromised mail accounts being abused for some kind of covert channel communication. These are the common denominators in the mail communication:

* Mail is submitted to the domain donotspamtoday.com, with a lowercase first name and single digit as local part of the address (kevin1@, joan4@, willy3@ etc)

* When sending mail, the client identifies with a HELO name of 10 random lowercase characters (helo=<ckvcjsgffo>, helo=<uhtcpnyawr>, helo=<mjjrdvztxq> etc)

* The mail's Subject is always three (3) garbled/hashed words, followed by MID: and 32 hex characters. Some examples:

Subject: tjx gxtew kfzmmi MID:b5d479449a955af149e251860092b089
Subject: yjlp tahfhg bhj MID:2067087fb3e302fbd357bc7ccfe2b495
Subject: oezdvlh nbqhhd smbhmh MID:88664d1750835a08d2594ab3fba92751
Subject: kiyeax ggsqfeb aucilpi MID:ac6b795067515742c5f8e025c2c3dff0
Subject: tqorgfj crngne jwkbljt MID:b86c50466b75f5b2a088365f60c87bdc
Subject: jhwz nchfa ujk MID:0178f3e6a51e6e4ad44baa0e54ba42cb
Subject: obem jzmep nju MID:1f22a1aa5038b0921874567d0f4a3012
Subject: zqfn gkdepn vjyuu MID:1696a6720ad44f8c38084eb923127f5e
Subject: ghod ttfq dqge MID:af8cad81ecaf028ad8019280d03f6a2b

* Each mail body contains exactly one line of eight hashed words. The words in the mail body are different from the words in the subject. Some examples (one line from each mail):

zjmviov rdktiw ovzdebu ysm htp kzhfsk escngpg oqcru
nklnhk lidwen qeujgbe lcrz krf xufkyb xqpjqbg nto
gcbvqzq dca wlfig prgrkg hmbzjn yxl uov lxwak
cxkw jnv vezxdkz sfvib rqnyze fqkfwgs memck cwm

* Most clients sending the mails connect from Brazil, but some connect from Iran, Turkey and others. The IPs observed exposing this behaviour are registered with Open Threat Exchange, https://otx.alienvault.com/pulse/5b2215e2eeaaee019b4e017b

 

If anyone has any additional information or is seeing this traffic, please let us know.  
 

Lorna Hutcheson
ISC Handler

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