just-because

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On the number of attempts on brute-force login attacks

Submitted by gwolf on Fri, 02/06/2015 - 12:51

I would expect brute-force login attacks to be more common. And yes, at some point I got tired of ssh scans, and added rate-limiting firewall rules, even switched the daemon to a nonstandard port... But I have very seldom received an IMAP brute-force attack. I have received countless phishing scams on my users, and I know some of them have bitten because the scammers then use their passwords on my servers to send tons of spam. Activity is clearly atypical.

Anyway, yesterday we got a brute-force attack on IMAP. A very childish atack, attempted from an IP in the largest ISP in Mexico, but using only usernames that would not belong in our culture (mosty English firstnames and some usual service account names).

What I find interesting to see is that each login was attempted a limited (and different) amount of times: Four account names were attempted only once, eight were attempted twice, and so on — following this pattern:

 1 •
 2 ••
 3 ••
 4 •••••
 5 •••••••
 6 ••••••
 7 •••••
 8 ••••••••
 9 •••••••••
10 ••••••••
11 ••••••••
12 ••••••••••
13 •••••••
14 ••••••••••
15 •••••••••
16 ••••••••••••
17 •••••••••••
18 ••••••••••••••
19 •••••••••••••••
20 ••••••••••••
21 ••••••••••••
22 ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••

(each dot represents four attempts)

So... What's significant in all this? Very little, if anything at all. But for such a naïve login attack, it's interesting to see the number of attempted passwords per login varies so much. Yes, 273 (over ¼ of the total) did 22 requests, and another 200 were 18 and more. The rest... Fell quite shorter.

In case you want to play with the data, you can grab the list of attempts with the number of requests. I filtered out all other data, as i was basically meaningless. This file is the result of:

  1. $ grep LOGIN /var/log/syslog.1 |
  2. grep FAILED.*201.163.94.42|
  3. awk '{print $7 " " $8}'|
  4. sort|uniq -c

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