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Custom IPS signature for Fortigate / block smtp auth failure

Posted on 2015-01-20
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Last Modified: 2015-01-21
We are regularly getting smtp auth failures on our email gateway. it used to be 535 and have changed to 504 ever since we forced smtp/tls and blocked ssl and weak cipher.

Is there a custom signature to monitor and block this on the fortigate firewall. running 5.0.10 on 60D fortigate.
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Question by:Rajkumar Kamath
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btan earned 500 total points
ID: 40563466
Typically as mentioned the smtp error 535 and 504 are pertaining to authentication issue, in this case the former is having incorrect password or account name and latter is likely unrecognized authentication type or need to authenticate first.

You can check out the below for custom signature, here is one example shared the custom signature that allows for 10 failures within a 2 minutes period. Typically to trigger a brute force attack against a SMTP user mailbox. Good to normalise it with your environment and monitor prior to blocking as required. There are other in the blog as well based on src ip
config ips custom
    edit "SMTP_AUTH01"
        set signature "F-SBID( --attack_id 7393;  --revision 1; --name \"SMTP_AUTH_FAILURE01\"; --service SMTP; --protocol tcp; --tcp_flags PA; --pattern \"535 Authentication failed. Restarting authentication process\"; --flow from_server,reversed; --track dst_ip; --rate 10,120; )"
    next
end

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http://socpuppet.blogspot.sg/2014/07/example-fo-smpauth-protection-fortigate.html
For custom signature, there is another generic from Fortinet doc for reference in steps. This case shared creating custom signature looking for signature pattern of "vrfy" in service/protocol SMTP/tcp
http://docs-legacy.fortinet.com/fos50hlp/50/index.html#page/FortiOS%205.0%20Help/ips_signatures.153.6.html
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Author Closing Comment

by:Rajkumar Kamath
ID: 40563703
Mnay thanks for the suggestion. have put them in and seems to be doing it properly. Can see quite a lot of attempts beingblocked.
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