[BlueOnyx:21683] Re: mailserver; possible security issue?

Thu Jan 25 08:19:15 -05 2018

Hello Brian,

thank you for your email.
Yes, I am aware of this. But in this case some of your suggestions are not applicable.
For example for an official mailserver it makes no sense to limit the ip address for port 25.
Yes smtp_auth is enabled at the server of course. But you can try. It is not working if you use an email address which is existing at the server and the recipient address also.

I did also try with an exchange server and this was working. Also with a sendmail at freebsd. So it seems to be a more  or less general "feature" which in my opinion is a security issue in our days.

Best regards
Dirk

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-----Ursprüngliche Nachricht-----
Von: Blueonyx [mailto:blueonyx-bounces at mail.blueonyx.it] Im Auftrag von Fungal Style
Gesendet: Donnerstag, 25. Januar 2018 13:55
An: BlueOnyx General Mailing List <blueonyx at mail.blueonyx.it>
Betreff: [BlueOnyx:21682] Re: mailserver; possible security issue?

Hi Dirk,

Well, there are often a few things that can be done, although normally you need to also consider email is one of the (if not the) most insecure methods of communication.

Simple things which some you may have enabled already include:
- Limiting the IP ranges that can send email (of course can be spoofed, but it requires more work from the scammer side)
- Authenticated sending (to ensure they do not send via your server, if they do then look for the account being exploited)
- SPF records can help a little too I believe (have nto played too much with them)

Note: I am no expert myself but the above should get you pointed in the right direction to start with.

I am sure there are other ways to harden the security like with RBLs, SPAM filetering such as SPAM ASSASIN, etc. I suppose some geo blocking may also help, which would go more hand in hand with the initial comment on limiting the IP ranges.

As always, staff training on cyber threats in invaluable.

Hope this helps for the future.

I suspect someone with more knowledge will reply also soon enough, but thought this may provide a little light reading to start with.

Regards
Brian

On 25/1/18, 11:07 pm, "Blueonyx on behalf of Dirk Estenfeld" <blueonyx-bounces at mail.blueonyx.it on behalf of dirk.estenfeld at blackpoint.de> wrote:

    Hello,

    we have one customer who was victim of a CEO fraud.
    Some of his employees got a message from the email address of the CEO with the order to send xx money to a specific bank account. He did :(

    Now we found out that it is possible to send email with sendmail at centos/blueonyx (also other distributions) from an existing email address to an existing email address.

    Example:
    telnet 208.77.xx.xx 25
    Trying 208.77.xx.xx...
    Connected to 208.77.xx.xx
    Escape character is '^]'.
    220 sol ESMTP Sendmail Ready; Thu, 25 Jan 2018 06:37:59 -0500
    EHLO blackpoint.de
    250-sol.xxx Hello ns3.xxx [xx.xx.xx.xx], pleased to meet you
    250-ENHANCEDSTATUSCODES
    250-PIPELINING
    250-8BITMIME
    250-SIZE
    250-DSN
    250-ETRN
    250-AUTH LOGIN PLAIN
    250-STARTTLS
    250-DELIVERBY
    250 HELP
    MAIL FROM:mstxxx at solxxx.net
    250 2.1.0 mstxxx at solxxx.net... Sender ok
    RCPT TO: mstxxx at solxxx.net
    451 4.7.1 Greylisting in action, please come back later
    RCPT TO: mstxxx at solxxx.net
    250 2.1.5 mstxxx at solxxx.net... Recipient ok
    DATA
    354 Enter mail, end with "." on a line by itself
    Some content for example send money to yx
    .
    250 2.0.0 w0PBbxN1026335 Message accepted for delivery
    QUIT
    221 2.0.0 sol.xxx closing connection
    Connection closed by foreign host.

    Unfortunately it is not only possible from the same to the same user. It is also possible from an (at the server existing) email address to an (at the server existing) email address.

    Does someone else did see something similar.
    In my opinion in days with CEO fraud it is a security issue.
    Do someone know how to change settings in sendmail to prevent this behaviour?

    Best regards,
    Dirk Estenfeld

    ---

    blackpoint GmbH - Friedberger Straße 106b - 61118 Bad Vilbel

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