How can I prevent spoofed emails from outside thats using my internal accepted domain

Question

I'm receiving spam emails sent from my own domain to my own domain. I'm using Exchange 2013.

Example:

[email protected] is being used to send spam to [email protected].

I can successfully replicate the issue by telneting to the server from any external IP.

telnet <external-ip-of-server> 25
helo anydomain.com
250 myserver.mydomain.com Hello [External-IP]
mail from:[email protected]
250 2.1.0 Sender OK
rcpt to:[email protected]
250 2.1.5 Recipient OK
data
354 Start mail input; end with <CRLF>.<CRLF>
some text here
.
250 2.6.0 <[email protected]> [InternalId=20890720927751, Hostname=myserver.mydomain.com] Queued mail for delivery

I have a SPF-record setup like this: v=spf1 ip4:External.IP.of.MyServer -all

I also have SenderID enabled on the Exchange 2013-server like this:

[PS] C:\Windows\system32>get-senderidconfig | fl


RunspaceId            : 9be45249-1186-42b4-9e4e-3bc5a56c0c63
SpoofedDomainAction   : Reject
TempErrorAction       : StampStatus
BypassedRecipients    : {}
BypassedSenderDomains : {}
Name                  : SenderIdConfig
Enabled               : True
ExternalMailEnabled   : True
InternalMailEnabled   : False
AdminDisplayName      :
ExchangeVersion       : 0.1 (8.0.535.0)
DistinguishedName     : CN=SenderIdConfig,CN=Message Hygiene,CN=Transport Settings,CN=MyOrganization,CN=Microsoft Exchange,CN=S
                        ervices,CN=Configuration,DC=mydomain,DC=com
Identity              : SenderIdConfig
Guid                  : e85c9acb-579e-4d92-bde7-03ac2dd9beac
ObjectCategory        : mydomain.com/Configuration/Schema/ms-Exch-Message-Hygiene-Sender-ID-Config
ObjectClass           : {top, msExchAgent, msExchMessageHygieneSenderIDConfig}
WhenChanged           : 2015-12-08 10:23:24
WhenCreated           : 2014-02-15 13:37:30
WhenChangedUTC        : 2015-12-08 09:23:24
WhenCreatedUTC        : 2014-02-15 12:37:30
OrganizationId        :
Id                    : SenderIdConfig
OriginatingServer     : mydc.mydomain.com
IsValid               : True
ObjectState           : Unchanged

How can I prevent this type of spam without using any External Anti-Spam services?

Solution

You need to remove permission to bypass the sender address spoofing check by running:

Get-ReceiveConnector "name of the internet receive connector" | Get-ADPermission -user "NT AUTHORITY\Anonymous Logon" | where {$_.ExtendedRights -like "ms-exch-smtp-accept-authoritative-domain-sender"} | Remove-ADPermission

If that doesn't solve the problem (i.e for Exchange 2013 CU5+), you should do the following:

1. Block your own domain with

   Set-SenderFilterConfig -BlockedDomains mydomain.com

   Set-SenderFilterConfig -InternalMailEnabled $true

2. Remove ms-Exch-SMTP-Accept-Any-Sender for anonymous users with

   Get-ReceiveConnector "name of the internet receive connector" | Get-ADPermission -user "NT AUTHORITY\Anonymous Logon" | where {$_.ExtendedRights -like "ms-Exch-SMTP-Accept-Any-Sender"} | Remove-ADPermission

3. Allow open relay from LAN (if needed) with:

   Get-ReceiveConnector "name of your LAN Open Relay connector" | add-ADPermission -user "NT AUTHORITY\Anonymous Logon" -ExtendedRights "ms-Exch-SMTP-Accept-Any-Sender"

P.S. Make sure to restart transport service after those operations.

Comments

[gxx]

Sorry, I've overlooked the fact that SPF is already in place, so my answer was quite pointless. I've deleted it.

[Niklas J. MacDowall]

I did find that too by Googling around for half a day. It didnt fix it. Do I need to restart the transport service after perhaps?

[Anubioz]

Yes, you should restart the transport service (according to this)

[Niklas J. MacDowall]

I do not see it listed as a requirement for configuring the above in your answer. But I will try it anyway.

[Anubioz]

Basically transport service restart is only required when you enable anti-spam agents/functionality (which is required for using any of the built-in anti-spam features). After that you should be able to change anti-spam settings without restarting the transport. I assumed you have restarted it already, so I didn't list it as a requirement in my answer. Anyway, can you try restarting it now and see if it helps?

[Niklas J. MacDowall]

I removed ms-exch-smtp-accept-authoritative-domain-sender and restarted Exchange Transport Service, but the issue is still the same.

[Anubioz]

Can you please run Set-SenderIdConfig -InternalMailEnabled $True and Set-SenderfilterConfig -InternalMailEnabled $True and see if it helps?

[Anubioz]

Let us continue this discussion in chat.

[Niklas J. MacDowall]

I managed to solve the issue by blocking my own accepted domain in the SenderFilterConfig. Set-SenderFilterConfig -BlockedDomains mydomain.com Set-SenderFilterConfig -InternalMailEnabled $true