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122 lines
8.3 KiB
Markdown
122 lines
8.3 KiB
Markdown
# ExOTA-Milter - Exchange Online Tenant Authorisation Milter (Mail-Filter)
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The **ExOTA-[Milter](https://en.wikipedia.org/wiki/Milter)** application is written in python3 and derives from **[sdgathman´s pymilter](https://github.com/sdgathman/pymilter)**.
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# Abstract/problem/motivation
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Fact is that more and more companies are migrating their Outlook/Exchange environments to the [Microsoft cloud](https://www.microsoft.com/microsoft-365).
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The **ExOTA-Milter** is kind of an authentication helper which plays an essential as well as security-related role in the so called [*smarthost connector for Microsoft Exchange-Online*](https://docs.microsoft.com/de-de/exchange/mail-flow-best-practices/use-connectors-to-configure-mail-flow/set-up-connectors-to-route-mail) scenario. The milter tries to close the gap of missing [client SMTP-Auth capability](https://tools.ietf.org/html/rfc4954) of MS Exchange-Online when there´s a need to relay emails over a third party smarthost. According to this the MS Exchange-Online platform expects the smarthost to accept emails without any further authentication of users/tenants, which can lead to many misconfigured SMTP-relays!
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So, the question is: *How can an Exchange-Online user/tenant be identified by a smarthost?*
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# Tenant identification options provided by Microsoft
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## Client-IP ranges specified by Microsoft
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Microsoft provides an ACL as [JSON file (ID: 10)](https://endpoints.office.com/endpoints/worldwide?clientrequestid=b10c5ed1-bad1-445f-b386-b919946339a7), which looks like this.
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```
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[...]
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{
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"id": 10,
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"serviceArea": "Exchange",
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"serviceAreaDisplayName": "Exchange Online",
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"urls": [
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"*.mail.protection.outlook.com"
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],
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"ips": [
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"40.92.0.0/15",
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"40.107.0.0/16",
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"52.100.0.0/14",
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"104.47.0.0/17",
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"2a01:111:f400::/48",
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"2a01:111:f403::/48"
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],
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"tcpPorts": "25",
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"expressRoute": true,
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"category": "Allow",
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"required": true
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}
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[...]
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```
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The problem of IP based ACLs is that many other Exchange-Online customers/tenants are sending from the same IP-ranges as well! **This means that many smarthost configured to relay mails comming from Exchange-Online tend to act as open relays (for Microsoft customers) unless additional authentication mechanism on a higher layer than IP takes place! IP-address based ACLs are definitely not the right way to achieve this!**
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## x509 client certificate presented by Exchange-Online
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The Exchange-Online platform also *presents* a x509 client certificate to identitfy onself to the smarthost. Taking a closer look at the received header shows on that the certificates common name (CN) *mail.protection.outlook.com* is not realy tenant specific. Although the certificate provides additional security regarding the identity of the client system, it does not provide identity regarding the tenant. **IMHO that´s stil not enough to permit relaying!**
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```
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Received: from DEU01-FR2-obe.outbound.protection.outlook.com (mail-fr2deu01lp2173.outbound.protection.outlook.com [104.47.11.173])
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(using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits))
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(Client CN "mail.protection.outlook.com", Issuer "GlobalSign Organization Validation CA - SHA256 - G3" (verified OK))
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by some.secure.smarthost.example.com (Postfix) with ESMTPS id 4CjqCQ2WRCzGjg6
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for <blah.blubb@example.com>; Sat, 28 Nov 2020 12:34:26 +0100 (CET)
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```
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Client certificate verification is the job of the underlying MTA. So the **ExOTA-Milter** does not validate the client certificate itself, but it can be enabled (disabled per default) to match for the *expected* client CN. The mail server software [Postfix](http://postfix.org), for example, only presents the client CN to the milter API if the client certificate was successfully validated before. Otherwise the **ExOTA-Milter** will not *see* the client CN over the milter protocol ([postfix milter macro](http://www.postfix.org/MILTER_README.html#macros) `{cert_subject}`) which results in a milter reject action.
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## DKIM - DomainKey Identified Message
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Nevertheless, as [Microsoft supports DKIM-signing for outbound email traffic](https://docs.microsoft.com/de-de/microsoft-365/security/office-365-security/use-dkim-to-validate-outbound-email?view=o365-worldwide) the **ExOTA-Milter** can be used to authenticate sending tenants, respectively their sender domains, based on the cryptographic capabilities of [DKIM](https://tools.ietf.org/html/rfc6376). In fact the **ExOTA-Milter** does not validate the DKIM-signatures itself. Instead it simply parses DKIM-specific *Authentication-Results* headers produced by any previously DKIM-validating milter (like [OpenDKIM](http://www.opendkim.org/), [Rspamd](https://rspamd.com/) or [AMavis](https://www.ijs.si/software/amavisd/)) in the chain. I personally prefer OpenDKIM as it´s lightweight and fully focused on DKIM.
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**To use DKIM for tenant/sender domain authentication, DKIM must be enabled in the milter as well as in each policy!**
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**Worth to know when using OpenDKIM as AR provider:** As Microsoft already signs with 2kRSA keys be sure to use a version of OpenDKIM, which is linked against a DNS resolver library that is able to handle such large DNS responses! Further the resolver library should be aware of DNSSEC! **[libunbound](https://nlnetlabs.nl/documentation/unbound/libunbound/) meets all of these requirements :-)**. A libunbound-linked version of OpenDKIM is provided by [Debian](https://wiki.debian.org/opendkim#DNS_resolution).
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*DKIM-Signature* headers appended by the Exchange-Online platform look like this:
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```
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[...]
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DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;
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d=tenantdomain.onmicrosoft.com; s=selector1-tenantdomain-onmicrosoft-com;
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h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
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bh=KWHUKEKZxeQyFbKe45TV1PJMC2XrVCMTFkdwSYWR6o0=;
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b=DYTLJtLFjvVrSZtZQagTwuEe5PQYqrNGi7hR5bkhO[...snip...]
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[...]
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```
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*Authentication-Results* headers provided by OpenDKIM (signature valid, public key not DNSSEC signed) look like this:
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```
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[...]
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Authentication-Results: trusted.dkim.validating.relay;
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dkim=pass (2048-bit key; unprotected) header.d=tenantdomain.onmicrosoft.com header.i=@tenantdomain.onmicrosoft.com header.b=mmmjFpv8";
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dkim-atps=neutral
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[...]
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```
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## X-MS-Exchange-CrossTenant-Id header (policy binding)
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Further each Microsoft Exchange-Online tenant has a unique tenant-ID in form of a UUID ([RFC 4122](https://tools.ietf.org/html/rfc4122)). **ExOTA-Milter** extracts the tenant-ID from the *X-MS-Exchange-CrossTenant-Id* email header and uses it as a *mandatory* authentication factor.
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```
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[...]
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X-MS-Exchange-CrossTenant-Id: <UUID-of-tenant>
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[...]
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```
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At last the **ExOTA-Milter** needs an additional policy (JSON file), that provides a mapping of *sender-domain <-> tenant-id* and if DKIM-signatures must be taken under consideration or not. The JSON policy file itself looks like this:
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```
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{
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"yad.onmicrosoft.com": {
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"tenant_id": "1234abcd-18c5-45e8-88de-123456789abc",
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"dkim_enabled": true
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}
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}
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```
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Actually I´m also working on a LDAP-based version as policy backend.
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# The solution
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So, *how can an Exchange-Online user/tenant be identified by a third party smarthost?*
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Finally it´s the combination of all of the above discussed aspects which may result in a robust-enough smarthost setup used by the Exchange-Online platform:
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* restriction of client IPs via ACL (MTA)
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* verification of Microsoft´s x509 client certificate (MTA)
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* matching for client certificate´s CN (ExOTA-Milter)
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* verification of DKIM signatures providing *Authentication-Results* header (another milter, e.g. OpenDKIM)
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* consideration of DKIM verification results per sender domain (ExOTA-Milter)
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* matching for tenant-id provided in *X-MS-Exchange-CrossTenant-Id* header (ExOTA-Milter)
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# How about a docker/OCI image?
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## Using prebuilt images from [dockerhub](https://hub.docker.com/)
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* AMD64: https://hub.docker.com/r/chillout2k/exota-milter-amd64
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* ARM32v6: https://hub.docker.com/r/chillout2k/exota-milter-arm32v6
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The images are built on a weekly basis. The corresponding *Dockerfile* is located [here](OCI/Dockerfile)
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## Build your own image
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Take a look [here](OCI/README.md)
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# How to test?
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First of all please take a look at how to set up the testing environment, which is described [here](tests/README.md)
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