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Protect Exchange Server OWA/ECP from brute force attacks

A lot of bots attack the Exchange Server OWA and ECP page constantly. It’s the (security) engineers task to secure and protect the Exchange Server OWA/ECP URLs from attacks. Measures such as Multi-Factor Authentication (MFA) or only making the OWA/ECP accessible through VPN are excellent ideas. Another method to stop Exchange Server brute force attacks is integrating Google reCAPTCHA to the OWA/ECP page. It’s FREE and works excellently.

Protect Exchange Server OWA/ECP from attacks

There are a lot of methods to protect Exchange Server OWA/ECP from attacks. Here are the best ones to protect Exchange Server OWA/ECP from brute force attacks:

  1. Configure a third-party MFA solution
  2. Configure VPN so only users connected to the internal network can access Exchange OWA/ECP
  3. Enable selected countries only in the firewall that can access Exchange OWA/ECP
  4. Configure Azure app proxy (Azure AD Premium P1 or P2 license)

If you don’t want to spend money on an MFA solution or you don’t want to pay Azure AD Premium P1 or P2 license, you can protect Exchange OWA/ECP from brute force attacks by adding a reCAPTCHA in the sign-in page.

Exchange Server Google reCAPTCHA integration

To create a Google reCAPTCHA site and integrate it into Exchange Server OWA/ECP, go through the below steps:

Create Google reCAPTCHA site

The first step is to create a new Google reCAPTCHA site. Once we have the reCAPTCHA keys, we can integrate them into Exchange Server.

Sign in to Google reCAPTCHA and fill in the below details:

  • Label: Exchange Server
  • reCAPTCHA type: reCAPTCHA v2 – “I’m not a robot” tickbox
  • Domains: mail.exoip.com (your Exchange URL) and localhost

Check both the checkboxes:

  • Accept the reCAPTCHA Terms of Service
  • Send alerts to owners

Click on Submit.

Protect Exchange OWA ECP from brute force attacks register reCAPTCHA

The site key and secret key will appear. Copy both keys and save them because you need them in the next step.

Protect Exchange OWA ECP from brute force attacks add reCAPTCHA

Create Exchange Server reCAPTCHA page

Let’s integrate the site key into Exchange Server.

Note: Do the below steps on every Exchange Server (CAS).

Sign in to the Exchange Server and go to the below path. Suppose you have an older Exchange Server version, change V15 to another version. For example, V14.

C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\owa\auth

Create a new file with Notepad with the name recaptcha.aspx in that folder.

Protect Exchange OWA ECP from brute force attacks recaptcha file

Copy/paste the below information into it.

<% @ Page AspCompat=True Language = "VB" %>
<%
Dim strPrivateKey As String = "SECRET_KEY"
Dim strResponse = Request("response")
Dim objWinHTTP As Object
objWinHTTP = Server.CreateObject("WinHTTP.WinHTTPRequest.5.1")
objWinHTTP.Open("POST", "https://www.google.com/recaptcha/api/siteverify", False)
objWinHTTP.SetRequestHeader("Content-type", "application/x-www-form-urlencoded")
Dim strData As String = "secret=" & strPrivateKey & "&response=" & strResponse
objWinHTTP.Send(strData)
Dim strResponseText = objWinHTTP.ResponseText
Response.Write(strResponseText)
%>

Paste the Google reCAPTCHA secret key you copied in the previous step and paste it in the field SECRET_KEY.

Protect Exchange OWA ECP from brute force attacks secret key

Configure Exchange Server OWA/ECP logon page

Let’s integrate the secret key into Exchange Server.

Make a backup of the logon.aspx file, just in case you have to revert the changes. Copy logon.aspx and rename it to logon.aspx.bak.

Note: The file logon.aspx will be rewritten to its original state when you install Exchange Server CU. So write down in your manual to replace the file after the CU and test that the Google reCAPTCHA works.

Protect Exchange OWA ECP from brute force attacks logon.aspx backup

Open the logon.aspx file with Notepad and modify the below:

Find:

<form action="/owa/auth.owa"
Protect Exchange OWA ECP from brute force attacks form action before

Change to:

<form action=""
Protect Exchange OWA ECP from brute force attacks form action after

Find:

<div onclick="clkLgn()"
Protect Exchange OWA ECP from brute force attacks div onclick before

Change to:

<div onclick="myClkLgn()"
Protect Exchange OWA ECP from brute force attacks div onclick after

Find:

<div><input id="passwordText"

Next, press the Enter button and create a couple of empty lines.

Protect Exchange OWA ECP from brute force attacks div input id passwordtext before

Copy/paste the below information into the empty lines.

<script type="text/javascript">
function myClkLgn()
{
var oReq = new XMLHttpRequest();
var sResponse = document.getElementById("g-recaptcha-response").value;
var sData = "response=" + sResponse;
oReq.open("GET", "/owa/auth/recaptcha.aspx?" + sData, false);
oReq.send(sData);
if (oReq.responseText.indexOf("true") != -1)
{
document.forms[0].action = "/owa/auth.owa";
clkLgn();
}
else
{
alert("Invalid CAPTCHA response");
}
}
</script>
<script src="https://www.google.com/recaptcha/api.js" async defer></script>
<div class="g-recaptcha" data-sitekey="SITE_KEY"></div>

Paste the Google reCAPTCHA site key you copied in the previous step and paste it in the field SITE_KEY.

Protect Exchange OWA ECP from brute force attacks div input id passwordtext after

Restart IIS on Exchange Server

You don’t need to restart IIS on the Exchange Server, and the Google reCAPTCHA will immediately appear on the Exchange Server OWA/ECP page.

Suppose it doesn’t, start Command Prompt as administrator and restart Internet Information Services (IIS) on the Exchange Server.

iisreset

Verify Exchange Server OWA/ECP with Google reCAPTCHA

Start your favorite web browser and go to the Exchange Server OWA address.

The Google reCAPTCHA is visible.

Protect Exchange OWA ECP from brute force attacks reCAPTCHA visible

Fill in the user credentials and check the Google reCAPTCHA checkbox. Next, click on Sign in.

Protect Exchange OWA ECP from brute force attacks reCAPTCHA valid

The user will successfully sign in.

Protect Exchange OWA ECP from brute force attacks signed in

If the user fills in the wrong credentials, it will show the standard message: The user name or password you entered isn’t correct. Try entering it again.

Protect Exchange OWA ECP from brute force attacks wrong password

Suppose the user doesn’t accept the Google reCAPTCHA box and clicks on sign in. A message will appear with text: Invalid CAPTCHA response.

Protect Exchange OWA ECP from brute force attacks reCAPTCHA invalid

The same applies to the Exchange Admin Center (ECP) page.

Protect Exchange OWA ECP from brute force attacks ECP reCAPTCHA

It will also work when browsing to https://localhost/owa or https://localhost/ecp. That’s because you did add localhost in the first step when creating the site in Google reCAPTCHA.

Protect Exchange OWA ECP from brute force attacks localhost reCAPTCHA

That’s it!

Read more: Secure Active Directory passwords from breaches »

Conclusion

You learned how to protect Exchange Server OWA/ECP from brute force attacks. First, create a free Google reCAPTCHA. After that, adjust the Exchange Server file so it will display and use the Google reCAPTCHA box on the OWA/ECP page. Combine this with an MFA solution, and you are good to go.

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ALI TAJRAN

ALI TAJRAN

ALI TAJRAN is a passionate IT Architect, IT Consultant, and Microsoft Certified Trainer. He started Information Technology at a very young age, and his goal is to teach and inspire others. Read more »

This Post Has 16 Comments

  1. Good day.
    And what about the cost from Google? Can I install this for free, on a private account? Or do I need some kind of enterprise solution?

      1. Hi Ali,

        Not able to get it working, only receive “Verification expired. Check the checkbox again” after a long wait after pressing sign-in.
        Localhost for ECP is not working either.
        When registering domains at google, should I register:
        mysite.com or the actual I use mail.mysite.com?

        Thanks,
        Ase

          1. Hi Ali,
            Did this as well now, but no change in behaviour.
            Double checked everything, but could not find errors in code.
            Ideas how to go on troubleshooting?
            //AseKarlsson

  2. Will there be a problem connecting to the Exchange server from remote Outlook clients accessing the message over HTTPS?

  3. Hi,

    I have a Exchange 2019 with DAG. I want to install reCAPTCHA. Will I have problems with installation?

      1. thank your answer. I created Google reCaptcha in Owa. I marked reCaptcha. I write my password. But I can’t enter. why could it be ?

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