---
title: "Why You Need to Pay Attention to Email Security Vulnerabilities Such as the Autodiscover Feature of The Microsoft Exchange Server | DuoCircle"
description: "Autodiscover, a Microsoft Exchange protocol, now has a vulnerability that miscreants can exploit."
image: "https://www.duocircle.com/images/og-default.png"
canonical: "https://www.duocircle.com/blog/email-security/why-you-need-to-pay-attention-to-email-security-vulnerabilities-such-as-the-autodiscover-feature-of-the-microsoft-exchange-server/"
---

Quick Answer

Microsoft Exchange's Autodiscover protocol leaked credentials at scale because of how its back-off algorithm constructed lookup domains. When an Outlook client could not reach the configured Autodiscover endpoint, it walked up the domain hierarchy and sometimes ended up querying autodiscover.tld domains owned by third parties. Guardicore Labs registered eleven such domains and captured 372,000 Windows domain credentials plus roughly 100,000 unique credentials from other applications, all sent over HTTP Basic Authentication. The fix has two halves. Users and IT teams should block the autodiscover. top-level domains (autodiscover.com.br, .com.cn, .es, .fr, .in, .it, .sg, .uk, and others on Guardicore's published list) at the firewall or hosts file, and disable HTTP Basic Authentication so credentials are not sent in clear text. Developers using Autodiscover should not let the back-off algorithm fail upward to public TLD-level domains. 

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![email security](https://media.mailhop.org/duocircle/images/2021/11/spf-validator-9734.jpg) 

Autodiscover, _a Microsoft Exchange protocol, now has a vulnerability that miscreants can exploit_, according to a security firm that discovered the loophole as part of their [email security](/) research efforts. If anyone uses the vulnerability, they can access sensitive credentials from the Exchange-connected client, in a threat akin to [spear phishing](/content/spear-phishing-prevention/spear-phishing-best-practices). These sensitive credentials are Windows domain credentials that can authenticate Exchange servers. And malicious actors [using the vulnerability](https://www.csoonline.com/article/3634388/exchange-autodiscover-feature-can-cause-outlook-to-leak-credentials.html) for their nefarious activities can be a nightmare to any organization.

_The design of the Autodiscover protocol and its incorrect implementation in some applications are the primary causes of this flaw_ that can put thousands of organizations at risk. Guardicore Labs, the firm that conducts [email security](/) and [phishing protection](/email/phishing-protection) research, _captured more than [372,000 Windows domain credentials](https://www.guardicore.com/labs/autodiscovering-the-great-leak/) apart from almost **100,000 unique credentials** from various other applications._ However, adequate awareness and appropriate configuration can solve the issue.

## What is Microsoft Exchange Autodiscover?

The [Autodiscover](https://docs.microsoft.com/en-us/exchange/architecture/client-access/autodiscover?view=exchserver-2019) service reduces configuration and allows email clients to discover and obtain configurations from Exchange servers automatically. It is used to secure the connection with mailboxes of Exchange users to access features like [email forwarding](/email/email-forwarding), offline address book, and unified messaging. Email clients can discover servers and provide credentials to receive the correct configurations for various email services.

_Microsoft initially introduced the Autodiscover service to solve a significant problem_. Administrators had to configure Exchange servers manually, and more configurations were needed to use the latest features offered by Outlook 2003\. These configurations were cumbersome, repetitive tasks. As manual configuration became difficult and risked **email security**, Microsoft launched Outlook 2007 with Autodiscover.

It became an integral protocol in all Outlook clients as it quickly discovers the server holding a user’s mailbox and configures the client to connect to the server.

[![email security](https://media.mailhop.org/duocircle/images/2021/11/spf-record-7891.jpg)](https://media.mailhop.org/duocircle/images/2021/11/spf-record-7891.jpg)

## How Was This Loophole Discovered?

Guardicore, a firm that offers security solutions, such as **phishing and ransomware protection**, discovered this _credential-exposing loophole leak during its email security research_. They obtained eleven domains and assigned them to a web server. After waiting for web requests for Autodiscover endpoints, the research team observed several requests from multiple IP addresses, domains, and clients to Autodiscover endpoints. These requests were for the relative path _/Autodiscover/Autodiscover.xml._

Surprisingly, the Authorization header associated with these requests included credentials in HTTP Basic Authentication. The organization captured many credentials by sending packets to establish server-client sessions.

The logs of the HTTP server revealed that the server requested HTTP Basic Authentication. The victim did not realize that they were under attack and was redirected to the Autodiscover server, requesting authentication through a prompt. The victim entered their credentials in the prompt dialog box, and the information was sent to the attacking firm’s server, as in a [spear-phishing attack](/phishing-protection/north-korea-attacks-united-stateswith-spear-phishing/).

## Why Should Organizations Be Concerned & What Are The Consequences?

_The scale of a possible attack exploiting this email security vulnerability can be enormous_. The implications of this flaw in design can be massively grave if an attacker accesses high-level Autodiscover domains. The malicious actor can exploit Autodiscover requests to possess critical and sensitive domain credentials in such an event. These credentials in the hands of threat actors can lead to many undesirable consequences, including massive data breaches, holding something ransom, DDOS attacks, and much more. Even though Microsoft introduced the feature with the [Exchange 2007](https://techcommunity.microsoft.com/t5/exchange-team-blog/exchange-2007-autodiscover-and-certificates/ba-p/593753) version, it is unclear how long the vulnerability existed.

[![email security](https://media.mailhop.org/duocircle/images/2021/11/spf-record-tester-1973.jpg)](https://media.mailhop.org/duocircle/images/2021/11/spf-record-tester-1973.jpg)

## How To Plug The Loophole?

Two parties must implement [email security](/) measures to protect against the Autodiscover leak. _One party is the general users and the other, developers who use the Autodiscover feature in their system_. The public users have devices with Exchange-based software (Outlook, ActiveSync, etc.) installed. They must ensure that they are actively blocking domains in their firewall. The domains to be blocked are Autodiscover domains, such as Autodiscover.com, Autodiscover.com.cn, etc. Guardicore has compiled a comprehensive list of top-level domains, which is available [here](https://github.com/guardicore/labs%5Fcampaigns/tree/master/Autodiscover). Users can add these domains to the host’s file or firewall configuration.

A list of examples of Autodiscover domains to be blocked is given below. They belong to various countries across the world.

- Autodiscover.com.br, Brazil
- Autodiscover.com.cn, China
- Autodiscover.com.co, Columbia
- Autodiscover.es, Spain
- Autodiscover.fr, France
- Autodiscover.in, India
- Autodiscover.it, Italy
- Autodiscover.sg, Singapore
- Autodiscover.uk, The United Kingdom

Users should also ensure that basic authentication support, HTTP basic authentication, is disabled. _It should be done to ensure that user credentials are not sent to the server_. Developers implementing the Autodiscover feature should ensure that they are not letting the protocol fail upwards. The “back-off” algorithm should not be allowed to construct domains such as “Autodiscover.”

## Final Words

_The design flaw in the Autodiscover feature can have grave consequences and put thousands of organizations at risk_. The risk is heightened as many devices are used remotely outside organizational networks. Even though this feature aims to help organizations set up easy connections to Exchange, it puts many users at an [email security](/) risk. If malicious actors use the vulnerability to attack organizations, the magnitude of the resultant loss could be enormous.

However, the good news is that _organizations can plug this vulnerability through proper configurations_. Organizations should continue securing their network and devices from such vulnerabilities and take steps towards keeping the organization’s critical information assets secure from threats such as **ransomware and phishing attempts**. Software manufacturers should also ensure that they have educated developers onboard, trained on creating and testing secure code. Software manufacturers should also keep analyzing their old and new products for lurking Autodiscover vulnerabilities and other possible email security risks.

![Brad Slavin](https://media.mailhop.org/dmarcreport/images/team/brad-slavin.jpg) 

Brad Slavin 

General Manager

General Manager at DuoCircle. Product strategy and commercial lead across the email security portfolio.

## Secure your email infrastructure

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[Contact Sales](/contact/) [Explore Products](/products/) 

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