UPDATE TO: Exchange Still Suffers from Document Handling Flaws

Yesterday, we released an alert warning you about vulnerabilities in Exchange Server, as well as a new update to fix those flaws. Today, we have learned that there are problems with the Exchange Server 2013 version of that update. Microsoft has confirmed this problem and has pulled the affected patch. If you use Exchange 2013 and have already installed the broken update, we recommend you follow the steps outlined in Microsoft blog post. If you have not installed the update yet, we expect Microsoft to released the corrected version soon.

As an aside, this is a perfect example of why you should always test server patches before deploying them to production environments (as we noted in the original alert).

We have included the original alert below for your convenience.

Severity: High

Summary:

• These vulnerabilities affect: Exchange Server 2007, 2010, and 2013
• How an attacker exploits it: By enticing a user to preview a specially crafted email attachment using OWA
• Impact: An attacker can execute code with the restricted privileges of the LocalService account
• What to do: Deploy the appropriate Exchange Server update as soon as possible.

Exposure:

Microsoft Exchange is one of the most popular email servers used today. It includes many advanced features and capabilities. One such feature, called WebReady Document Viewing, allows your email users to preview attached documents as web pages. Exchange leverages Oracle’s Outside In technology to parse these documents and provide these previews.

According to today’s bulletin, Exchange suffers from two remote code execution vulnerabilities related to Oracle’s Outside In technology, and a Denial of Service (DoS) flaw involving their Data Loss Protection (DLP) feature.  All three vulnerabilities have to do with how Exchanges parses files and documents when users view or preview them. By enticing one of your web-based users to preview an email with a specially crafted attachment, an attacker can exploit the worst of these flaws to execute code directly on your Exchange server. Luckily, the code only runs with LocalService account permissions, which has limited privileges.

Also, this attack only works against victims who check and preview mail using Exchange’s Outlook Web App (OWA). If your users only download email from Exchange using email clients, like Outlook, attackers may not be able to leverage these flaws against your server.

Nonetheless, we still recommend Exchange administrators update as soon as possible.

Solution Path:

Microsoft has released Exchange updates to correct these vulnerabilities. You should download, test, and deploy the appropriate update as soon as possible, or let Windows Update do it for you. You can find the updates in the “Affected and Non-Affected Software” section of Microsoft’s Exchange bulletin.

NOTE: In the past, one of our readers reported having technical issues when installing an Exchange update very similar to this one. This is a perfect example of why we highly recommend you test server updates before deploying them to production machines. Most virtualization platforms allow you to make a copy of a running computer, which you can then spin up in a virtual environment. Doing this allows you to create an ideal virtual environment in which you can test these sorts of updates on a virtual server, before installing them on your real one. 

For All WatchGuard Users:

Though you can configure our XTM and XCS appliances to strip certain attachments from email, this sort of attack may arrive as many types of attachments, including ones you may want to allow for business. We recommend you apply the patches instead.

Status:

Microsoft has released patches to fix these vulnerabilities.

References:

• MS Security Bulletin MS13-061

This alert was researched and written by Corey Nachreiner, CISSP (@SecAdept).

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