Free Micropatches For Microsoft Access Forced Authentication Through Firewall (0day)

 

Update 2/14/2024: Either January 30 or February 1 Office update brought a fix for this issue: now, Access warns the user for any ODBC connection to SQL Server. Our patch only shows a warning when such connection is made on non-standard ports 80 or 443, because these would carry user’s NTLM hash through a company firewall, so Microsoft’s patch might display more – in our view unnecessary – warnings. So what CVE ID did this issue get? Well, it doesn’t seem to have gotten one: neither January 30 nor February 1 Office update mention any changes in Access, and February Windows Updates also have no suitable match. So far, this issue seems to have been fixed silently. With official patch available, our patches for this issue are no longer FREE and require a PRO or Enterprise license. Our patch was available 66 days before Microsoft’s.

On November 9, 2023, Check Point Research published an article about an “information disclosure” / “forced authentication” vulnerability in Microsoft Access that allows an attacker to obtain the victim’s NTLM hash by having them open a Microsoft Office document (docx, rtf, accdb, etc.) with an embedded Access database.

Many similar vulnerabilities have been disclosed in the last few years, all having a common theme of forcing a Windows process to authenticate to attacker’s server and thereby disclose credentials of Windows user or a privileged service account to the attacker. Microsoft has patched some of them, but decided not to patch others: DFSCoerce, PrinterBug/SpoolSample and PetitPotam still don’t have an official patch today and our micropatches remain the only patches available for these (our customers who can’t stop using NTLM really appreciate them). RemotePotato0 was initially experiencing a similar fate but was then silently fixed 9 months after publication. ShadowCoerce was just as silently fixed 6 months after publication. On the other hand, a WordPad vulnerability from this same category, leaking user’s NTLM hash to a web share upon opening an RTF document, was openly patched by Microsoft just last month.

It’s hard to tell how Microsoft decides whether to patch a forced authentication vulnerability or not – and this one in Microsoft Access just adds to the confusion. Let’s see how.

The Vulnerability

As Haifei Li, security researcher at Check Point, describes in their detailed article, a remote SQL Server database link can be inserted to a Microsoft Access database with “Windows NT authentication”, which will force such authentication – and leak user’s NTLM hash – every time the table with such link is refreshed in Access. So far, this would be a “classic” forced authentication issue, only different from most others in the fact that the connection isn’t established on “classic” SMB and RPC ports but on SQL Server’s port 1433. Which is expected to be filtered for outbound traffic from internal network towards the Internet.

But as Haifei noted, the database link can override the default port and specify an arbitrary port, including 80 or 443 – which are both typically allowed by firewalls for outbound connections so users in the network can browse the Internet. This makes things more interesting, and impact-wise almost comparable to the previously mentioned WordPad issue. Why “almost”? Because it’s not enough just to open a Word document with such Access database embedded: to force a refresh of the database link, the user has to “open” the linked Access table by clicking on it.

To overcome this limitation, Haifei found that the AutoExec macro can be used to automatically open the table and force a refresh. But hey, macros have already been blocked for documents coming from the Internet, so how would this even work?

Well, we first need to understand something Haifei calls “simple Access macros”. Admittedly, we did not know that Access macros come in two flavors, and we couldn’t find any relevant results on this phrase on the Internet, but that’s probably because Haifei usually knows more about the targeted product than the Internet does.

“Simple Access macros” are limited macros that only allow you to perform a set of predefined harmless actions, in contrast to “full-fledged regular macros” (sorry, another unofficial term) that are actual VBScript code capable of doing pretty much anything on the computer, including downloading and executing ransomware. It turned out that simple Access macros are blocked neither by the Access macro policy nor by the Protected View. In addition, if you name such macro AutoExec, it will get executed upon opening the Access database.

Putting two and two together, Haifei created an Access database with a remote SQL Server database link, Windows NT authentication, and an AutoExec macro that opened the linked table – and embedded that in a Word document because users prefer opening Word documents to Access databases. Now, there is no “almost” there anymore: this issue is impact-wise identical to the WordPad issue.

Checkpoint reported this vulnerability to Microsoft in January 2023 and were in July still “unable to obtain conclusive answer because the issue is considered as “low/none severity”, according to the MSRC reply.” They did notice, however, that at some point during this period, Access started showing this security dialog when opening their PoC file:

 

So, was this issue silently fixed too? Nope, at least not successfully: while the above dialog is certainly triggered by the presence of the AutoExec macro (it shows even when AutoExec is the only active content), closing this informational dialog either by pressing OK or clicking the X still leads to the AutoExec macro being executed and user’s credentials being sent to attacker’s server. The only way to block the exploit when this dialog is displayed is to forcefully kill the msaccess.exe process, e.g. using Windows Task Manager.

In summary, we have active content that is detected, user informed about it being blocked, and then still getting executed unless the user kills Access with Task Manager. Not ideal.

Our Analysis

We tried to make sense of all this and here’s what we think happened.

We think Microsoft never intended to patch the reported issue due to its “low/none severity” assessment, which we think was wrong because its impact is comparable to the WordPad issue they had patched last month with severity “important”.

We think the security dialog that started appearing in Access is part of Microsoft’s slow and painful process of gradually restricting malicious macros while not getting hammered by customers whose Office documents they might break along the way. (See here and here for examples.) Microsoft is doing the right but difficult thing here, addressing a very popular attack avenue, and they deserve huge credit for that.

We think that the current macro-blocking logic in Access is flawed: it clearly detects the AutoExec “simple” macro, it tells the user that macros are blocked – but then doesn’t block it. Microsoft needs to fix this, but it’s not hard to imagine thousands of enterprises using “simple” macros on a daily basis, and thousands of angry calls to the Office PM the next day if they actually start getting blocked. Still, this needs to be fixed this because it’s confusing and useless: either don’t trip on simple macros, or trip on them and block them.

We expect Microsoft will do something about this all; they will probably fix the macro logic and the dialog, but will they revise the severity of the issue reported by CheckPoint and fix it too?

Maybe they will, maybe they won’t – but we did.

Our Micropatch

We pondered on how to address this: shall we fix Microsoft’s macro logic so that simple macros will indeed be blocked when the dialog says they would be? If we did, and broke “simple” macros for our users, they would probably blame Microsoft and make angry calls to the surprised Office PM. We don’t presume we understand the complex dynamics between a huge software vendor who decades ago made a convenient powerful feature that boosted product usability but has since become a major security risk, and organizations that have this feature embedded in critical processes but can at the same time be seriously harmed because of it.

So we decided on a different approach: we would block database connections from Access to SQL Server on ports 80 and 443. While it is not impossible for some real, legitimate SQL Server to be accessible on port 80/443 and some real, legitimate Access database being linked to it, we think it’s realy unlikely. Note that such patch would not block SQL Server connections on port 80/443 from any other client, just Microsoft Access.

“How about other database servers that could be linked to remotely,” you ask, “some of them may also support Windows NT authentication and be accessible on arbitrary port?”

Well, the main risk is posed by the ones supported on Windows by default, without a non-default ODBC driver having to be installed on user’s computer. And SQL Server is the only one that fits the bill.

This is our micropatch:

 

Let’s see our micropatch in action. In the video below we can see attacker’s computer on the left and user’s computer on the right. The user is running fully updated Office 365. On attacker’s computer we can see Wireshark, a network monitoring tool, that is filtered to only show communication with the IP address of user’s computer.

First, with 0patch disabled, the user opens a malicious Access file in Microsoft Access, and as described above, a security dialog is displayed informing them that active content in the file has been blocked (we know it wasn’t). As the user closes this dialog, the linked database connection is established with attacker’s computer on port 80 due to the AutoExec macro being executed.

Next, with 0patch enabled, the user again opens attacker’s Access file. This time, as the “blocked active content” security dialog is closed, our patch detects that a connection to a SQL Server is attempted on port 80 and blocks it. It also records an “Exploit blocked” event and shows an alert to the user.

Micropatch Availability

Micropatches were written for the following versions of Microsoft Office with all available Updates installed:

1. Office 2010*
2. Office 2013*
3. Office 2016
4. Office 2019
5. Office 2021
6. Office 365
   

These micropatches have already been distributed to, and applied on, all
online 0patch Agents (unless Enterprise group settings prevented that). 

Vulnerabilities like this one get discovered on a regular basis, and
attackers know about them. If you’re using Windows or Office that aren’t
receiving official security updates anymore, 0patch will make sure these
vulnerabilities won’t be exploited on your computers. (By the way, still using Windows Server 2012? 0patch has you covered!)

If you’re new to 0patch, create a free account
in 0patch Central, then install and register 0patch Agent from 0patch.com, and email sales@0patch.com for a trial. Everything else will happen automatically. No computer reboot will be needed.

We would like to thank Haifei Li with Check Point for sharing the details of this vulnerability, which made it possible for us to create a
micropatch for our users.

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