Microsoft Windows 11 Caches Exploitable Malware

I am a life-long gamer and from experimentation over the years, I know I have the best rig, a customisable gaming PC. It’s very nice hardware it looks stunning with rainbow glowing fans and it sounds like a mini jet engine, rendering most graphics on Ultra with a graphics card which is at most 2 years old. The only issue I have had with it is in the operating system, which of course for a big gamer is Windows 11 due to its age. After just two weeks of operation I was alerted to a dropper located in my Windows Appdata folder, it was not dropping any further viruses yet as it had been caught by my expensive AV - which is why it sounds like a jet engine due to the large use of CPU!!

Was it a false positive? Well I ran the offline scan myself as my machine took 8x longer to boot. It behaved as if rebooting after a major update. There were no updates carried out, which made me suspect something amiss.

The virus location was not new to me, it is an appdata folder present in Windows 10 and Windows 11 for Windows mail in particular it processes mail syncing across devices; the full path is:

C:\Users\’Username’\AppData\Local\Packages\microsoft.windowscommunicationsapps_8wekyb3d8bbwe\LocalState\Files\SO\

Now I have had viruses popping up here before, in fact it has been an ongoing problem since I adopted the Windows 11 operating system (OS) in 2022 on the household laptops. Since then, both of my son’s laptops have alerted me to droppers and Trojans in the same Appdata folder. I initially assumed my children were not so good with their cybersecurity (being aged 5 and 9 that makes sense). Maybe they had clicked an email notification. Maybe they had downloaded some Trojan in the style of a game from a requested and shoddy gaming store all parents know about, stealthy and all whilst under supervision. That was until my brand new gaming rig got one.

Examining the attack timeline of my gaming rig in detail showed that a phish had been ‘clicked’ from my Hotmail account which was logged in on my Outlook App, running in the background processes (not running in my taskbar) whilst I played some epic title the night before. This ‘click’ put an infected PDF invoice on my PC, and on boot the next day activated the dropper, slowing the machine right down - which gave me a clue.

Now let's be clear about how Microsoft works, as I understand it, and why this is a gripe serious enough to warrant its own blog post.

You must sign in online to a Microsoft account to access the PC at all times, then it creates session keys for all applications that come installed as standard with the OS. So, unless you take some drastic measures I will discuss in a moment, you will undoubtedly be running sessions of Windows apps in the background; Outlook, OneDrive, Teams, Xbox, Photos, Office, Store, the list is quite extensive.

Moving on, I look for the phish email I had. According to my AV "clicked on" log, I located the suspicious subject of the email. It was something akin to;

AMAZON ACCOUNT ###U$IIHknDBWON38383y4y29~~~

Now, you do not need to be a cybersecurity expert to tell that is a phish from the subject which was located using the now foreground Outlook app. And as expected, it was unread. It showed me that within a few minutes of receiving the unread email, the attached PDF invoice was added to the Windows Communication Appdata folder, which led to the dropper found by the offline scan.

It seemed that Outlook App was saving unread attachments to the appdata folder where the virus was located. Some of the located viruses over the past two years were found on multiple devices that used the same MS credentials which unless stated otherwise, auto sync application data. That is exactly what the

microsoft.windowscommunicationsapps_8wekyb3d8bbwe

folder is for. It contains both the application data for Windows apps like Outlook to run and sync as well as data obtained in the process. I synced between my children's laptops as my account was the administrator for the network, it kept them safer online. Or so I thought.

So I start down the usual process of researching the vulnerability to find a permanent mitigation. I find nothing at first, no CVE, no reports. I do find some details on the MS community website like this one from "2020 Trojan Found and Deleted"… but it Returns (Trojan: HTML/Phish.AB!MSR) . It is here I see the same problem and I see a response from an expert.

So I search all around this topic and I cannot find anything from Microsoft about this issue right away, but I do find many victims. Anti-Virus companies, affected users and experts alike are all drawing attention to this problem. Eventually, thanks to Reddit I find a similar experience from a commentor who managed to find the reported exploit listed by Microsoft. You can find many more threads on the issue on Reddit with a search.

It is a known software error marked as severe on their own security intelligence website, with 24 exploits under the O97M CVE. I’d like to say I have more details from Microsoft but I do not, instead they offer very little default cybersecurity steps, such as "remain up to date and don’t click a phish".

This does seem rather severe. It locally caches attachments from emails including those which have not been opened to the windowscommunicationsapps folder from the Outlook App. It will then auto sync the data in the folder to all devices using the same credentials in their Outlook App on their phones, laptops, desktops, anywhere the Outlook App runs.

The only step missing for a full compromise is that infected files are auto-run… a feature I feel sure Microsoft are working on at this exact moment!

Attacks are becoming more complex, in 2022 they were able to add the dropper, which very slowly downloaded a fairly rubbish Trojan, which was easily removed. Recently there were 74 password protected files and multiple Trojans appearing as game applications in the same appdata folder. These were located only weeks after a fresh OS install and could not be explained by known activities on the machine or entirely resolved by AV due to the encryption of the folders they were located in.

Take a moment to think about how many commercial users of the Outlook application have auto-sync enabled in daily use. All those using Office 365 who sync between devices on their smartphone, personal devices and company equipment, or amongst family member's tablets and laptops. You should be concerned. And then get mad as hell, because it seems Microsoft have created their own quite special service for propagating malware, one that's been ongoing since at least 2020.

I have wasted countless hours re-installing OS’s, searching files, reading reports and researching this issue to find my only salvation in Reddit. Reddit of all places! This looks like something Microsoft rather wanted to sweep under the rug.

If you sync with Outlook App between devices with the same MS account, you are vulnerable to this malware propagation. Microsoft insist users take advantage of auto-synced features across devices and use this as a clear marketing tool especially for commercial settings. It seems my trust in this feature was misplaced.

Some will say that this is "just a feature" of sync. I disagree because like so many Microsoft processes it feels out of control. It does not just synchonise expected user data. It inappropriately populates and copies undocumented files into system folders and, without any knowledge or intervention from the user, replicates them across devices.

The mitigation is you must live without synchronisation in Microsoft applications. So far it has worked 100%. Turning off sync across applications does work. This can be done during an OS install by refusing all sync options when prompted. You must also make sure it is off in the account settings for the user. This can be done from the settings panel when logged on to Windows. There are many guides available like this one from Process.st. Even with sync off, you can still access email and other services using the web applications, which will sync files and emails but will not store these to the local machine.

If like myself, you have lost trust in the applications themselves, removing windows apps entirely may be more fitting, this allows the folder

microsoft.windowscommunicationsapps_8wekyb3d8bbwe

to be deleted and does not appear, like a lurking background threat at a later date just in case you change your mind. My gaming rig has only one MS app remaining, Xbox and that is the way it will stay until the situation is openly discussed by MS and the vulnerability resolved. No more Appdata Phishes please.

In summary, no amount of phishing training will prevent a bad design in the operating system. Caching malware infected attachments to system folders and replicating them is bad design in my opinion. In this case, the operating system has been phished, not the human.