Outlook 365 damages PDF and Office files since April 2023

[German]Short addendum from this week. I received a tip that Microsoft Outlook 365 has been constantly corrupting Office files – and probably also PDF documents – since April 2023. If you don't have a backup, you may lose attachments – deadly if you have to archive mails including attachments. Microsoft's assumption that this is related to a Windows update and was fixed in June 2023 on patchday is unfortunately incorrect. Here is an overview of the problem.

A reader's tip

German blog reader Christoph Stauffer alerted me this week in a private message on Facebook that there was a serious problem with Microsoft Outlook (thanks for that – I won't get around to picking it up here on the blog until the end of the week). Christoph wrote about it:

Hello Günter,

maybe interesting for you. We have a problem at a customer (150 users) that since April [2023] attachments in Outlook suddenly break.

If you go to Page 3 in the [linked post], you see that Microsoft only confirms on 05/31 that it is an update problem (Page 3 at the bottom).

On page 4, the first voices are already coming in that the June update is not helping. We hope for the best with our customer and roll out today.

Christoph has posted a link to the Microsoft Answers forum thread Outlook 365 keeps corrupting PDF and Office files, dated May 10, 2023. Someone there writes:

Since April we started a new EA agreement with Microsoft and added all of our users to E5 licenses of M365 on top of the E3 licenses they already had – we were told that the E3 licenses would disappear anyway.

We don't know if that is related, however since mid-April we started to have numerous users who find their email attachments to get corrupt in Outlook. Whether it's with Outlook 2016 or 365, and Windows 10 or 11 Pro, the symptoms are always the same: the size of the attachments decrease significantly and they cannot be opened in their usual application (Adobe PDF Viewer, Word, Excel, PowerPoint, etc.).

Attachments in emails are oftentimes received already corrupt and are therefore unusable, sometimes they are good to be viewed or opened, then later on get corrupt. Sometimes

the attachments get corrupt when their related emails get forwarded to other users (who also find corrupt attachments in the emails they received).

We noticed that Outlook sometimes get even in a state where no file can get attached to a new email, or no attachment from a received email can get saved on any drive.

Whenever a user tries to open a corrupt attachment, an error message similar to this appear: "there was an error opening this document. The file is damaged and could not be repaired". No useful details get logged in the Event Viewer, all we see at the time of the error is "An unexpected error has occurred".

We tried the SaRa diagnosis program on some users' workstations with no luck, the SaRa program gets to vanish before the issue is seen again.

We tried to run Desktop Outlook in Safe Mode, the issue has not been seen. We tried to disable all of our add-ins one by one in normal mode, the issue was seen with every add-in being disabled….

We have hundreds of users and are out of ideas. Can you please help.

So in this thread the observation, that Outlook breaks mailboxes, since mid-April 2023,  has been confirmed by several users. One user wrote that Microsoft has confirmed that this is a known issue. It was suggested that people use Office for Web app (OWA) to access mailboxes.

Windows bugs as cause?

In a post on this page, user Stephane Lacasse suspects it's related to the save bug in Windows update KB5023773. However, this bug affects Windows 10 and Windows 11. I had addressed the issue in May 2023 in the section "File operations (copy, save) fail" in the blog post Windows 10/11: Known issues (May 2023). There it is stated that there are temporary problems with file operations like copying or saving files.

This bug occurs in a special scenario (saving, copying or appending files with 32-bit applications that consider large addresses and use the CopyFile API). However, it also states that Microsoft Office applications such as Microsoft Word or Microsoft Excel are only affected if you use 32-bit versions. Then users may get during saving a document the error message "Document not saved". If the error occurs, one should repeat the last file operation, Microsoft writes in its recommendation for a workaround.

In the MS Answers forum thread, David Williams8 writes on this page that the update KB5026513 (Preivew for Windows 10 version 22H2 from May 2023) was found to be the cause of the problem

Our early testing has identified that if KB5026513 is uninstalled followed by a restart, PDFs can be opened without an issue. 3 Microsoft engineers we have spoken to did not know whether any KB was the cause of the issue. process was we ran in all May updates and then took KB5026513 out.

Microsoft employee Scott Xie then wrote on page 3: This is caused by a known problem in the Windows patch in April and May. He also makes reference to the copying problem I mentioned above. KB5026361 also lists this as a "Know Issue". It was said by the Microsoft employee that this copying issue would be fixed with the June 2023 security update for the Windows versions in question.

No fix through Windows June 2023 updates

I can confirm the copying bugs has been fixed – at least as far as Microsoft's description of the June 13, 2023 updates is right. However, I wondered "if this is really the core of the issue described above" and I had some doubts that installing the Windows 11 22H2 June 2023 update really works.

Within the Microsoft answers forum thread user Menyelek Thomas wrote on June 14, 2023 (after the June patchday) on this page, that the June 2023 Windows updates won't help:

Hi All,

We have installed the latest KB KB5027215 onw a windows 10 22h2 multi session vdi however we still appear to be experiencing the same issue, has anyone else tested the latest update ?

And at this point comes a twist in the theme that confirms my gut feeling that it has nothing to do with Microsoft's theories on the file copy problem with 32-bit apps. Because Menyelek Thomas writes:

I can confirm that we are running the 64 bit outlook client Microsoft 365 MSO (version 2208 Build 15601.20660). Just to circle back on your comment are we saying that all previously opened attachments are now permanently corrupted and will experience the intermittent failure of opening / downloading from an email?

He is using a 64-bit version of Microsoft Outlook 365 (version 2208 build 15601.20660), which according to the above theory rules out the 32-bit bug. Nevertheless, broken attachments sporadically occur in Outlook. More users confirm in the thread linked above that the Windows June 2023 updates do not fix the bugs. Samuel Westen writes that the problem persists and that the Windows updates did not prevent the corruption of newly attached PDF files.

He points out that the problem with corrupted PDF files occurs for him on a web platform that uses AmazonSES as its email service provider. And he asks, "Is there something about the relationship between AmazonSES, Outlook, and the CopyFile API that is causing this problem?"

As of writing this article, I haven't found any hint of a fix in the Microsoft Answers forum thread. Is anyone else in the readership affected? If so, is there a workaround that is known?