Forum Discussion
Excel repair strips all formulas from large .xlsm after March 2026 security update (KB5002849)
Hi everyone,
I'm a master's student at Karolinska Institutet in Stockholm. My thesis is a health economic cost-effectiveness model built entirely in Excel — a gender-neutral static Markov cohort model with 34 worksheets. The file has become completely unusable after what I believe is the March 2026 security update, and I'm running out of options.
The file:
- .xlsm, ~46.5 MB compressed, ~370 MB uncompressed XML
- 34 worksheets, four of which are 73–92 MB each (Markov trace sheets)
- ~65,000 formulas, ~33,500 shared formulas
- Heavy use of LET, LAMBDA, XLOOKUP, XMATCH, CHOOSECOLS, TAKE, MAP, SWITCH
- 771 defined names including ~147 hidden _xlpm.* LET/LAMBDA variable placeholders
- Stored on OneDrive via KI SharePoint, 34,000+ AutoSave revisions
- Contains VBA (vbaProject.bin)
The problem:
Every time I open the file — on Excel for Mac or Excel Online — the repair engine triggers and strips ALL formulas from every sheet, replacing them with cached values. The file shrinks from ~46.5 MB to ~26 MB. Clicking "No" on the repair dialog just closes the file. There is no way to bypass the repair.
What I've verified:
- Extracted the .xlsm as a ZIP and confirmed all formulas (<f> tags) are fully intact in the raw XML
- Libr€Office Calc can read the formulas but cannot execute them (Err:508 — no LET/LAMBDA support)
- Removed 158 broken named ranges (#REF! and #NAME? entries) from workbook.xml and rebuilt the archive — repair engine still strips all formulas
- The issue reproduces on every OneDrive version history copy (up until I largely used LET formulas in my sheets - but there is still 1,5months of changes lost)
- The issue reproduces on both Excel for Mac and Excel Online
Suspected cause:
The March 10, 2026 security update (KB5002849) patched CVE-2026-26108, a heap overflow in Excel's file parsing during loading. The same patch was applied to Office Online Server (KB5002846). I believe the tightened parsing now rejects or flags my file's large XML structures as potentially malicious, triggering the repair engine to strip all formulas. This is consistent with:
- The known _xlfn. namespace bug on Excel for Mac (reported by multiple users on Microsoft Q&A since late 2024)
- The timing - the file was working before this update flawlessly up until March 16th
- The fact that Excel Online is also affected (same server-side patch)
My questions to the community:
1. Has anyone else experienced formula stripping on large workbooks after the March 2026 update?
2. Is there a way to bypass the repair engine on Mac, or roll back the specific security patch without downgrading all of Office?
3. Would opening this file on Windows Excel (pre-patch or current) preserve the formulas? If anyone with a Windows PC would be willing to try opening and re-saving this file, I would be incredibly grateful.
4. Is there now effectively a size/complexity ceiling for Excel workbooks that makes models like this unviable? If so - should I be migrating this to another environment (R, Python, etc.) going forward?
This file represents six months of thesis work. The formulas are all there in the XML. I just need Excel to stop destroying them on open.
Any help, pointers, or similar experiences would be hugely appreciated.
Thank you,
Florian Boschek
4 Replies
3. Would opening this file on Windows Excel (pre-patch or current) preserve the formulas? If anyone with a Windows PC would be willing to try opening and re-saving this file, I would be incredibly grateful.
Yes, this is worth a try. If you can open the file without a repair notice, you might be home free.
My recommendation:
- Open the file in Safe mode on a PC
- File | Save As to create a new workbook with a different name
- Test on the Mac
The reasoning:
I work out of an AFE module daily for hours. I've gotten pretty good at finding ways to bonk Excel through stress testing. One thing Excel does not like doing is repeatedly calculating large spill arrays and re-sizing large spill areas. It's one way to get the workbook stuck where it doesn't want to even do the simplest calculations. I don't know if your workbook does this but it's worth a try. It's my understanding Excel will re-constitute the calculation chain when a workbook is re-named.
Have you tried reporting the problem back to Microsoft?
They certainly should be made aware of issues caused by a security update (assuming your diagnosis is correct) and may be able to help. The only time I have had to switch to .xlsb format is when I exceeded an 8k limit on the number of characters forming a single formula, so not that relevant.
Honestly, this doesn’t look like file corruption. It feels more like something changed in Excel after that March 2026 update.
If your formulas are still visible in the XML (<f> tags), then Excel is probably removing them during the repair process on purpose likely because of stricter security rules.
A couple of things you can try:
- Open the file on a different system (preferably Windows if you're on Mac)
- Download it and open it offline instead of OneDrive/Excel Online
- Try an older Excel version if you have access
- If the file is very large/complex, try simplifying it a bit (remove unused stuff, split sheets, etc.)
This update seems to have tightened how Excel handles big or complex .xlsm files, so it might be hitting some internal limit or flagging something as unsafe.
If nothing works, you might have to rebuild parts of it since the formulas are technically still there in the XML.
Hellodjflobo
Oh Boy I sure do not want to be in your shoes! But my suggestion is to switch to the .xlsb binary format of Excel. It seems to be better for large files. Not that it will not crash and ruin your day, but maybe less often.
I have seen plenty of great examples of Excel VBA code to backup your workbooks, and some are simply FANTASTIC!
I suggest you start rebuilding the workbook from a clean up, and then Utilize the .xlsb file format with the VBA code that would automatically make a backup as you open the workbook. Some code exmples out there also prompt you to delete old versions of the workbook.
And yes, one of the options in recovery is to recover the data by fixing all the formulas to their values.
Try safe mode and see if the Repair process does not trigger.
and yes Excel for Windows and the Mac should be identical in terms of functionality. Try an unpatched version and see if you can make 25 backups 🤪 but try the local library that may have PCs for the public to use. Here in the USA, most public libraries have computers for the public to use.
I’m don’t have any information about size. But it’s been an issue from Excel’s day one! The XML format did help, but a little…
As you are searching for the backup code, also search for the code that would list all the Conditional Formatting rules and see if you have duplicate rules. Sometimes Excel makes these duplicate rules when you copy/paste data. Cleaning these will make your workbook smaller and perform better.
several years back, I started a project to rebuild a workbook and I got a lot done, but then abandoned this project for some reason or another. Maybe I’ll dig up the code and try again.
Keep us posted as to your progress.
