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Is Your Excel Broken After the Latest Update? The Costly Impact of [ ] in Filenames
Following the Microsoft Office security updates released on May 13, 2025, users have reported a significant issue: Excel can no longer open files if their filenames begin with square brackets [ ] or contain them in certain positions. This affects Office 2016, Office 2016-2021 C2R, Office 2021, and Microsoft 365.
What Changed?
After installing the May 2025 update, attempting to open Excel files with names like [Test]Report.xlsx results in errors.
Excel truncates the filename at the bracket, searching only for the portion after the bracket, which leads to the file not being found or opened.
This problem did not occur in previous Office versions and is specific to the latest update. Older versions and other spreadsheet software (like LibreOffice) do not have this issue.
Why Are Square Brackets a Problem?
Square brackets [ ] are reserved characters in Excel, used internally for workbook references in formulas (e.g., [WorkbookName]Sheet1!A1).
Including these characters in file names confuses Excel’s file handling logic, especially after the recent update that appears to enforce stricter parsing rules.
Microsoft documentation now explicitly warns against using [ ], along with other special characters, in Excel filenames and folder paths.
Real-World Impact
Organizations with legacy systems or automated processes that generate files with brackets in their names are particularly affected.
Users report that double-clicking such files or opening them from the jump list fails, while opening them via Excel’s File > Open dialog sometimes works but is not reliable.
This can disrupt workflows, especially in businesses relying on automated file generation or strict naming conventions.
Security Risks
There is a potential security risk: if two files exist, such as [Test]anyFileName.xlsx and Test.xlsx, Excel may open Test.xlsx when the user intends to open the bracketed file, leading to confusion or possible exploitation if an attacker manipulates filenames to disguise malicious files.
Hidden files with similar names could be leveraged to trick users, especially if macros or other embedded content are present.
Workarounds
Rename Files
Use File Explorer to remove square brackets from filenames before attempting to open them in Excel.
Open via Excel Dialog
Sometimes, opening the file from within Excel (File > Open) bypasses the issue, but this is not guaranteed and not recommended as a permanent fix.
Update Naming Conventions
Adjust automated systems and scripts to avoid generating filenames with [ ].
Key Takeaways
- Avoid using [ ], as well as other special characters like ?, <, >, :, |, and *, in Excel filenames and folder paths.
- This change is likely permanent, as Microsoft’s documentation and user reports confirm the restriction is now actively enforced.
- For organizations with legacy systems, urgent updates to file naming conventions are necessary to prevent workflow disruptions.
Next Steps
- Review and update file naming practices to ensure compatibility with the latest Excel versions.
- Communicate this change to all users and IT teams to prevent future file access issues.
- Monitor Microsoft’s official support channels for any updates or patches addressing this behavior.
This issue highlights the importance of adhering to best practices for file naming in business-critical applications, ensuring smooth and secure operations with every update.