Kimai vulnerable to formula Injection via tag names in XLSX export

Summary

Any ROLE_USER can create a tag with a formula string as its name (e.g. =SUM(54+51)) via POST /api/tags and assign it to a timesheet. When an admin exports timesheets to XLSX, ArrayFormatter.formatValue() joins tag names with implode() and returns the result unchanged. OpenSpout promotes any =-prefixed string to a FormulaCell, writing <f>SUM(54+51)</f> into the XLSX archive. Excel evaluates the formula when the file is opened.

Details

1. ArrayFormatter does not sanitize before returning

sanitizeDDE() exists on StringHelper and is called by TextFormatter, but ArrayFormatter never calls it.

// src/Export/Package/CellFormatter/ArrayFormatter.php:24
return implode(', ', $value);  // no sanitizeDDE() call

2. Tag name validation does not block formula trigger characters

The API blocks commas in tag names but permits =, +, -, and @ - all valid formula prefixes in Excel and LibreOffice Calc.

3. OpenSpout silently promotes strings to formula cells

Cell::fromValue("=SUM(54+51)") returns a FormulaCell with no warning.

PoC

1. It logs in as normal user, creates tag =SUM(54+51), assigns it to a timesheet.
2. Admin has to export timesheets to Excel version via /en/export/ endpoint.

<img width="1339" height="700" alt="image" src="https://github.com/user-attachments/assets/884c7943-5e3b-4647-8bcc-e264d6719d66" />

<img width="1304" height="128" alt="formula_injection_tags" src="https://github.com/user-attachments/assets/ef28f2ad-7491-4a15-bb18-1fcd6ff5e55a" />

Impact

• Any ROLE_USER can plant a formula that executes on the workstation of any user who exports and opens timesheet data
• A single malicious tag poisons all future exports across all users and date ranges until the tag is deleted

Fixes

1. Prevent = being part of the tag name (and other fields as well)
2. Use OpenSpout TextCell for everything that is a string