Excelize ColumnNameToNumber: int64 overflow yields an out-of-domain coordinate with nil error, causing negative slice index panic on r="0" rows

Summary

ColumnNameToNumber (lib.go:220-237) accumulates the bijective base-26 value of a column name in an int64 with only an upper-bound check (col > MaxColumns) applied after the loop and no overflow detection. A 14-letter column name whose true value is 3·2⁶⁴ — e.g. VGWQHXLSDVIKWV — wraps to col = 0 and passes the guard, so CellNameToCoordinates("VGWQHXLSDVIKWV1") returns (col=0, row=1, err=nil).

When normalizing a r="0" row, checkSheetR0 (excelize.go:417-443, called from checkSheet at excelize.go:405) runs its checkRow closure with col = 0: colIdx := col - 1 becomes -1, and sheetData.Row[rowIdx].C[-1] (excelize.go:424) raises an unrecovered panic: runtime error: index out of range [-1], killing the host process.

Details

• The row side of the same coordinate gate is enforced (checkRowNum at excelize.go:342-350 bounds r before the make([]xlsxRow, row) allocation; this includes the fix for GHSA-h69g-9hx6-f3v4 / CVE-2026-54063, present in the audited commit). The column side is not: checkSheet/lastRowNum/checkSheetR0 treat err == nil from CellNameToCoordinates as proof of an in-domain coordinate (excelize.go:356, :438), which is unsound because of the wrap-around above.
• The same unsound gate also feeds ws.SheetData.Row[rowIdx].C[colNum-1] in xlsxWorksheet.checkRow (rows.go:969): a row combining a valid large column (e.g. XFD1) with an overflowed column panics identically.
• Reachable from any workSheetReader-based API on an attacker-supplied worksheet: GetCellValue, GetCellFormula, SetCellValue, GetMergeCells, GetSheetDimension, GetColWidth, AddTable, etc.
• Probe on pristine master: ColumnNameToNumber("VGWQHXLSDVIKWV") returns (0, nil).
• This is a distinct root cause from GHSA-h69g-9hx6-f3v4 (row-index allocation): different mechanism (int64 wrap-around → negative index, not oversized allocation), different sink, different fix.

PoC

A standalone program (public API only) was provided to the maintainer by email (3-column-overflow): it builds a workbook in memory whose xl/worksheets/sheet1.xml contains <sheetData><row r="0"><c r="VGWQHXLSDVIKWV1" t="inlineStr"><is><t>pwn</t></is></c></row></sheetData> (<1 KB of attacker XML), calls OpenReader, then GetCellValue("Sheet1", "A1") → PANIC_REPRODUCED: runtime error: index out of range [-1] on master ecd99d761fe0 (2026-09-08). With the proposed patch the same program prints NO_PANIC_BLOCKED.

Impact

A <1 KB crafted .xlsx crashes any service that opens a user-supplied spreadsheet and reads it — upload processing, mail-scanning pipelines, spreadsheet conversion endpoints. Remote, unauthenticated, no privileges.

Proposed fix

Bound the accumulated value inside the loop: check col > MaxColumns after each digit. Every digit is at least 1, so any name whose true value exceeds MaxColumns crosses the bound inside the loop, before the accumulation can wrap or overflow — this provably covers all cases, including wraps that would land back inside [1, MaxColumns]. A complete patch has been provided to the maintainer.