PhpSpreadsheet has CPU Denial of Service via Unbounded Row Index in SpreadsheetML XML Reader
Date: 2026-04
Affected: composer/phpoffice/phpspreadsheet, composer/phpoffice/phpspreadsheet, composer/phpoffice/phpspreadsheet
Attack vector: dos
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-40863
CWE: CWE-400
CWE-770
CVSS: 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)

## Summary The SpreadsheetML XML reader (`Reader\Xml`) does not validate the `ss:Index` row attribute against the maximum allowed row count (`AddressRange::MAX_ROW = 1,048,576`). An attacker can craft a SpreadsheetML XML file with `ss:Index="999999999"` on a `<Row>` element, which inflates the internal `cachedHighestRow` to ~1 billion. Any subsequent call to `getRowIterator()` without an explicit end row will attempt to iterate ~1 billion rows, causing CPU exhaustion and denial of service. ## Details In `src/PhpSpreadsheet/Reader/Xml.php`, the `loadSpreadsheetFromFile` method processes `<Row>` elements: ```php // Xml.php:397-402 if (isset($row_ss['Index'])) { $rowID = (int) $row_ss['Index']; // No validation against MAX_ROW } if (isset($row_ss['Hidden'])) { $rowVisible = ((string) $row_ss['Hidden']) !== '1'; $spreadsheet->getActiveSheet()->getRowDimension($rowID)->setVisible($rowVisible); } ``` The `$rowID` value read from `ss:Index` is cast to int with no upper bound check. It is then passed to `getRowDimension()`: ```php // Worksheet.php:1342-1351 public function getRowDimension(int $row): RowDimension { if (!isset($this->rowDimensions[$row])) { $this->rowDimensions[$row] = new RowDimension($row); $this->cachedHighestRow = max($this->cachedHighestRow, $row); } return $this->rowDimensions[$row]; } ``` This inflates `cachedHighestRow` to the attacker-controlled value. Additionally, at line 412, `$cellRange = $columnID . $rowID` i


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