Cross-Site Scripting (XSS) in phpoffice/phpspreadsheet (CVE-2026-40296)
Summary
Cross-site scripting in phpoffice/phpspreadsheet (CVE-2026-40296). Risk of unauthorized operations or information disclosure. Exploitable via General. Mitigation: upgrade to 1.30.4 or later.
Original description
The description as published by the source.
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():
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
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', '', DataType::TYPE_STRING);
$sheet->getStyle('A1')->getNumberFormat()->setFormatCode('. @');
$writer = new Html($spreadsheet);
$writer->save('output.html');
The generated HTML contains:
.
The XSS payload is completely unescaped.
Tested Bypass Formats
| Format Code | Result | Escaped? |
|---|---|---|
General (default) |
Original value | YES (safe) |
. @ |
. + value |
NO (XSS!) |
@ (trailing… |
Source: GitHub Security Advisories (Global) NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-40296 has been found in phpoffice/phpspreadsheet.
Attackers can target a specific entry point like General over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 5.4/10.
What to do: upgrade phpoffice/phpspreadsheet to 1.30.4 or later.
If unsure, ask your IT team or search "phpoffice/phpspreadsheet CVE-2026-40296" on the vendor's site.
CVE-2026-40296 (phpoffice/phpspreadsheet) — CWE-79 / CVSS v3 5.4
Attack vector: remote (network-reachable)
Attack surface: General
Patched: 1.30.4 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
Cross-site scripting (XSS) (CWE-79) exists in phpoffice/phpspreadsheet. Attackers reach the vulnerable code path via General.
Check the vendor advisory for the affected versions of phpoffice/phpspreadsheet. Attack surface: General.
Severity: Medium (CVSS 5.4/10). Risk of unauthorized operations or information disclosure.
Update to 1.30.4.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'phpoffice/phpspreadsheet' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
-
1Identify exposure identify
grep -r 'phpoffice/phpspreadsheet' . | grep -v node_modulesリポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で
phpoffice/phpspreadsheetを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade phpoffice/phpspreadsheet to 1.30.4ステージング環境で 1.30.4 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。
-
3Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
Affected packages
References
- advisory nvd.nist.gov
- package github.com
- web github.com
- web github.com