CVE-2026-40296 — phpoffice/phpspreadsheet
MEDIUMCVE-2026-40296 is a medium-severity (CVSS 5.4) Cross-site Scripting (XSS) vulnerability in phpoffice/phpspreadsheet. A fix is available for phpoffice/phpspreadsheet — see the affected versions and patch details below.
PhpSpreadsheet vulnerable to XSS in HTML writer via custom number format codes
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-40296.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-40296 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 376,715 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheetReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
PhpSpreadsheet is a pure PHP library for reading and writing spreadsheet files. The HTML writer skips htmlspecialchars escaping when a cell's formatted value differs from the original value. When a cell has a custom number format containing the text placeholder @ along with any additional literal characters (for example ". @", "@ ", or "x@"), the formatter replaces @ with the cell value and adds the extra characters, causing the formatted value to differ from the original and bypassing HTML escaping entirely. An attacker who can control the cell value and number format of an uploaded spreadsheet that is later converted to HTML and displayed to other users can achieve stored cross-site scripting. This issue is fixed in versions 5.7.0, 3.10.5, 2.4.5, 2.1.16, and 1.30.4.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | phpoffice/phpspreadsheet | ≥ 4.0.0&&< 5.7.0 | 5.7.0composer require phpoffice/phpspreadsheet:^5.7.0 |
| 🐘Packagist | phpoffice/phpspreadsheet | ≥ 3.3.0&&< 3.10.5 | 3.10.5composer require phpoffice/phpspreadsheet:^3.10.5 |
| 🐘Packagist | phpoffice/phpspreadsheet | ≥ 2.2.0&&< 2.4.5 | 2.4.5composer require phpoffice/phpspreadsheet:^2.4.5 |
| 🐘Packagist | phpoffice/phpspreadsheet | ≥ 2.0.0&&< 2.1.16 | 2.1.16composer require phpoffice/phpspreadsheet:^2.1.16 |
| 🐘Packagist | phpoffice/phpspreadsheet | all versions | 1.30.4composer require phpoffice/phpspreadsheet:^1.30.4 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for phpoffice/phpspreadsheet, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update phpoffice/phpspreadsheet to 5.7.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40296 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-40296 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-40296. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-40296 in your dependencies?
O3 Security finds CVE-2026-40296 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.