CVE-2026-40296: PhpSpreadsheet vulnerable to XSS in HTML writer via custom number format codes

Severity: medium (CVSS 5.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-05-06 · updated 2026-10-26
Weakness: CWE-79

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 products

PHPOffice PhpSpreadsheet: before 1.30.4; from 2.0.0 before 2.1.16; from 2.2.0 before 2.4.5; from 3.3.0 before 3.10.5; from 4.0.0 before 5.7.0 — fixed in 1.30.4, 2.1.16, 2.4.5, 3.10.5, 5.7.0

How to fix

Upgrade to a fixed version: PHPOffice PhpSpreadsheet 1.30.4, 2.1.16, 2.4.5, 3.10.5. Check the vendor advisory for mitigations.

References

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