CVE-2026-35453: PhpSpreadsheet XSS via number format text substitution in HTML Writer

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: 2026-05-05 · updated 2026-07-24
Weakness: CWE-79

Description

PhpSpreadsheet is a library for reading and writing spreadsheet files. In versions 1.30.3 and earlier, 2.0.0 through 2.1.15, 2.2.0 through 2.4.4, 3.3.0 through 3.10.4, and 4.0.0 through 5.6.0, the HTML Writer skips htmlspecialchars() output escaping when a cell uses a custom number format containing the @ text placeholder with additional literal text (e.g., @ "items"). The escaping is only applied when the formatted output strictly equals the original cell value. When the format code contains @ with quoted literal text, the formatter substitutes the raw cell value into the format string and returns early without invoking the escaping callback. An attacker who can control cell content in a spreadsheet processed by the HTML Writer can inject arbitrary HTML and JavaScript into the generated output. This issue has been fixed in versions 1.30.4, 2.1.16, 2.4.5, 3.10.5, and 5.7.0.

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

Get alerted when CVE-2026-35453 or the next vulnerability affects your stack