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

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 containin...

PhpSpreadsheet has XSS via number format code with @ text placeholder bypasses htmlspecialchars in HTML writer

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 ===...