XSS via unescaped custom number format substitution
Published May 5, 2026 · Updated May 5, 2026
Cross-site scripting in the PHPOffice PhpSpreadsheet HTML Writer allows attackers to inject script via crafted spreadsheet cells. Html.php escapes formatted cell data only when it strictly equals the original value, while NumberFormat replaces the @ placeholder with raw cell content and returns before the escaping callback. The attacker must control cell data and its custom format, and the generated HTML must be served or opened in a browser, where the script runs in that document's context.
Summary
What happened
Cross-site scripting in the PHPOffice PhpSpreadsheet HTML Writer allows attackers to inject script via crafted spreadsheet cells. Html.php escapes formatted cell data only when it strictly equals the original value, while NumberFormat replaces the @ placeholder with raw cell content and returns before the escaping callback. The attacker must control cell data and its custom format, and the generated HTML must be served or opened in a browser, where the script runs in that document's context.
The record
- CVE
- CVE-2026-35453
- Published
- May 5, 2026
- Updated
- May 5, 2026
- Vendor
- PHPOffice
- Product
- PhpSpreadsheet
- Classifications
- CWE-80, CWE-79, T1203
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- May 5, 2026CVE publishedPublication date reported by the CVE source.
- May 5, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=<= 1.30.3
- Affected versionversion=>= 2.0.0, <= 2.1.15
- Affected versionversion=>= 2.2.0, <= 2.4.4
- Affected versionversion=>= 3.3.0, <= 3.10.4
- Affected versionversion=>= 4.0.0, <= 5.6.0
What conditions does exploitation require?
What is affected?
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Upstream HTML Writer proof of conceptproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
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Your stack
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