How severe is CVE-2026-35453?

No CVSS score has been assigned to CVE-2026-35453 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-35453?

CVE-2026-35453 affects the following packages: phpoffice/phpspreadsheet (Packagist), phpoffice/phpspreadsheet (Packagist), phpoffice/phpspreadsheet (Packagist), phpoffice/phpspreadsheet (Packagist), phpoffice/phpspreadsheet (Packagist). Ecosystems affected: Packagist.

How do I fix CVE-2026-35453?

Update phpoffice/phpspreadsheet to 5.7.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-35453 is resolved across your whole dependency graph.

How do I detect CVE-2026-35453 in my Packagist dependencies?

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.

How do I mitigate CVE-2026-35453 if there is no patch (or I can't update yet)?

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 does O3 Security protect against CVE-2026-35453?

O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-35453 can be triaged on real exposure rather than presence alone.

Is CVE-2026-35453 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-35453 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-35453?

CVE-2026-35453 has an EPSS (Exploit Prediction Scoring System) score of 0.2%, placing it in the 10th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-35453?

CVE-2026-35453 is classified as Cross-site Scripting (XSS) (CWE-79). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-35453 published, and has it been updated?

CVE-2026-35453 was published on May 5, 2026 and was last updated on August 12, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2026-35453: phpoffice/phpspreadsheet

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-2026-35453.

EPSS Exploitation Probability

via FIRST.org ↗

0.2%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs10th percentile — riskier than 10% of all scored CVEsHighest risk

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.

Real-World Exposure

5 pkgs affected

🐘phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheet🐘phpoffice/phpspreadsheet

Real-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 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 Packages

5 total 5 fixed

Ecosystem	Package	Vulnerable range	Fix
🐘Packagist	`phpoffice/phpspreadsheet`	≥ 4.0.0&&< 5.7.0	5.7.0`composer require phpoffice/phpspreadsheet:^5.7.0`
🐘Packagist	`phpoffice/phpspreadsheet`	≥ 3.3.0&&< 3.10.5	3.10.5`composer require phpoffice/phpspreadsheet:^3.10.5`
🐘Packagist	`phpoffice/phpspreadsheet`	≥ 2.2.0&&< 2.4.5	2.4.5`composer require phpoffice/phpspreadsheet:^2.4.5`
🐘Packagist	`phpoffice/phpspreadsheet`	≥ 2.0.0&&< 2.1.16	2.1.16`composer require phpoffice/phpspreadsheet:^2.1.16`
🐘Packagist	`phpoffice/phpspreadsheet`	all versions	1.30.4`composer require phpoffice/phpspreadsheet:^1.30.4`

Detection & mitigation playbook

Open-source dependency

Detect
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-2026-35453 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-2026-35453 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-35453. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

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 re

O3 Security · Impact-Aware SCA

Is CVE-2026-35453 in your dependencies?

O3 Security finds CVE-2026-35453 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works

CVE-2026-35453 — phpoffice/phpspreadsheet