CVE-2026-40863

PhpSpreadsheet has CPU Denial of Service via Unbounded Row Index in SpreadsheetML XML Reader

Any PHP application that processes user-uploaded SpreadsheetML XML files using PhpSpreadsheet is vulnerable. An attacker can cause denial of service by:

• Exhausting server CPU with a single small XML file (~300 bytes)
• Blocking the PHP worker process, potentially affecting all concurrent users
• Triggering PHP max_execution_time limits that still consume resources before killing the process

The attack requires no authentication, only the ability to upload or cause the application to process a crafted SpreadsheetML file.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-40863 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.7.0, 3.10.5, 2.4.5, 2.1.16, 1.30.4); upgrading removes the vulnerable code path.

Kodem intelligence

Add MAX_ROW validation after reading the ss:Index attribute in src/PhpSpreadsheet/Reader/Xml.php:

// After line 398:
if (isset($row_ss['Index'])) {
    $rowID = (int) $row_ss['Index'];
    if ($rowID > AddressRange::MAX_ROW) {
        $rowID = AddressRange::MAX_ROW;
    }
}

Add the necessary import at the top of the file:

use PhpOffice\PhpSpreadsheet\Cell\AddressRange;

The same validation should also be applied to the ss:Index attribute on <Cell> elements (line 409) for the column dimension.

1. What is CVE-2026-40863? CVE-2026-40863 is a high-severity uncontrolled resource consumption vulnerability in phpoffice/phpspreadsheet (composer), affecting versions >= 4.0.0, <= 5.6.0. It is fixed in 5.7.0, 3.10.5, 2.4.5, 2.1.16, 1.30.4. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-40863? CVE-2026-40863 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of phpoffice/phpspreadsheet are affected by CVE-2026-40863? phpoffice/phpspreadsheet (composer) versions >= 4.0.0, <= 5.6.0 is affected.
4. Is there a fix for CVE-2026-40863? Yes. CVE-2026-40863 is fixed in 5.7.0, 3.10.5, 2.4.5, 2.1.16, 1.30.4. Upgrade to this version or later.
5. Is CVE-2026-40863 exploitable, and should I be worried? Whether CVE-2026-40863 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-40863 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-40863?
   • Upgrade phpoffice/phpspreadsheet to 5.7.0 or later
   • Upgrade phpoffice/phpspreadsheet to 3.10.5 or later
   • Upgrade phpoffice/phpspreadsheet to 2.4.5 or later
   • Upgrade phpoffice/phpspreadsheet to 2.1.16 or later
   • Upgrade phpoffice/phpspreadsheet to 1.30.4 or later