CVE-2026-40296: Phpoffice/phpspreadsheet Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hrmw-qprp-wgmc",
  "modified" : "2026-05-08T19:32:34Z",
  "published" : "2026-04-28T22:57:13Z",
  "aliases" : [ "CVE-2026-40296" ],
  "summary" : "PhpSpreadsheet has XSS via number format code with @ text placeholder bypasses htmlspecialchars in HTML writer",
  "details" : "It was discovered that there is a way to bypass HTML escaping in the HTML writer using custom number format codes.\n\n## The Problem\n\nIn `Writer/Html.php` around line 1592, the code checks if the formatted cell data equals the original data to decide whether to apply `htmlspecialchars()`:\n\n```php\nif ($cellData === $origData) {\n    $cellData = htmlspecialchars($cellData, ...);\n}\n```\n\nWhen a cell has a custom number format containing `@` (text placeholder) with any additional literal characters, the formatter replaces `@` with the cell value and adds the extra characters. This makes `$cellData !== $origData`, so `htmlspecialchars()` is **skipped entirely**.\n\nEven a single trailing space in the format (`@ `) is enough to bypass the escape.\n\n## Proof of Concept\n\n```php\nuse PhpOffice\\PhpSpreadsheet\\Spreadsheet;\nuse PhpOffice\\PhpSpreadsheet\\Writer\\Html;\nuse PhpOffice\\PhpSpreadsheet\\Cell\\DataType;\n\n$spreadsheet = new Spreadsheet();\n$sheet = $spreadsheet->getActiveSheet();\n\n// XSS payload with malicious number format\n$sheet->setCellValueExplicit('A1', '<img src=x onerror=alert(document.cookie)>', DataType::TYPE_STRING);\n$sheet->getStyle('A1')->getNumberFormat()->setFormatCode('. @');\n\n$writer = new Html($spreadsheet);\n$writer->save('output.html');\n```\n\nThe generated HTML contains:\n```html\n<td>. <img src=x onerror=alert(document.cookie)></td>\n```\n\nThe XSS payload is **completely unescaped**.\n\n## Tested Bypass Formats\n\n| Format Code | Result | Escaped? |\n|---|---|---|\n| `General` (default) | Original value | YES (safe) |\n| `. @` | `. ` + value | **NO (XSS!)** |\n| `@ ` (trailing space) | value + ` ` | **NO (XSS!)** |\n| `x@` | `x` + value | **NO (XSS!)** |\n\nThis was tested with PhpSpreadsheet 4.5.0 and confirmed the XSS executes in the browser.\n\n## Impact\n\nAny application that:\n1. Accepts uploaded XLSX files from users\n2. Converts them to HTML using PhpSpreadsheet's HTML writer\n3. Displays the HTML to other users\n\n...is vulnerable to stored XSS. The attacker embeds the payload in a cell value and sets a custom number format in the XLSX file's `xl/styles.xml`.\n\n## Suggested Fix\n\nAlways apply `htmlspecialchars()` regardless of whether formatting changed the value:\n\n```php\n// Instead of conditional escaping:\n$cellData = htmlspecialchars($cellData, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');\n```\n\nOr escape AFTER formatting, not conditionally based on equality.\n\n## Reporter\nKeyvan Hardani",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "5.7.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 5.6.0"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.3.0"
      }, {
        "fixed" : "3.10.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.10.4"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.2.0"
      }, {
        "fixed" : "2.4.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.4.4"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "2.1.16"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.1.15"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.30.4"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.30.3"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/security/advisories/GHSA-hrmw-qprp-wgmc"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-40296"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-28T22:57:13Z",
    "nvd_published_at" : "2026-05-06T22:16:25Z"
  }
}