Description

PHPSpreadsheet: XLS/OLE sector-chain self-loop causes memory exhaustion

Summary

PhpSpreadsheet'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.

This 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.

Vulnerability details

OLERead::read() loads the input and builds sector chains from attacker-controlled OLE header and allocation-table values:

src/PhpSpreadsheet/Shared/OLERead.php:82 reads the entire file after validating only the OLE magic.

src/PhpSpreadsheet/Shared/OLERead.php:84-97 reads sector-chain metadata from the file header.

src/PhpSpreadsheet/Shared/OLERead.php:132-146 builds bigBlockChain and then follows the small-block depot chain.

The vulnerable loop is:

$sbdBlock = $this->sbdStartBlock;
$this->smallBlockChain = '';
while ($sbdBlock != -2) {
    $pos = ($sbdBlock + 1) * self::BIG_BLOCK_SIZE;

    $this->smallBlockChain .= substr($this->data, $pos, 4 * $bbs);
    $pos += 4 * $bbs;
...
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There 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.

The issue is reachable during normal reader detection/loading:

src/PhpSpreadsheet/Reader/XlsBase.php:153-165 calls OLERead::read() from canRead().

src/PhpSpreadsheet/Reader/Xls.php:376-383 calls OLERead::read() from loadOLE().

src/PhpSpreadsheet/IOFactory.php:181-213 calls canRead() while creating a reader for a file, so automatic format detection can trigger the issue.

Similar unbounded sector-chain walks exist later in stream reading:

src/PhpSpreadsheet/Shared/OLERead.php:175-180

src/PhpSpreadsheet/Shared/OLERead.php:198-202

src/PhpSpreadsheet/Shared/OLERead.php:218-222

The proof of concept below confirms the small-block depot chain loop; the same remediation pattern should be applied to all sector-chain walks.

Impact

A 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.

The issue occurs before the file is recognized as a valid workbook stream, so even detection/probing paths are affected.

Safe local proof of concept

This proof of concept uses only Docker with --network none; it creates the malformed OLE file inside the container and does not contact external infrastructure.

docker run --rm --network none -i \
  -v /home/sondt23/Github/CVE/ares/github-repo/PhpSpreadsheet:/app \
  -w /app ghcr.io/typo3/core-testing-php82:1.15 sh <<'SH'
set -eu
php -r '
$data = str_repeat("\0", 1024);
$set = function (int $off, string $bytes) use (&$data): void { $data = substr_replace($data, $bytes, $off, strlen($bytes)); };
$set(0, hex2bin("D0CF11E0A1B11AE1"));...
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Observed output:

ole_size=1024
PHP Fatal error:  Allowed memory size of 67108864 bytes exhausted (tried to allocate 48234528 bytes) in /app/src/PhpSpreadsheet/Shared/OLERead.php on line 143
PHP Stack trace:
PHP   1. {main}() Command line code:0
PHP   2. PhpOffice\PhpSpreadsheet\Reader\XlsBase->canRead($filename = '/tmp/phpspreadsheet-ole-selfloop.xls') Command line code:4
PHP   3. PhpOffice\PhpSpreadsheet\Shared\OLERead->read($filename = '/tmp/phpspreadsheet-ole-selfloop.xls') /app/src/PhpSpreadsheet/Reader/XlsBase.php:164

Suggested remediation

Validate every OLE sector-chain walk with:

a visited-sector set to reject cycles;

maximum chain length based on file size and sector size;

bounds checks before reading from $this->data, $this->bigBlockChain, or $this->smallBlockChain;

rejection of negative sector IDs other than the documented end-of-chain marker.

Replace fatal memory exhaustion with a recoverable Reader\Exception for malformed OLE chains.

Apply the same guarded chain-walk helper to:

small-block depot chain construction;

small-block stream extraction;

big-block stream extraction;

readData().

Add regression tests with self-looping and out-of-range SAT/SSAT chains.

Mitigation

Aliases

References