PHPSpreadsheet: Gnumeric reader unbounded gzip expansion causes memory exhaustion

Summary

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.

Impact

A small .gnumeric upload can crash a PHP worker during spreadsheet type detection or import. This can cause denial of service in web applications, queue workers, preview services, document converters, or any service that runs PhpSpreadsheet against untrusted spreadsheet files.

In the local reproduction below, a 97,811-byte file expands to about 96 MiB and crashes Gnumeric::canRead() under memory_limit=64M at Reader/Gnumeric.php:195.