Medium [CVE-2026-39244] Denial of Service via crafted ZIP file leading to excessive memory allocation

adm-zip before 0.5.18 is vulnerable to denial of service via a crafted ZIP file with a manipulated uncompressed size header field.

In zipEntry.js line 103, Buffer.alloc(_centralHeader.size) allocates memory based on the declared uncompressed size from the ZIP central directory header without validating it against the actual compressed data size or imposing any upper bound. The size value is read directly from the binary header at entryHeader.js line 266 with no bounds check.

An attacker can craft a ~120-byte ZIP file that declares ~4GB uncompressed size, causing a memory allocation amplification ratio of over 33 million to 1. The allocation occurs before CRC validation, so the malicious payload cannot be rejected early.

All extraction and read methods are affected: readFile(), readAsText(), extractEntryTo(), extractAllTo(), extractAllToAsync(), test(), and entry.getData(). Any application accepting untrusted ZIP files via adm-zip is vulnerable to immediate process crash.

A remote attacker can exploit this vulnerability by providing a specially crafted ZIP file. The file's header can be manipulated to declare an extremely large uncompressed size, causing the application to allocate an excessive amount of memory.

A denial of service flaw exists in adm-zip, affecting applications that process untrusted ZIP archives.