Sample ZIP file (valid flat)

Handle it in code

Each snippet below was run against this exact file with Python 3.14.4 and Node 24.15.0 when this page was built, and the output shown is what it printed.

Python

Check every entry before extracting anything. Python's extractall already strips "../" and leading slashes, but silently, so the file lands somewhere the archive did not say. Rejecting makes the attack visible and matches what other languages need.

import os, zipfile

def safe_extract(archive, dest):
    dest = os.path.realpath(dest)
    with zipfile.ZipFile(archive) as z:
        for info in z.infolist():
            target = os.path.realpath(os.path.join(dest, info.filename))
            if os.path.commonpath([dest, target]) != dest:
                raise ValueError(f"blocked: {info.filename!r} escapes the target folder")
        z.extractall(dest)

try:
    safe_extract("valid-flat.zip", "out")
    print("extracted")
except ValueError as e:
    print(e)

Run against this file, it prints: extracted