Path traversal lab: Serve an uploaded file without path traversal

Stripping ".." misses encodings, symlinks and absolute paths. Let os.path.realpath compute the real target, then check where it landed.

os.path.commonpath([base, target]) == base is true only when target is inside base. realpath both paths first so a symlink cannot hide the destination.

import os


def read_upload(base_dir, name):
    base = os.path.realpath(base_dir)
    target = os.path.realpath(os.path.join(base, name))
    if base != target and os.path.commonpath([base, target]) != base:
        raise ValueError("path escapes the upload directory")
    with open(target, "r", encoding="utf-8") as handle:
        return handle.read()

notes.txt and sub/data.txt resolve to paths inside the upload directory and return their contents. "../secret.txt" resolves to the secret one level up, so commonpath is the parent, not base, and the function raises; an absolute path such as /etc/hostname does too, because os.path.join drops base when the second argument is absolute. The symlink inside the directory resolves through realpath to the secret outside, so it is refused even though its own name looks ordinary. A file named upload beside the uploads directory is refused too: its path is a prefix of the directory’s as text, but commonpath compares whole components. base_dir given with a trailing separator still reads notes.txt, because realpath normalizes it as well. A name for a file that does not exist raises as well, so nothing is read. Dotless, well-behaved names keep working, because the fix checks the resolved location rather than editing the name.

Compare it with your own fix, then rerun the tests. The editor is not changed when you open this explanation.

The browser and the lab download run the same cases, including cases that catch common wrong fixes, such as comparing with one fixture value instead of the real rule. The download includes the complete test adapter and cryptographic fixtures.