rclone archive/zip: Zip Slip via unsanitized zip entry names lets a malicious archive escape its own namespace

Summary

backend/archive mounts a zip file as a browsable, syncable rclone Fs (e.g. rclone lsf :zip:downloaded.zip or rclone copy :zip:downloaded.zip dest:). Go's archive/zip package does not sanitize file.Name - it is taken verbatim from the untrusted zip's central directory. readZip() in backend/archive/zip/zip.go applies path.Clean to the entry name, but this alone cannot fully neutralize a name with more .. components than real segments preceding them (e.g. "../../etc/cron.d/evil" stays exactly as-is after cleaning). When the archive is mounted with an empty root (the common case), there was no check at all that the resulting name stayed inside the archive's own namespace, so it was stored verbatim and returned unchanged by Object.Remote().

fs/sync/fs/operations use srcObj.Remote() directly as the destination-relative path when copying between filesystems, so a maliciously crafted zip file can cause rclone copy/sync to attempt writes outside the intended destination directory on whatever backend it targets - this is the well-known "Zip Slip" vulnerability class (https://security.snyk.io/research/zip-slip-vulnerability) applied to rclone's own zip-mounting backend. It is distinct from cmd/archive/extract, which already validates via its own destPath() choke point and is not affected.

Details

Vulnerable code (before fix), backend/archive/zip/zip.go, (*Fs).readZip:

for _, file := range zr.File {
	remote := strings.Trim(path.Clean(file.Name), "/")
	if remote == "." { remote = "" }
	remote = path.Join(f.prefix, remote)
	if f.root != "" {
		// Ignore all files outside the root
		if !strings.HasPrefix(remote, f.root) { continue }
		...
	}
	...
	o := &Object{f: f, remote: remote, ...}
	dt.Add(o)
}

The escape check only ran when f.root != "", and even then used a bare strings.HasPrefix with no boundary check (so f.root="foo" incorrectly also matched a sibling entry "foobar").

PoC

Built a zip in memory with Go's real archive/zip writer (entry name "../../etc/cron.d/evil", not sanitized by the writer either), wrote it to disk, and mounted it via the actual production constructor zip.New(ctx, localFs, "evil.zip", "", ""):

zip entry Name="../../etc/cron.d/evil" -> Object.Remote()="../../etc/cron.d/evil"

Fully outside the archive's own namespace - confirmed via a regression test that mounts the malicious zip through the real local backend and inspects the resulting Fs's internal dirtree and every Object's Remote().

Impact

A user who runs rclone copy/sync/mount against an untrusted zip file (downloaded, e-mailed, etc.) can have files written outside the intended destination directory on the destination backend, depending on that backend's own confinement. No server compromise or custom remote configuration is required from the attacker - only a crafted zip file and a normal rclone copy/sync invocation by the victim.

Fix

Skip any zip entry whose cleaned+prefixed name still escapes the archive's own namespace, rather than exposing it. Also tightened the pre-existing root filter's weak prefix check.