archive: fix zip subdirectory root matching sibling directories GHSA-66hp-wgxq-6f5q

When a zip archive was mounted at a subdirectory root, readZip used a bare
strings.HasPrefix to decide which entries fell inside the root. This
matched on a raw string prefix rather than a path boundary, so mounting
root "foo" also exposed sibling entries such as "foobar/..." with their
names left uncorrected.

Require a path boundary when filtering by root.

backend/archive/zip/zip.go

@@ -146,8 +146,10 @@ func (f *Fs) readZip() (singleObject bool, err error) {
 		}
 		remote = path.Join(f.prefix, remote)
 		if f.root != "" {
-			// Ignore all files outside the root
-			if !strings.HasPrefix(remote, f.root) {
+			// Ignore all files outside the root, requiring a path
+			// boundary so that root "foo" does not also match a
+			// sibling entry such as "foobar"
+			if remote != f.root && !strings.HasPrefix(remote, f.root+"/") {
 				continue
 			}
 			if remote == f.root {

backend/archive/zip/zip_internal_test.go

@@ -83,3 +83,24 @@ func TestReadZipSlip(t *testing.T) {
 	}
 	assert.Equal(t, []string{"good.txt"}, remotes)
 }
+
+// Mounting with a non-empty root must only expose entries within that
+// root directory, not sibling directories that merely share a name
+// prefix (root "foo" must not match "foobar").
+func TestReadZipRootBoundary(t *testing.T) {
+	ctx := context.Background()
+	dir := t.TempDir()
+	name := writeZip(t, dir, "test.zip",
+		"foo/a.txt",
+		"foobar/b.txt",
+	)
+
+	localFs, err := cache.Get(ctx, dir)
+	require.NoError(t, err)
+
+	f, err := New(ctx, localFs, name, "", "foo")
+	require.NoError(t, err)
+
+	remotes := allRemotes(t, f)
+	assert.Equal(t, []string{"a.txt"}, remotes)
+}