diff --git a/lib/rest/url.go b/lib/rest/url.go
index 3e6b7188d..4de634ad1 100644
--- a/lib/rest/url.go
+++ b/lib/rest/url.go
@@ -9,6 +9,10 @@ import (
 // URLJoin joins a URL and a path returning a new URL
 //
 // path should be URL escaped
+//
+// path is resolved as a URL reference so an absolute URL or a
+// network-path reference ("//host/path") replaces the scheme or host
+// of base. Use URLJoinRoot for a path which must stay on base.
 func URLJoin(base *url.URL, path string) (*url.URL, error) {
 	rel, err := url.Parse(path)
 	if err != nil {
@@ -17,6 +21,22 @@ func URLJoin(base *url.URL, path string) (*url.URL, error) {
 	return base.ResolveReference(rel), nil
 }
 
+// URLJoinRoot joins root, an unescaped path, onto base returning a new URL.
+//
+// root may only change the path of base. A root which carries a
+// scheme, host or userinfo of its own, such as the network-path
+// reference "//host/path", returns an error.
+func URLJoinRoot(base *url.URL, root string) (*url.URL, error) {
+	rel, err := url.Parse(URLPathEscape(root))
+	if err != nil {
+		return nil, fmt.Errorf("error parsing %q as URL: %w", root, err)
+	}
+	if rel.Scheme != "" || rel.Opaque != "" || rel.User != nil || rel.Host != "" {
+		return nil, fmt.Errorf("root %q must not change the scheme, host or user of %q", root, base.Redacted())
+	}
+	return base.ResolveReference(rel), nil
+}
+
 // URLPathEscape escapes URL path the in string using URL escaping rules
 //
 // This mimics url.PathEscape which only available from go 1.8
diff --git a/lib/rest/url_test.go b/lib/rest/url_test.go
index df3519c13..aad664007 100644
--- a/lib/rest/url_test.go
+++ b/lib/rest/url_test.go
@@ -44,6 +44,46 @@ func TestURLJoin(t *testing.T) {
 	}
 }
 
+func TestURLJoinRoot(t *testing.T) {
+	for i, test := range []struct {
+		base   string
+		root   string
+		wantOK bool
+		want   string
+	}{
+		{"http://example.com/dir/", "", true, "http://example.com/dir/"},
+		{"http://example.com/dir/", "potato", true, "http://example.com/dir/potato"},
+		{"http://example.com/dir/", "sub/potato/", true, "http://example.com/dir/sub/potato/"},
+		{"http://example.com/dir/", "/potato", true, "http://example.com/potato"},
+		{"http://example.com/dir/", "../potato", true, "http://example.com/potato"},
+		{"http://example.com/dir/", "With colon :", true, "http://example.com/dir/With%20colon%20:"},
+		{"http://example.com/dir/", "http://example.com/dir/", true, "http://example.com/dir/http://example.com/dir/"},
+		{"http://user:pass@example.com/dir/", "potato", true, "http://user:pass@example.com/dir/potato"},
+		// network-path references are rejected, even for the same host
+		{"http://example.com/dir/", "//evil.example/", false, ""},
+		{"http://example.com/dir/", "//evil.example", false, ""},
+		{"http://example.com/dir/", "//evil.example/potato", false, ""},
+		{"http://example.com/dir/", "//example.com/dir/", false, ""},
+		{"http://example.com/dir/", "//example.com:8080/dir/", false, ""},
+		{"http://example.com/dir/", "//user:pass@example.com/dir/", false, ""},
+		{"http://user:pass@example.com/dir/", "//other:pass@example.com/dir/", false, ""},
+		{"http://user:pass@example.com/dir/", "//example.com/dir/", false, ""},
+		{"http://example.com/dir/", "//[::1]/potato", false, ""},
+	} {
+		u, err := url.Parse(test.base)
+		require.NoError(t, err)
+		got, err := URLJoinRoot(u, test.root)
+		gotOK := err == nil
+		what := fmt.Sprintf("test %d base=%q, root=%q", i, test.base, test.root)
+		assert.Equal(t, test.wantOK, gotOK, what)
+		var gotString string
+		if gotOK {
+			gotString = got.String()
+		}
+		assert.Equal(t, test.want, gotString, what)
+	}
+}
+
 func TestURLPathEscape(t *testing.T) {
 	for i, test := range []struct {
 		path string