fix(lib): mend case where domainless redirects could allow cross-domain

Ref: AWOO-009
Signed-off-by: Xe Iaso <me@xeiaso.net>

docs/docs/CHANGELOG.md

@@ -33,6 +33,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Limit naive honeypot r9k delay to one second.
 - Fix an obscure case where adding query values to a subrequest match could cause an invalid rule match when using path based matching for protected resources.
 - Fix an edge case where load average expression values could nil pointer dereference when Anubis just started up.
+- Fix an obscure case where Anubis in subrequest mode could allow redirects to invalid domains with strange instructions.
 - Fix `path_regex` and CEL `path` rules not matching when using Traefik `forwardAuth` middleware. Anubis now checks `X-Forwarded-Uri` (Traefik) in addition to `X-Original-URI` (nginx) when resolving the request path in subrequest mode ([#1628](https://github.com/TecharoHQ/anubis/issues/1628)).
 
 ## v1.25.0: Necron

lib/http.go

@@ -403,14 +403,15 @@ func (s *Server) ServeHTTPNext(w http.ResponseWriter, r *http.Request) {
 		localizer := localization.GetLocalizer(r)
 
 		redir := r.FormValue("redir")
-		urlParsed, err := url.ParseRequestURI(redir)
-		if err != nil {
-			// if ParseRequestURI fails, try as relative URL
-			urlParsed, err = r.URL.Parse(redir)
+		urlParsed, err := url.Parse(redir)
 		if err != nil {
 			s.respondWithStatus(w, r, localizer.T("redirect_not_parseable"), makeCode(err), http.StatusBadRequest)
 			return
 		}
+
+		if urlParsed.Opaque != "" || (urlParsed.Scheme == "" && strings.HasPrefix(redir, "//")) {
+			s.respondWithStatus(w, r, localizer.T("invalid_redirect"), "", http.StatusBadRequest)
+			return
 		}
 
 		// validate URL scheme to prevent javascript:, data:, file:, tel:, etc.

lib/http_test.go

@@ -223,3 +223,17 @@ func TestNoCacheOnError(t *testing.T) {
 		})
 	}
 }
+
+func TestRejectsHostlessRedirect(t *testing.T) {
+	pol := loadPolicies(t, "testdata/useragent.yaml", 0)
+	srv := spawnAnubis(t, Options{Policy: pol, RedirectDomains: []string{"allowed.example"}})
+	req := httptest.NewRequest(http.MethodGet, "https://anubis.example/.within.website/?redir=%2f%2fevil.example%2fphish", nil)
+	rr := httptest.NewRecorder()
+	srv.ServeHTTPNext(rr, req)
+	if rr.Code != http.StatusBadRequest {
+		t.Fatalf("expected hostless redirect to be rejected, got HTTP %d body %q", rr.Code, rr.Body.String())
+	}
+	if got := rr.Header().Get("Location"); got != "" {
+		t.Fatalf("expected no Location header on rejected redirect, got %q", got)
+	}
+}