libxslt: Fix use-after-free in xsltApplyTemplates

Fixes: CVE-2021-30560

pkgs/development/libraries/libxslt/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl
+{ lib, stdenv, fetchurl, fetchpatch
 , pkg-config
 , libxml2, findXMLCatalogs, gettext, python, libgcrypt
 , cryptoSupport ? false
@@ -14,6 +14,15 @@ stdenv.mkDerivation rec {
     sha256 = "0zrzz6kjdyavspzik6fbkpvfpbd25r2qg6py5nnjaabrsr3bvccq";
   };
 
+  patches = [
+    (fetchpatch {
+      # Fixes use-after-free in xsltApplyTemplates
+      name = "CVE-2021-30560.patch";
+      url = "https://gitlab.gnome.org/GNOME/libxslt/-/commit/50f9c9cd3b7dfe9b3c8c795247752d1fdcadcac8.patch";
+      hash = "sha256-XJD9SBo8xzztQQ6g13h4IzID7HV7u3xWSQdb2rVCJBQ=";
+    })
+  ];
+
   outputs = [ "bin" "dev" "out" "man" "doc" ] ++ lib.optional pythonSupport "py";
 
   nativeBuildInputs = [