diff --git a/pkgs/by-name/xm/xmlsec/package.nix b/pkgs/by-name/xm/xmlsec/package.nix
index f1b8f9b472e3..2a2b52ee337a 100644
--- a/pkgs/by-name/xm/xmlsec/package.nix
+++ b/pkgs/by-name/xm/xmlsec/package.nix
@@ -1,6 +1,7 @@
 {
   stdenv,
   fetchurl,
+  fetchpatch,
   libxml2,
   gnutls,
   libxslt,
@@ -33,6 +34,11 @@ lib.fix (
     patches = [
       ./lt_dladdsearchdir.patch
       ./remove_bsd_base64_decode_flag.patch
+      (fetchpatch {
+        # xmlDoc.encoding is no longer const in libxml 2.15, so fetch the fix
+        url = "https://github.com/lsh123/xmlsec/commit/ef0e3b5cac04db13ce070b1e5bcad7dd7b0eb49b.patch?full_index=1";
+        hash = "sha256-Hv8PaJXkXLq++NuCAJ4IvsYBPj8wkN7dBTniYucq18o=";
+      })
     ];
 
     postPatch = ''