diff --git a/pkgs/tools/system/at/default.nix b/pkgs/tools/system/at/default.nix
index 4c1a5431f3e..501a1bde33b 100644
--- a/pkgs/tools/system/at/default.nix
+++ b/pkgs/tools/system/at/default.nix
@@ -1,12 +1,13 @@
 { fetchurl, stdenv, bison, flex, pam, sendmailPath ? "/run/wrappers/bin/sendmail" }:
 
-stdenv.mkDerivation {
-  name = "at-3.1.16";
+stdenv.mkDerivation rec {
+  name = "at-${version}";
+  version = "3.1.20";
 
   src = fetchurl {
     # Debian is apparently the last location where it can be found.
-    url = mirror://debian/pool/main/a/at/at_3.1.16.orig.tar.gz;
-    sha256 = "1hfmnhgi95vsfaa69qlakpwd22al0m0rhqms6sawxvaldafgb6nb";
+    url = "mirror://debian/pool/main/a/at/at_${version}.orig.tar.gz";
+    sha256 = "1fgsrqpx0r6qcjxmlsqnwilydhfxn976c870mjc0n1bkmcy94w88";
   };
 
   patches = [ ./install.patch ];