diff --git a/build.sh b/build.sh
index 5d777a9b73e47401253d84d206c5a8e0fa85b427..fcf0dd81863e9a481626c2e4e977129e0e1905ea 100755
--- a/build.sh
+++ b/build.sh
@@ -91,6 +91,11 @@ done
 mkdir /var/run/mailman
 chmod 1777 /var/run/mailman
 
+# We don't need the setgid bit on Mailman CGI scripts.
+# Everything runs as the same user in the container, including
+# the web server.
+chmod g-s /usr/lib/cgi-bin/mailman/*
+
 # Create empty dir for webroot.
 mkdir /var/empty