diff --git a/buildserver/provision-apt-get-install b/buildserver/provision-apt-get-install
index 870e5f1e..e29a786a 100644
--- a/buildserver/provision-apt-get-install
+++ b/buildserver/provision-apt-get-install
@@ -38,6 +38,9 @@ if echo $debian_mirror | grep '^https' 2>&1 > /dev/null; then
     apt-get install ca-certificates
 fi
 
+# ensure that only the configured sources are active
+rm -f /etc/apt/sources.list /etc/apt/sources.list.d/*.*
+
 cat << EOF > /etc/apt/sources.list.d/debian.sources
 Types: deb
 URIs: ${debian_mirror}