diff --git a/auto/config b/auto/config
index bb8f6e2fc25e38a9b98b802f22989fbb25aba98b..edf6bcf62a0441b2fad812d978e048b575f68b29 100755
--- a/auto/config
+++ b/auto/config
@@ -114,6 +114,7 @@ TORPROJECT_MIRROR="$(apt-mirror torproject)"
 perl -pi \
      -E \
        "s|^(deb(?:-src)?\s+)https?://ftp[.]us[.]debian[.]org/debian/?(\s+)|\$1$DEBIAN_MIRROR\$2| ; \
+        s|^(deb(?:-src)?\s+)https?://security[.]debian[.]org/debian-security/?(\s+)|\$1$DEBIAN_SECURITY_MIRROR\$2| ; \
         s|^(deb(?:-src)?\s+)https?://deb[.]torproject[.]org/torproject[.]org/?(\s+)|\$1$TORPROJECT_MIRROR\$2|" \
     config/chroot_sources/*.chroot \
     || fatal "APT mirror substitution failed with exit code $?"
diff --git a/config/chroot_sources/buster.chroot b/config/chroot_sources/buster.chroot
index 9e060978278caaecd21cac4ba56896f37dc47cc6..2635d936c409957a0882cebb9c962f67b6886016 100644
--- a/config/chroot_sources/buster.chroot
+++ b/config/chroot_sources/buster.chroot
@@ -1 +1,2 @@
 deb http://ftp.us.debian.org/debian/ buster main contrib non-free
+deb http://security.debian.org/debian-security buster/updates main contrib non-free