This file is indexed.

/usr/lib/update-notifier/cddistupgrader is in update-notifier-common 3.192.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#!/bin/sh
#

CDROM_MOUNT="$1"

for d in "$CDROM_MOUNT"/dists/*/main/dist-upgrader/binary-all/; do
    if [ -d "$d" ]; then
        UPGRADER_DIR="$d"
        break
    fi
done

TAR=$(basename "$UPGRADER_DIR"/*.tar.gz)
CODENAME=${TAR%.tar.gz}

# get a tempdir
TMPDIR=$(mktemp -t -d distupgrade.XXXXXX)
cd $TMPDIR

# extract the tar
tar xzf "$UPGRADER_DIR"/"$TAR"

# apply any patches needed before running the upgrade
for apatch in /usr/share/update-notifier/upgrader-patches/*.diff; do
    if patch --dry-run < "$apatch"; then
        patch < "$apatch"
    fi
done

# run it
"$TMPDIR"/"$CODENAME" --cdrom "$CDROM_MOUNT"