Complex sentence: Successful attacks often occur when the CMTS is configured for backwards compatibility with early pre-standard DOCSIS 1.1 modems which were "software upgradeable in the field," but did not include valid DOCSIS or EuroDOCSIS root certificates.
Split into simpler sentences: Successful attacks often occur when the CMTS is configured for backwards compatibility with early pre-standard DOCSIS 1.1 modems. These modems were "software upgradeable in the field", but did not include valid DOCSIS or EuroDOCSIS root certificates.