"Publicly, Belgacom has played down the extent of the compromise, insisting that only its internal systems were breached and that customers’ data was never found to have been at risk. But secret GCHQ documents show the agency gained access far beyond Belgacom’s internal employee computers and was able to grab encrypted and unencrypted streams of private communications handled by the company.
The spy agency was able to obtain data that was being sent between Belgacom and other operators through encrypted tunnels known as “virtual private networks.” GCHQ boasted that its work to conduct “exploitation” against these private networks had been highly productive, noting “the huge extent of opportunity that this work has identified.” Another document, dated from late 2011, added: “Network Analysis on BELGACOM hugely successful enabling exploitation.”
GCHQ had accomplished its objective. The agency had severely compromised Belgacom’s systems and could intercept encrypted and unencrypted private data passing through its networks. The hack would remain undetected for two years, until the spring of 2013."