Grid computing through malware

Hello,

I would like to build my license project on the grid computing
subject. I want to add a twist to it though, for this i was hoping for
some input on this scenario:

Say that a new vulnerability is discovered and a malicious attacker
uses that vulnerability to install an inconspicuous grid client that
uses the system's resources to compute whatever you throw at it. Would
the combined power of thousands or even millions of systems be enough
to break a secured online transaction?

Thanks.

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