estaban infectando
-they/you were infecting
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infectar

En esta ocasión, los atacantes lograron comprometer el sitio oficial de la agencia polaca de regulación del sector financiero, una web visitada frecuentemente por empleados de diversos bancos polacos que, sin saberlo, estaban infectando sus computadoras corporativas con malware.
On that occasion, the attackers managed to compromise the official website of the Polish financial regulatory agency, which is visited frequently by employees of various Polish banks, who were unwittingly infecting their work computers with malware.
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