Asia-Pacific recorded a growth decline of 7.4%.
Email to staff says information, including bank and payroll number, salary, National Insurance number, and personal contact details have been taken.
All six networks were invited to pitch earlier this year.
The company also downgraded its net revenue forecast for the year.
The appointments follow the arrival of new chief executive Cindy Rose on 1 September.
Dentsu is reported to be exploring divestment options including sale of a minority stake to a full sale of its overseas operations.
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