From Bank Info Security -
Were distributed denial-of-service attacks against major American banks in 2012 waged in retribution for U.S. government actions? A top-secret memo prepared in 2013 for Keith Alexander, who was then director of the National Security Agency, seems to confirm that's the case.
It's long been widely assumed that Iranians waged the DDoS attacks, and that it was retribution for earlier assaults on Iranian IT (see More U.S. Banks Report Online Woes). This memo seems to confirm that and also answers the question of why the Iranians launched those attacks.
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