A single vulnerability in one shared platform cascaded across six Japanese internet providers, exposing up to 14.2 million email accounts. Telecoms giant KDDI has confirmed a breach of a shared email system used by KDDI, JCOM, NIFTY, BIGLOBE and others — an intrusion via a flaw in third-party software. We look at "shared-infrastructure blast radius": how concentration risk turns one weakness into millions of victims, why exposed email logins are gold for credential-stuffing, and the transparency questions KDDI has left unanswered.

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Sources: BleepingComputer; SecurityAffairs; The Japan Times.