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FYI...here’s the Emergency Directive 19-01 itself:
https://cyber.dhs.gov/ed/19-01/
Excerpt/summary:
Using the following techniques, attackers have redirected and intercepted web and mail traffic, and could do so for other networked services.
1.The attacker begins by compromising user credentials, or obtaining them through alternate means, of an account that can make changes to DNS records.
2.Next, the attacker alters DNS records, like Address (A), Mail Exchanger (MX), or Name Server (NS) records, replacing the legitimate address of a service with an address the attacker controls. This enables them to direct user traffic to their own infrastructure for manipulation or inspection before passing it on to the legitimate service, should they choose. This creates a risk that persists beyond the period of traffic redirection.
3.Because the attacker can set DNS record values, they can also obtain valid encryption certificates for an organization’s domain names. This allows the redirected traffic to be decrypted, exposing any user-submitted data. Since the certificate is valid for the domain, end users receive no error warnings.