Email Bombs Exploit Lax Authentication in Zendesk

Summary

Cybercriminals are abusing a widespread lack of authentication in the customer service platform Zendesk to flood targeted email inboxes with menacing messages that come from hundreds of Zendesk corporate customers simultaneously. Zendesk is an automated help desk service designed to make it simple for people to contact companies for customer support issues. Earlier this week, KrebsOnSecurity started receiving thousands of ticket creation notification messages through Zendesk in rapid succession, each bearing the name of different Zendesk customers, such as CapCom, CompTIA, Discord, GMAC, NordVPN, The Washington Post, and Tinder. The abusive missives sent via Zendesk’s platform can include any subject line chosen by the abusers. In my case, the messages variously warned about a supposed law enforcement investigation involving KrebsOnSecurity.com, or else contained personal insults. Moreover, the automated messages that are sent out from this type of abuse all come from customer domain names — not from Zendesk. In the example below, replying to any of the junk customer support responses from The Washington Post’s Zendesk installation shows the reply-to address is help@washpost.com. One of dozens of messages sent to me this week by The Washington Post. Notified about the mass abuse of their platform, Zendesk said the emails were ticket creation notifications from customer accounts that configured their Zendesk instance to allow anyone to submit support requests — including anonymous users. “These types of support tickets can be part of a customer’s workflow, where a prior verification is not required to allow them to engage and make use of the Support capabilities,” said Carolyn Camoens, communications director at Zendesk. “Although we recommend our customers to permit only verified users to submit tickets, some Zendesk customers prefer to use an anonymous environment to allow for tickets to be created due to various business reasons.” Camoens said requests that can be...