Mimecast Says SolarWinds Attackers Accessed Its Source Code Repositories

Hackers who gained access to Mimecast's systems via a poisoned SolarWind's software update late last year appear to have caused more damage than originally thought.

The email security vendor's continuing investigation of the breach has revealed that the attackers accessed and downloaded at least some of its source code repositories and also email addresses, contact information and hashed, salted credentials belonging to some customers.

In an update this week — at least the third since news of the breach first broke — Mimecast described the source code theft as limited in scope and unlikely to have any negative consequences for customers.

"We believe that the source code downloaded by the threat actor was incomplete and would be insufficient to build and run any aspect of the Mimecast service," the company said.

There is also no evidence that the threat actor used their access to modify Mimecast source code or impact products in any way, the security vendor noted.

Mimecast is one of many organizations around the world that was impacted when a believed nation-state backed threat actor installed malware called SUNBURST on their networks by quietly hiding the malicious code in legitimate software updates from SolarWinds. Some 18,000 SolarWinds customers — such as Mimecast — received and downloaded the poisoned updates. But relatively few of them were subsequently targeted for further exploits.

Mimecast discovered the attack in January when Microsoft notified the company about a compromise involving certificates used to authenticate Mimecast security products to Microsoft 365 Exchange Web Services environments. Along with the certificates, the attackers also accessed related customer server connection information.

Mimecast said Microsoft had discovered the threat actor using the compromised certificates to illegally access networks that belonged to a handful of their mutual customers. In its initial advisory, Mimecast said about 10% of its customers used the compromised certificates. But it said less than 10 of those customers had their Microsoft 365 environments illegally accessed via the compromised certificate-based connection. Mimecast asked all customers to delete their existing connection to their M365 tenant and re-establish a fresh one with the company's newly issued keys.

Mimecast's subsequent investigation — with FireEye Mandiant's help — showed the attackers had used their initial foothold to move laterally to the company's production environment, which contained "a small number" of Windows servers. The attackers then queried and likely extracted encrypted service account credentials from the servers that potentially gave them access to on-premises and cloud-hosted systems belonging to mutual customers of Microsoft and Mimecast in the US and UK.

According to Mimecast, it did not find any evidence to suggest that the encrypted credentials were later decrypted and misused in any way.

"The update from Mimecast reiterates the fact that the recent attack did not stop with the initial target," says John Morgan, CEO of Confluera.The threat actors used their access on Mimecast's network to steal certificates and keys that allowed them to further expand attacks beyond Mimecast's own environment and affiliated systems, he says. This was a scenario that played out at many of the organizations that were impacted in the SolarWinds supply chain attack.

"Another takeaway from the Mimecast report is how critical lateral movement was to the overall attack," Morgan notes. "As with many modern attacks, after gaining initial access, the attacker moved from the point of access to the targeted servers via lateral movement."

One of the reasons why many modern attacks are so effective is because organizations often cannot detect such lateral movement, he says.

Mimecast Reiterates Earlier Recommendation
In its advisory this week, Mimecast reiterated its recommendation that all of its customers reset server connection credentials being used on the Mimecast platform. The company said it had reset the hashed and salted credentials that were accessed, changed all impacted certificates and encryption keys, and implemented closer monitoring of all its certificates and encryption keys.

In addition, the company has decommissioned the SolarWinds Orion platform, changed credentials for all employees, and implemented two-factor authentication for employees requiring access to production systems. It has also implemented measures to ensure its source code is secure and cannot be tampered with and is currently working on developing a new OAuth-based authentication and connection mechanism between Microsoft and Mimecast environments.

A Mimecast spokesman declined to say when the new mechanism would become available.

Dirk Schrader, global vice president of security research at New Net Technologies, says Mimecast's response and remediation measures have been laudable, but its failure to detect the breach in the first place is concerning. Measures such as host monitoring, system and file integrity checks, and change control are essential and should have been implemented, he says. They would have helped Mimecast detect the intrusion, instead of having to be alerted by Microsoft days later, he adds.

The fact that the credentials stolen from Mimecast's environment gave threat actors a way to potentially attack both cloud and on-premises systems is troubling as well, Confluera's Morgan says.

"This should be a wake-up call for any organizations who have preconceived notions about the security of the servers based on its deployment models," he says. "It reiterates the need for organizations to adopt a security model that can detect and respond to threats in real-time across their entire environment."