Authored by: Max Heinemeyer, Director of Threat Hunting, Darktrace

On March 11 and 12, 2021, Darktrace detected multiple attempts by a broad campaign to attack vulnerable servers in customer environments. The campaign targeted Internet-facing Microsoft Exchange servers, exploiting the recently discovered ProxyLogon vulnerability (CVE-2021-26855).
 
While this exploit was initially attributed to a group known as Hafnium, Microsoft has announced that the vulnerability is also being rapidly weaponised by other threat actors. These new, unattributed campaigns, which have never been seen before, have been disrupted by Cyber AI in real time.
 
Hafnium copycats
 
As soon as a vulnerability is made public it is common for there to be an influx of attacks as hackers capitalise on the chaos and attempt to compromise vulnerable networks.
 
Patches are rapidly reverse-engineered by hackers once they have been published by the vendor, leading to mass high-impact exploits. At the same time, the offensive tooling trickles down from the first adopters, such as nation-state actors, to ransomware gangs and other opportunistic attackers. Darktrace has observed this exact phenomenon as a result of Hafnium’s attacks against vulnerable Microsoft Exchange email servers this month.
 
Exchange servers attacked: AI analysis
 
Cyber AI has observed threat actors attempting to download and install malware using ProxyLogon as the initial attack vector. For customers with Autonomous Response, the malicious payload was intercepted at this point, stopping the attack before any developments.
 
In other Darktrace customer environments, the Darktrace Immune System identified and alerted on every stage of the attack. Generally, the malware has been observed acting as a generic backdoor, without much follow-up activity. Various forms of command and control (C2) channels were detected, including Telegraph. In a few intrusions, the attackers installed cryptocurrency miners.
 
Once a foothold has been established in the digital environment, it is likely that the actors will begin a hands-on-keyboard attack, exfiltrating data, moving laterally, or deploying ransomware.

After the ProxyLogon vulnerability was exploited, the Exchange servers reached out to the malicious domain microsoftsoftwaredownload.com, utilising a PowerShell User Agent. Darktrace flagged this anomalous behaviour as the particular User Agent had never been used before by the Exchange server, let alone to access a malicious domain which had never been observed in the network.

The malware executable was masqueraded as a ZIP file, further trying to obfuscate the attack. Darktrace identified this highly anomalous file download and the masqueraded file.

In some cases, Darktrace AI also observed cryptocurrency mining seconds or minutes after the initial malware download.

In terms of C2 traffic, Darktrace has observed various potential channels. Around the time of the malware download, some of the Exchange servers began to beacon out to several external destinations using unusual SSL or TLS encrypted connections.
• Telegraph — popular messenger application
• dev.opendrive.com — cloud storage service
• od.lk — cloud storage service
 
In this case, Darktrace recognised that none of these three external domains had ever been contacted before by anybody in the organisation, let alone in a beaconing fashion. The fact that these communications started around the same time as the malware downloads strongly suggests a correlation. Darktrace’s Cyber AI Analyst automatically began an investigation into the incident, stitching together these events into one coherent narrative.
 
Investigating with AI
 
Cyber AI Analyst then automatically created a summary incident report about the activity, covering the malware download as well as the various C2 channels observed.

Looking at an infected Exchange server ([REDACTED].local) from a birds-eye perspective shows that Darktrace created various alerts when the attack hit. Every one of the coloured dots in the graph below represents a major anomaly detected by Darktrace.

This activity was prioritised as the most urgent incident in Cyber AI Analyst among a full week’s worth of data. In this particular organisation, there were only four incidents for that week in total in Cyber AI Analyst. Such precise and clear alerting allows security teams to immediately understand the top threats facing their digital environment, without being overwhelmed by unnecessary alerts and false positives.
 
Machine-speed response
 
For customers with Darktrace Antigena, Antigena autonomously acted to block all outgoing traffic to malicious external endpoints on the relevant ports. This behaviour is held for several hours to interrupt the threat actor from escalating the attack, while giving security teams time to react and remediate.
 
Antigena responded within seconds of the attack starting, effectively containing the attack in its earliest stage – without interrupting regular business activity (emails could still be sent and received), and despite this being a zero-day campaign.

Catching a zero-day exploit
 
This is not the first time Darktrace has stopped an attack leveraging a zero-day or a freshly released n-day vulnerability. Back in March 2020, Darktrace detected APT41 exploiting the Zoho ManageEngine vulnerability, two weeks before public attribution.
 
It is highly likely that there will be more cyber-criminals exploiting ProxyLogon in the wake of Hafnium. And while the recent Exchange server vulnerabilities were today’s threat, next time it might be a software or hardware supply chain attack, or a different zero-day. Novel threats are emerging every week. In this climate we now find ourselves in, where ‘known unknowns’ which are difficult or impossible to pre-define are the new norm, we need to be more adaptable and proactive than ever.
 
As soon as an attacker begins to exhibit unusual activity, Darktrace AI will detect it, even if there is no threat intelligence associated with the attack. This is where Darktrace works best, autonomously detecting, investigating and responding to advanced and never-before-seen threats in real time.