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Stories from the SOC is a blog series that describes recent real-world security incident investigations conducted and reported by the AT&T SOC analyst team for AT&T Managed Threat Detection and Response customers.

Executive summary

WannaCry malware was first discovered in May 2017 and a patch was released roughly two months prior to its public release. However, 230,000 computers were globally affected by WannaCry as of 3/31/2021. It is unfortunate to hear, but many companies remain vulnerable to this attack due to unpatched systems. We often see that by the time some companies update their systems, they have already experienced a breach.

The Managed Threat Detection and Response (MTDR) SOC analyst team received 56 alarms related to the suspicious use of port 445 within a 24-hour timeframe. Given the high influx of alarms, our team created an Investigation to reveal which assets were using port 445, the destinations that were being communicated with, and the frequency of the connections. The customer quickly identified that the source assets were unpatched Windows 7 production servers affected by WannaCry. They were able to segment the infected computers, block SMB port 445, use Trend Micro’s Anti-Threat Toolkit to clean the machines, and then return the assets to the network.


Initial alarm review

Indicators of compromise (IOC)

The initial alarms that triggered this investigation were created from a custom alarm. The MTDR team can create custom alarms specific to the customers environment to help improve time to response. The alarms were triggered when events from Trend Micro showed assets using Server Message Block (SMB) port 445 in which a single source was communicating with multiple destinations.

This initial alarm was one of many that was generated. The alarms came in with a priority of “Low” because use of SMB port 445 is common within the customer’s organization. Our team and the customer began to suspect that a breach had occurred due to the high volume of internal connections as well as those connections attempting to reach external IP’s.

Expanded investigation

Events search

Upon further investigation, we searched for events “CnC Callback” and “Suspicious Connection”. The team then analyzed these events over a 24-hour period. This analysis revealed all of the internal assets and their events’ sources and destinations. These assets were communicating over port 445 and were likely compromised systems.

Event deep dive

Continuing with the investigation, we learned that the affected assets were communicating with unknown external IP’s. Many of these outbound connections were blocked at the firewall; however, at this point, we were able to pivot from the external IP’s to look for more affected assets.

Reviewing for additional indicators

We then made a complete list of all potentially affected internal assets. After individually inspecting the assets, we discovered the following event: “Ransom_WCRY.SM2” on a few of the assets. This particular event confirmed our suspicion that this was, indeed, the WannaCry malware.


Building the investigation

Within minutes of the team creating the investigation, the customer escalated the case. The customer noticed that all of the associated assets were part of a single subnet isolated to one sector of their business. The customer then isolated the subnet of potentially affected assets from the rest of the network in order to begin reviewing the machines.

While the assets were being scanned for further indicators of compromise, we involved the customer’s Threat Hunter (TH). The TH helped generate additional reports of all internal assets that were associated with the malicious events.

At this point, the customer blocked port 445 on the assets, used Trend Micro’s Anti-Threat Toolkit to clean the machines, and then returned the assets to the network.

We continued to closely monitor the customer’s network for further signs of compromise from the WannaCry malware. We maintained this vigilance until the team ensured the situation had been fully resolved.

Customer interaction

Our team worked closely with the customer to ensure we were up to date with any changes being made to their systems. Because of the close communication between our team and the customer, we were able to quickly assess the situation, investigate appropriate assets, and resolve the issue before any systems could be encrypted for ransomware.

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