Blog
/
/
November 4, 2020

Mimecast Link Rewriting: A False Sense of Security Exposed

Gain insight into modern email security methods to ensure you avoid pitfalls of traditional email gateways. Learn why rewriting links isn't the best approach.
Inside the SOC
Darktrace cyber analysts are world-class experts in threat intelligence, threat hunting and incident response, and provide 24/7 SOC support to thousands of Darktrace customers around the globe. Inside the SOC is exclusively authored by these experts, providing analysis of cyber incidents and threat trends, based on real-world experience in the field.
Written by
Dan Fein
VP, Product
Default blog imageDefault blog imageDefault blog imageDefault blog imageDefault blog imageDefault blog image
04
Nov 2020

Many organizations feel secure in the knowledge that their email gateway is rewriting all of the harmful links targeting their employees. Link rewriting is a common technique that involves encoding URLs sent via email into a link that redirects the user to the gateway’s own servers. These servers contain some unique codes that then track the user and perform later checks to determine whether the link is malicious.

This blog reveals why the sense of protection this gives is a fallacy, and how rewriting links does not equate to protecting the end user from actual harm. In fact, gateways’ reliance on this technique is actually an indicator of one of their fundamental flaws: their reliance on rules and signatures of previously recognized threats, and their consequent inability to stop threats on the first encounter. The reason these tools pre-emptively rewrite links is so they can make a determination later on: with the link now pointing to their own servers, they can leverage their updated assessment of that link and block a malicious site, once more information has become available (often once ‘patient zero’ has become infected; and the damage is already done).

Email security that recognizes and blocks threats on the first encounter has no need to rewrite every link.

How to measure success

If the sheer number of links rewritten is to be our measure of success, then traditional gateways win every time. For instance, Mimecast will usually rewrite 100% of the harmful links that Antigena Email locks. In fact, it rewrites nearly 100% of all links. That even includes links pointing to trusted websites like LinkedIn and Twitter, and even emails containing links to the recipient’s own website. So when tim.cook[at]apple.com receives a link to apple.com, for example, ‘mimecast.com’ will still dominate the URL.

Some organizations suffering from low first-encounter catch rates with their gateways have responded by increasing employee education: training the human to spot the giveaways of a phishing email. With email attacks getting more targeted and sophisticated, humans should never be considered the last line of defense, and rewriting links makes the situation even worse. If you’re training your users to watch which links they’re clicking, and every one of those links reads ‘mimecast.com’, how are your users supposed to learn what’s good, bad, or sketchy when every URL looks the same?

Moreover, when Mimecast’s URL gateway is down, these rewritten links don’t work (and the same applies to protected attachments). This results in business downtime which is intolerable for businesses in these critical and challenging times.

We can see the effect of blanket rewriting through Darktrace’s user interface, which shows us the frequency of rewritten links over time. Looking back over three days, this particular customer – who was trialing Antigena Email alongside Mimecast, received 155,008 emails containing rewritten links. Of those, 1,478 were anomalous, and Darktrace’s AI acted to immediately lock those links, protecting even the first recipient from harm. The remaining 153,530 links were all unnecessarily rewritten.

Figure 1: Over 155,000 inbound emails contained rewritten Mimecast links

If it comes to actually stopping the threat when a user goes to click that rewritten link, gateway tools fail. Their reliance on legacy checks like reputation, deny-lists, and rules and signatures mean that malicious content will sometimes sit for days or weeks without any meaningful action, as the technology requires at least one – and usually many – ‘patient zeros’ before determining a URL or an attachment as malicious, and updating their deny-lists.

Let’s look at the case of an attack launched from entirely new infrastructure: from a freshly purchased domain, and containing a newly created malicious payload. None of the typical metrics legacy tools search for appear as malicious, and so of course, the threat gets through, and ‘patient zero’ is infected.

Figure 2: ‘Patient Zero’ denotes the first victim of an email attack.

It inevitably takes time for the malicious link to be recognized as malicious, and for that to be reported. By this point, large swathes of the workforce have also become infected. We can call this the ‘time to detection’.

Figure 3: The time to detection

As legacy tools then update their lists in recognition of the attack, the malware continues to infect the organization, with more users engaging in the contents of the email.

Figure 4: The legacy tool reacts

Finally, the legacy tool reacts, updating its deny-list and providing substantive action to protect the end user from harm. By this point, hundreds of users across multiple organizations may have interacted with the links in some way.

Figure 5: Many ‘patient zeros’ are required before the threat is deny-listed

Email gateways’ reliance on rewriting links is directly related to their legacy approach to detection. They do it so that later down the line, when they have updated information about a potential attack, they can take action. Until then, it’s just a rewritten link, and if clicked on, it will bring the user to whatever website was hiding underneath it.

These links are also rewritten in an attempt to grasp an understanding of what user network behavior looks like. But far from giving an accurate or in-depth picture of network activity, this method barely scratches the surface of the wider behaviors of users across the organization.

Alongside Darktrace’s Enterprise Immune System, Antigena Email can pull these insights directly from a unified, central AI engine that has complete and direct visibility over an organization’s entire digital estate – not just links accessed from emails, but network activity as a whole – and not a makeshift version where it is assumed people only visit links through emails. It also pulls insights from user behavior in the cloud and across SaaS applications – from Salesforce to Microsoft Teams.

Taking real action in real time

While gateways rewrite everything in order to leave the door open to make assessments later on, Darktrace is able to take action when it needs to – before the email poses a threat in the inbox. The technology is uniquely able to do this due to its high success rates for malicious emails seen on first encounter. And it’s able to achieve such high success rates because it takes a much more sophisticated approach to detection that uses AI to catch a threat – regardless of whether or not that threat has been seen before.

Darktrace’s understanding of ‘normal’ for the human behind email communications allows it to not only detect subtle deviations that are indicative of a cyber-threat, but respond to that threat at the point of delivery. This response is targeted, proportionate and non-disruptive, and varies according to the nature of the attack. While Darktrace’s unsupervised machine learning can accurately identify deviations from ‘normal’, its supervised machine learning models are able to classify the intention behind the email; what the attacker is trying to do (extort information, solicit a payment, harvest credentials, or convince the user to download a malicious attachment).

Crucially, organizations trialing both approaches to security find that Antigena Email consistently identifies threats that Mimecast and other tools miss. With the scale and sophistication of email attacks growing, the need for a proactive and modern approach to email security is paramount. Organizations need to ensure they are measuring their sense of protection with the right yardstick, and adopt a technology that can take meaningful action before damage is done.

Trial Antigena Email today

Inside the SOC
Darktrace cyber analysts are world-class experts in threat intelligence, threat hunting and incident response, and provide 24/7 SOC support to thousands of Darktrace customers around the globe. Inside the SOC is exclusively authored by these experts, providing analysis of cyber incidents and threat trends, based on real-world experience in the field.
Written by
Dan Fein
VP, Product

More in this series

No items found.

Blog

/

Network

/

June 5, 2025

Unpacking ClickFix: Darktrace’s detection of a prolific social engineering tactic

Woman on laptop in office buildingDefault blog imageDefault blog image

What is ClickFix and how does it work?

Amid heightened security awareness, threat actors continue to seek stealthy methods to infiltrate target networks, often finding the human end user to be the most vulnerable and easily exploited entry point.

ClickFix baiting is an exploitation of the end user, making use of social engineering techniques masquerading as error messages or routine verification processes, that can result in malicious code execution.

Since March 2024, the simplicity of this technique has drawn attention from a range of threat actors, from individual cybercriminals to Advanced Persistent Threat (APT) groups such as APT28 and MuddyWater, linked to Russia and Iran respectively, introducing security threats on a broader scale [1]. ClickFix campaigns have been observed affecting organizations in across multiple industries, including healthcare, hospitality, automotive and government [2][3].

Actors carrying out these targeted attacks typically utilize similar techniques, tools and procedures (TTPs) to gain initial access. These include spear phishing attacks, drive-by compromises, or exploiting trust in familiar online platforms, such as GitHub, to deliver malicious payloads [2][3]. Often, a hidden link within an email or malvertisements on compromised legitimate websites redirect the end user to a malicious URL [4]. These take the form of ‘Fix It’ or fake CAPTCHA prompts [4].

From there, users are misled into believing they are completing a human verification step, registering a device, or fixing a non-existent issue such as a webpage display error. As a result, they are guided through a three-step process that ultimately enables the execution of malicious PowerShell commands:

  1. Open a Windows Run dialog box [press Windows Key + R]
  2. Automatically or manually copy and paste a malicious PowerShell command into the terminal [press CTRL+V]
  3. And run the prompt [press ‘Enter’] [2]

Once the malicious PowerShell command is executed, threat actors then establish command and control (C2) communication within the targeted environment before moving laterally through the network with the intent of obtaining and stealing sensitive data [4]. Malicious payloads associated with various malware families, such as XWorm, Lumma, and AsyncRAT, are often deployed [2][3].

Attack timeline of ClickFix cyber attack

Based on investigations conducted by Darktrace’s Threat Research team in early 2025, this blog highlights Darktrace’s capability to detect ClickFix baiting activity following initial access.

Darktrace’s coverage of a ClickFix attack chain

Darktrace identified multiple ClickFix attacks across customer environments in both Europe, the Middle East, and Africa (EMEA) and the United States. The following incident details a specific attack on a customer network that occurred on April 9, 2025.

Although the initial access phase of this specific attack occurred outside Darktrace’s visibility, other affected networks showed compromise beginning with phishing emails or fake CAPTCHA prompts that led users to execute malicious PowerShell commands.

Darktrace’s visibility into the compromise began when the threat actor initiated external communication with their C2 infrastructure, with Darktrace / NETWORK detecting the use of a new PowerShell user agent, indicating an attempt at remote code execution.

Darktrace / NETWORK's detection of a device making an HTTP connection with new PowerShell user agent, indicating PowerShell abuse for C2 communications.
Figure 1: Darktrace / NETWORK's detection of a device making an HTTP connection with new PowerShell user agent, indicating PowerShell abuse for C2 communications.

Download of Malicious Files for Lateral Movement

A few minutes later, the compromised device was observed downloading a numerically named file. Numeric files like this are often intentionally nondescript and associated with malware. In this case, the file name adhered to a specific pattern, matching the regular expression: /174(\d){7}/. Further investigation into the file revealed that it contained additional malicious code designed to further exploit remote services and gather device information.

Darktrace / NETWORK's detection of a numeric file, one minute after the new PowerShell User Agent alert.
Figure 2: Darktrace / NETWORK's detection of a numeric file, one minute after the new PowerShell User Agent alert.

The file contained a script that sent system information to a specified IP address using an HTTP POST request, which also processed the response. This process was verified through packet capture (PCAP) analysis conducted by the Darktrace Threat Research team.

By analyzing the body content of the HTTP GET request, it was observed that the command converts the current time to Unix epoch time format (i.e., 9 April 2025 13:26:40 GMT), resulting in an additional numeric file observed in the URI: /1744205200.

PCAP highlighting the HTTP GET request that sends information to the specific IP, 193.36.38[.]237, which then generates another numeric file titled per the current time.
Figure 3: PCAP highlighting the HTTP GET request that sends information to the specific IP, 193.36.38[.]237, which then generates another numeric file titled per the current time.

Across Darktrace’s investigations into other customers' affected by ClickFix campaigns, both internal information discovery events and further execution of malicious code were observed.

Data Exfiltration

By following the HTTP stream in the same PCAP, the Darktrace Threat Research Team assessed the activity as indicative of data exfiltration involving system and device information to the same command-and-control (C2) endpoint, , 193.36.38[.]237. This endpoint was flagged as malicious by multiple open-source intelligence (OSINT) vendors [5].

PCAP highlighting HTTP POST connection with the numeric file per the URI /1744205200 that indicates data exfiltration to 193.36.38[.]237.
Figure 4: PCAP highlighting HTTP POST connection with the numeric file per the URI /1744205200 that indicates data exfiltration to 193.36.38[.]237.

Further analysis of Darktrace’s Advanced Search logs showed that the attacker’s malicious code scanned for internal system information, which was then sent to a C2 server via an HTTP POST request, indicating data exfiltration

Advanced Search further highlights Darktrace's observation of the HTTP POST request, with the second numeric file representing data exfiltration.
Figure 5: Advanced Search further highlights Darktrace's observation of the HTTP POST request, with the second numeric file representing data exfiltration.

Actions on objectives

Around ten minutes after the initial C2 communications, the compromised device was observed connecting to an additional rare endpoint, 188.34.195[.]44. Further analysis of this endpoint confirmed its association with ClickFix campaigns, with several OSINT vendors linking it to previously reported attacks [6].

In the final HTTP POST request made by the device, Darktrace detected a file at the URI /init1234 in the connection logs to the malicious endpoint 188.34.195[.]44, likely depicting the successful completion of the attack’s objective, automated data egress to a ClickFix C2 server.

Darktrace / NETWORK grouped together the observed indicators of compromise (IoCs) on the compromised device and triggered an Enhanced Monitoring model alert, a high-priority detection model designed to identify activity indicative of the early stages of an attack. These models are monitored and triaged 24/7 by Darktrace’s Security Operations Center (SOC) as part of the Managed Threat Detection service, ensuring customers are promptly notified of malicious activity as soon as it emerges.

Darktrace correlated the separate malicious connections that pertained to a single campaign.
Figure 6: Darktrace correlated the separate malicious connections that pertained to a single campaign.

Darktrace Autonomous Response

In the incident outlined above, Darktrace was not configured in Autonomous Response mode. As a result, while actions to block specific connections were suggested, they had to be manually implemented by the customer’s security team. Due to the speed of the attack, this need for manual intervention allowed the threat to escalate without interruption.

However, in a different example, Autonomous Response was fully enabled, allowing Darktrace to immediately block connections to the malicious endpoint (138.199.156[.]22) just one second after the initial connection in which a numerically named file was downloaded [7].

Darktrace Autonomous Response blocked connections to a suspicious endpoint following the observation of the numeric file download.
Figure 7: Darktrace Autonomous Response blocked connections to a suspicious endpoint following the observation of the numeric file download.

This customer was also subscribed to our Managed Detection and Response service, Darktrace’s SOC extended a ‘Quarantine Device’ action that had already been autonomously applied in order to buy their security team additional time for remediation.

Autonomous Response blocked connections to malicious endpoints, including 138.199.156[.]22, 185.250.151[.]155, and rkuagqnmnypetvf[.]top, and also quarantined the affected device. These actions were later manually reinforced by the Darktrace SOC.
Figure 8: Autonomous Response blocked connections to malicious endpoints, including 138.199.156[.]22, 185.250.151[.]155, and rkuagqnmnypetvf[.]top, and also quarantined the affected device. These actions were later manually reinforced by the Darktrace SOC.

Conclusion

ClickFix baiting is a widely used tactic in which threat actors exploit human error to bypass security defenses. By tricking end point users into performing seemingly harmless, everyday actions, attackers gain initial access to systems where they can access and exfiltrate sensitive data.

Darktrace’s anomaly-based approach to threat detection identifies early indicators of targeted attacks without relying on prior knowledge or IoCs. By continuously learning each device’s unique pattern of life, Darktrace detects subtle deviations that may signal a compromise. In this case, Darktrace's Autonomous Response, when operating in a fully autonomous mode, was able to swiftly contain the threat before it could progress further along the attack lifecycle.

Credit to Keanna Grelicha (Cyber Analyst) and Jennifer Beckett (Cyber Analyst)

Appendices

NETWORK Models

  • Device / New PowerShell User Agent
  • Anomalous Connection / New User Agent to IP Without Hostname
  • Anomalous Connection / Posting HTTP to IP Without Hostname
  • Anomalous Connection / Powershell to Rare External
  • Device / Suspicious Domain
  • Device / New User Agent and New IP
  • Anomalous File / New User Agent Followed By Numeric File Download (Enhanced Monitoring Model)
  • Device / Initial Attack Chain Activity (Enhanced Monitoring Model)

Autonomous Response Models

  • Antigena / Network::Significant Anomaly::Antigena Significant Anomaly from Client Block
  • Antigena / Network::Significant Anomaly::Antigena Enhanced Monitoring from Client Block
  • Antigena / Network::External Threat::Antigena File then New Outbound Block
  • Antigena / Network::External Threat::Antigena Suspicious File Block
  • Antigena / Network::Significant Anomaly::Antigena Alerts Over Time Block
  • Antigena / Network::External Threat::Antigena Suspicious File Block

IoC - Type - Description + Confidence

·       141.193.213[.]11 – IP address – Possible C2 Infrastructure

·       141.193.213[.]10 – IP address – Possible C2 Infrastructure

·       64.94.84[.]217 – IP address – Possible C2 Infrastructure

·       138.199.156[.]22 – IP address – C2 server

·       94.181.229[.]250 – IP address – Possible C2 Infrastructure

·       216.245.184[.]181 – IP address – Possible C2 Infrastructure

·       212.237.217[.]182 – IP address – Possible C2 Infrastructure

·       168.119.96[.]41 – IP address – Possible C2 Infrastructure

·       193.36.38[.]237 – IP address – C2 server

·       188.34.195[.]44 – IP address – C2 server

·       205.196.186[.]70 – IP address – Possible C2 Infrastructure

·       rkuagqnmnypetvf[.]top – Hostname – C2 server

·       shorturl[.]at/UB6E6 – Hostname – Possible C2 Infrastructure

·       tlgrm-redirect[.]icu – Hostname – Possible C2 Infrastructure

·       diagnostics.medgenome[.]com – Hostname – Compromised Website

·       /1741714208 – URI – Possible malicious file

·       /1741718928 – URI – Possible malicious file

·       /1743871488 – URI – Possible malicious file

·       /1741200416 – URI – Possible malicious file

·       /1741356624 – URI – Possible malicious file

·       /ttt – URI – Possible malicious file

·       /1741965536 – URI – Possible malicious file

·       /1.txt – URI – Possible malicious file

·       /1744205184 – URI – Possible malicious file

·       /1744139920 – URI – Possible malicious file

·       /1744134352 – URI – Possible malicious file

·       /1744125600 – URI – Possible malicious file

·       /1[.]php?s=527 – URI – Possible malicious file

·       34ff2f72c191434ce5f20ebc1a7e823794ac69bba9df70721829d66e7196b044 – SHA-256 Hash – Possible malicious file

·       10a5eab3eef36e75bd3139fe3a3c760f54be33e3 – SHA-1 Hash – Possible malicious file

MITRE ATT&CK Mapping

Tactic – Technique – Sub-Technique  

Spearphishing Link - INITIAL ACCESS - T1566.002 - T1566

Drive-by Compromise - INITIAL ACCESS - T1189

PowerShell - EXECUTION - T1059.001 - T1059

Exploitation of Remote Services - LATERAL MOVEMENT - T1210

Web Protocols - COMMAND AND CONTROL - T1071.001 - T1071

Automated Exfiltration - EXFILTRATION - T1020 - T1020.001

References

[1] https://www.logpoint.com/en/blog/emerging-threats/clickfix-another-deceptive-social-engineering-technique/

[2] https://www.proofpoint.com/us/blog/threat-insight/security-brief-clickfix-social-engineering-technique-floods-threat-landscape

[3] https://cyberresilience.com/threatonomics/understanding-the-clickfix-attack/

[4] https://www.group-ib.com/blog/clickfix-the-social-engineering-technique-hackers-use-to-manipulate-victims/

[5] https://www.virustotal.com/gui/ip-address/193.36.38.237/detection

[6] https://www.virustotal.com/gui/ip-address/188.34.195.44/community

[7] https://www.virustotal.com/gui/ip-address/138.199.156.22/detection

Continue reading
About the author
Keanna Grelicha
Cyber Analyst

Blog

/

Proactive Security

/

June 4, 2025

Beyond Discovery: Adding Intelligent Vulnerability Validation to Darktrace / Attack Surface Management

Man on computer doing workDefault blog imageDefault blog image

Introducing Exploit Prediction Assessment

Security teams are drowning in vulnerability alerts, but only a fraction of those issues pose a real threat. The new Exploit Prediction Assessment feature in Darktrace / Attack Surface Management helps teams cut through the noise by validating which vulnerabilities on their external attack surface can be actively exploited.

Instead of relying solely on CVSS scores or waiting for patch cycles, Exploit Prediction Assessment uses safe, targeted simulations to test whether exposed systems can be compromised, delivering fast, evidence-based results in under 72 hours.

This capability augments traditional pen testing and complements existing ASM workflows by transforming passive discovery into actionable insight. With EPA, security teams move from reacting to long lists of potential vulnerabilities to making confident, risk-based decisions on what actually matters.

Key highlights of Exploit Prediction Assessment

Simulated attacks to validate real risk

Exploit Prediction Assessment conducts safe, simulated attacks on assets with potential security vulnerabilities that have been identified by Darktrace / Attack Surface Management. This real-time testing validates your systems' susceptibility to compromise by confirming which vulnerabilities are present and exploitable on your attack surface.

Prioritize what matters most

Confirmed security risks can be prioritized for mitigation, ensuring that the most critical threats are promptly addressed. This takes the existing letter ranking system and brings it a step further by drilling down to yet another level. Even in the most overwhelming situations, teams will be able to act on a pragmatic, clear-cut plan.

Fast results, tailored to your environment

Customers set the scope of the Exploit Prediction Assessment within Darktrace / Attack Surface Management and receive the results of the surgical vulnerability testing within 72 hours. Users will see 1 of 2 shields:

1. A green shield with a check mark: Meaning no vulnerabilities were found on scanned CVEs for the asset.

2. A red shield with a red x: Meaning at least one vulnerability was found on scanned CVEs for the asset.

Why it's a game changer

Traditionally, attack surface management tools have focused on identifying exposed assets and vulnerabilities but lacked the context to determine which issues posed the greatest risk. Without context on what’s exploitable, security teams are left triaging long lists of potential risks, operating in isolation from broader business objectives. This misalignment ultimately leads to both weakened risk posture and cross team communication and execution.

This is where Continuous Threat Exposure Management (CTEM) becomes essential. Introduced by Gartner, CTEM is a framework that helps organizations continuously assess, validate, and improve their exposure to real-world threats. The goal isn’t just visibility, it’s to understand how an attacker could move through your environment today, and what to fix first to stop them.

Exploit Prediction Assessment brings this philosophy to life within Darktrace / Attack Surface Management. By safely simulating exploit attempts against identified vulnerabilities, it validates which exposures are truly at risk—transforming ASM from a discovery tool into a risk-based decision engine.

This capability directly supports the validation and prioritization phases of CTEM, helping teams focus on exploitable vulnerabilities rather than theoretical ones.  This shift from visibility to action reduces the risk of critical vulnerabilities in the technology stack being overlooked, turning overwhelming vulnerability data into focused, clear actionable insights.

As attack surfaces continue to grow and change, organizations need more than static scans they need continuous, contextual insight. Exploit Prediction Assessment ensures your ASM efforts evolve with the threat landscape, making CTEM a practical reality, not just a strategy.

Exploit Prediction Assessment in action

With Darktrace / Attack Surface Management organizations can get Exploit Prediction Assessment, and the cyber risk team no longer guesses which vulnerabilities matter most. Instead, they identify several externally exposed areas of their attack surface, then use the feature to surgically test for exploitability across these exposed endpoints. Within 72 hours, they receive a report:  

Positive outcome: Based on information in the html or the headers it seems that a vulnerable software version is running on an externally exposed infrastructure. By running a targeted attack on this infrastructure, we can confirm that it cannot be abused.

Negative outcome: Based on information in the html or the headers it seems that a vulnerable software version is running on an externally exposed infrastructure. By running a targeted attack on this infrastructure, we can confirm that it can be exploited, so we can predict it being exploited.

This second outcome changes everything. The team immediately prioritizes the exploitable asset for patching and takes the necessary adjustments to mitigate exposure until the fix is deployed.

Instead of spreading their resources thin across dozens of alerts, they focus on what poses a real threat, saving time, reducing risk, and demonstrating actionable results to stakeholders.

Conclusion

Exploit Predication Assessment bolsters Darktrace’s commitment to proactive cybersecurity. It supports intelligent prioritization of vulnerabilities, keeping organizations ahead of emerging threats. With this new addition to / Attack Surface Management, teams have another tool to empower a more efficient approach to addressing security gaps in real-time.

Stay tuned for more updates and insights on how Darktrace continues to develop a culture of proactive security across the entire ActiveAI Security Platform.

[related-resource]

Continue reading
About the author
Kelland Goodin
Product Marketing Specialist
Your data. Our AI.
Elevate your network security with Darktrace AI