Bytesize Security: A Guide to HTML Phishing Attachments
10
Aug 2022
Darktrace guides you through the common signs of HTML phishing attachments, including common phishing emails, clever impersonations, fake webpages, and more.
One of the most common types of phishing email seen by the Darktrace SOC, involves the use of HTML attachments (Figure 1). These emails make use of an attachment to hide redirects to overtly malicious or suspicious domains. Some even impersonate legitimate web pages and send any entered or captured information back to the attacker's infrastructure once opened or filled out by the recipient. Indicators of these attempts can be identified from a few key patterns found across multiple emails.
Figure 1: An example of a suspicious HTML attachment containing dynamic content
A typical feature of these HTML attachments is the use of a generic-sounding filename that relates to the message's subject line, but with no specific information pertaining to the recipient or their line of business. These files almost always contain some form of Javascript code, as they often make use of external Javascript libraries to accomplish whatever goal is being pursued. For example, an attacker might use Javascript to convincingly impersonate a trustworthy website and trick the recipient into providing credentials or sensitive information, or they might use it to deploy malware and get a foothold on the device for further compromise once opened. This can be further identified by the presence of certain links in the HTML file itself (Figure 2).
Figure 2: The HTML file previously referenced contained multiple rare and suspicious links
Figure 2 above is an example of an HTML file containing multiple links with calls for .js files. This shows that the attachment contains Javascript and is making calls for external libraries for an undetermined purpose.
Another common red flag is when the file contains links to common Product or Service images from domains wholly unrelated to those services, as seen below (Figure 3).
Figure 3: An example of an unusual .png call from a rare domain. The subsequent image called is for a company with no apparent relation to the hosting domain
The examples above imply an obvious (and poor) attempt by the HTML file to impersonate a Microsoft webpage, likely a fake login page set up for credential harvesting, as the ‘Microsoft’ logo is being pulled from a domain entirely unrelated to Microsoft or any common image-hosting service.
Rather than impersonating a website directly in the file and loading resources from external sources, these HTML files will instead directly point toward a webpage that already contains these elements. This comes with its own set of pros and cons: by hosting their phishing page in a public setting, they are far more likely to be taken down, however it may be easier to appear legitimate than if they were to build it all out in the HTML file itself.
The final routine element in these types of HTML phishing emails is the mechanism by which the attacker intends to receive any successfully scammed credentials or information. If the fake webpage is entirely contained in the HTML file, this often presents as a suspicious PHP link present in the file itself (Figure 4).
Figure 4: Phishing HTMLs often include links to rare domains with PHP destinations as an indication that it will engage in some form of HTTP POST communication
PHP calls suggest that some part of the webpage is intended to submit an HTTP POST or equivalent ‘submission’ call, often present in the ‘Login’ button in these scenarios. After the victim clicks this button, the webpage sends all the form-submission items to the endpoint hosting the PHP page, which is commonly an indicator of the webserver hosting the attacker infrastructure running the phishing attack.
If the HTML file redirects to an externally hosted phishing page, identical PHP links are often found in the source code of those pages (Figure 5). This serves the same function as sending any entered credentials back to the attacker.
Figure 5: The source-code of an external-hosted phishing page, showing calls for PHP pages hosted on alternate attacker infrastructure
The process of HTML attacks is so standardized that they are commonly released in the form of easily deployable phishing kits. These can be deployed on unsuspecting compromised webservers with little to no modification, resulting in virtually identical attacks being seen year-round. WordPress seems to be a prime target for hosting such attacks, with the site owners often becoming unsuspecting victims in propagating these phishing campaigns. An unfortunate side effect of these kits being readily available is that the attackers often don't bother to set any sort of access restrictions on their phishing servers once established, which can result in their entire setup being publicly viewable with a simple link modification. One example is seen below (Figure 6).
Figure 6: The parent directory of the website hosting a suspicious PHP page was fully accessible without restriction
In this incident, the website hosting the PHP link seen earlier had a publicly accessible parent directory structure, where both the PHP file above and an additional suspicious .txt file could be seen. This .txt file appears to be where any information submitted by victims ultimately ended up written to (Figure 7).
Figure 7: The TXT file in the parent directory above appeared to contain the login information that was likely submitted to the PHP page referred to in the initial HTML attachment
Figure 7 above presents the unusual risk of not only having the victims’ credentials at the disposal of the original attacker, but also potentially exposed to any malicious actor that can get creative with a web-crawler to identify key elements of the files used by these particular phishing kits.
Fortunately, due to the standardized nature of these ready-made phishing kits, these types of attacks often conform to a series of common behaviors that Darktrace / EMAIL excels in identifying. Despite being a popular technique, it is extremely rare for attempts using this HTML attachment method to successfully get through a correct Darktrace / EMAIL deployment. Overall, this means one less risk for the end user to worry about.
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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.
Phishing and Persistence: Darktrace’s Role in Defending Against a Sophisticated Account Takeover
The exploitation of SaaS platforms
As businesses continue to grow and evolve, the need for sharing ideas through productivity and cloud Software-as-a-Service (SaaS) platforms is becoming increasingly crucial. However, these platforms have also become prime targets for cyber attackers.
Threat actors often exploit these widely-used services to gain unauthorized access, steal sensitive information, and disrupt business operations. The growing reliance on SaaS platforms makes them attractive entry points for cybercriminals, who use sophisticated techniques such as phishing, social engineering, and malware to compromise these systems.
Services like Microsoft 365 are regularly targeted by threat actors looking for an entry point into an organization’s environment to carry out malicious activities. Securing these platforms is crucial to protect business data and ensure operational continuity.
Darktrace / EMAIL detection of the phishing attack
In a recent case, Darktrace observed a customer in the manufacturing sector receiving a phishing email that led to a threat actor logging in and creating an email rule. Threat actors often create email rules to move emails to their inbox, avoiding detection. Additionally, Darktrace detected a spoofed domain registered by the threat actor. Despite already having access to the customer’s SaaS account, the actor seemingly registered this domain to maintain persistence on the network, allowing them to communicate with the spoofed domain and conduct further malicious activity.
Darktrace / EMAIL can help prevent compromises like this one by blocking suspicious emails as soon as they are identified. Darktrace’s AI-driven email detection and response recognizes anomalies that might indicate phishing attempts and applies mitigative actions autonomously to prevent the escalation of an attack.
Unfortunately, in this case, Darktrace was not configured in Autonomous Response mode at the time of the attack, meaning actions had to be manually applied by the customer’s security team. Had it been fully enabled, it would have held the emails, preventing them from reaching the intended recipient and stopping the attack at its inception.
However, Darktrace’s Managed Threat Detection alerted the Security Operations Center (SOC) team to the compromise, enabling them to thoroughly investigate the incident and notify the customer before further damage could occur.
The Managed Threat Detection service continuously monitors customer networks for suspicious activities that may indicate an emerging threat. When such activities are detected, alerts are sent to Darktrace’s expert Cyber Analysts for triage, significantly speeding up the remediation process.
Attack Overview
On May 2, 2024, Darktrace detected a threat actor targeting a customer in the manufacturing sector then an unusual login to their SaaS environment was observed prior to the creation of a new email rule.
Darktrace immediately identified the login as suspicious due to the rarity of the source IP (31.222.254[.]27) and ASN, coupled with the absence of multi-factor authentication (MFA), which was typically required for this account.
The new email rule was intended to mark emails as read and moved to the ‘Conversation History’ folder for inbound emails from a specific domain. The rule was named “….,,,”, likely the attacker attempting to setup their new rule with an unnoteworthy name to ensure it would not be noticed by the account’s legitimate owner. Likewise, by moving emails from a specific domain to ‘Conversation History’, a folder that is rarely used by most users, any phishing emails sent by that domain would remain undetected by the user.
Figure 1: Darktrace’s detection of the unusual SaaS login and subsequent creation of the new email rule “….,,,”.
The domain in question was identified as being newly registered and an example of a typosquat domain. Typosquatting involves registering new domains with intentional misspelling designed to convince users to visit fake, and often malicious, websites. This technique is often used in phishing campaigns to create a sense of legitimacy and trust and deceive users into providing sensitive information. In this case, the suspicious domain closely resembled several of the customer’s internal domains, indicating an attempt to impersonate the organization’s legitimate internal sites to gain the target’s trust. Furthermore, the creation of this lookalike domain suggests that the attack was highly targeted at this specific customer.
Interestingly, the threat actor registered this spoofed domain despite already having account access. This was likely intended to ensure persistence on the network without having to launch additional phishing attacks. Such use of spoofed domain could allow an attacker to maintain a foothold in their target network and escalate their malicious activities without having to regain access to the account. This persistence can be used for various purposes, including data exfiltration, spreading malware, or launching further attacks.
Following this, Darktrace detected a highly anomalous email being sent to the customer’s account from the same location as the initial unsual SaaS login. Darktrace’s anomaly-based detection is able to identify threats that human security teams and traditional signature-based methods might miss. By analyzing the expected behavior of network users, Darktrace can recognize the subtle deviations from the norm that may indicate malicious activity. Unfortunately, in this instance, without Darktrace’s Autonomous Response capability enabled, the phishing email was able to successfully reach the recipient. While Darktrace / EMAIL did suggest that the email should be held from the recipients inbox, the customer was required to manually approve it.
Despite this, the Darktrace SOC team were still able to support the customer as they were subscribed to the Managed Threat Detection service. Following the detection of the highlight anomalous activity surrounding this compromise, namely the unusual SaaS login followed by a new email rule, an alert was sent to the Darktrace SOC for immediate triage, who then contacted the customer directly urging immediate action.
Conclusion
This case underscores the need to secure SaaS platforms like Microsoft 365 against sophisticated cyber threats. As businesses increasingly rely on these platforms, they become prime targets for attackers seeking unauthorized access and disruption.
Darktrace’s anomaly-based detection and response capabilities are crucial in identifying and mitigating such threats. In this instance, Darktrace detected a phishing email that led to a threat actor logging in and creating a suspicious email rule. The actor also registered a spoofed domain to maintain persistence on the network.
Darktrace / EMAIL, with its AI-driven detection and analysis, can block suspicious emails before they reach the intended recipient, preventing attacks at their inception. Meanwhile, Darktrace’s SOC team promptly investigated the activity and alerted the customer to the compromise, enabling them to take immediate action to remediate the issue and prevent any further damage.
Credit to Vivek Rajan (Cyber Security Analyst) and Ryan Traill (Threat Content Lead).
Lifting the Fog: Darktrace’s Investigation into Fog Ransomware
Introduction to Fog Ransomware
As ransomware attacks continue to be launched at an alarming rate, Darktrace’s Threat Research team has identified that familiar strains like Akira, LockBit, and BlackBasta remain among the most prevalent threats impacting its customers, as reported in the First 6: Half-Year Threat Report 2024. Despite efforts by law agencies, like dismantling the infrastructure of cybercriminals and shutting down their operations [2], these groups continue to adapt and evolve.
As such, it is unsurprising that new ransomware variants are regularly being created and launched to get round law enforcement agencies and increasingly adept security teams. One recent example of this is Fog ransomware.
What is Fog ransomware?
Fog ransomware is strain that first appeared in the wild in early May 2024 and has been observed actively using compromised virtual private network (VPN) credentials to gain access to organization networks in the education sector in the United States.
Darktrace's detection of Fog Ransomware
In June 2024, Darktrace observed instances of Fog ransomware across multiple customer environments. The shortest time observed from initial access to file encryption in these attacks was just 2 hours, underscoring the alarming speed with which these threat actors can achieve their objectives.
Darktrace identified key activities typical of a ransomware kill chain, including enumeration, lateral movement, encryption, and data exfiltration. In most cases, Darktrace was able to successfully halt the progression Fog attacks in their early stages by applying Autonomous Response actions such as quarantining affected devices and blocking suspicious external connections.
To effectively illustrate the typical kill chain of Fog ransomware, this blog focuses on customer environments that did not have Darktrace’s Autonomous Response enabled. In these cases, the attack progressed unchecked and reached its intended objectives until the customer received Darktrace’s alerts and intervened.
Darktrace’s Coverage of Fog Ransomware
Initial Intrusion
After actors had successfully gained initial access into customer networks by exploiting compromised VPN credentials, Darktrace observed a series of suspicious activities, including file shares, enumeration and extensive scanning. In one case, a compromised domain controller was detected making outgoing NTLM authentication attempts to another internal device, which was subsequently used to establish RDP connections to a Windows server running Hyper-V.
Given that the source was a domain controller, the attacker could potentially relay the NTLM hash to obtain a domain admin Kerberos Ticket Granting Ticket (TGT). Additionally, incoming NTLM authentication attempts could be triggered by tools like Responder, and NTLM hashes used to encrypt challenge response authentication could be abused by offline brute-force attacks.
Darktrace also observed the use of a new administrative credential on one affected device, indicating that malicious actors were likely using compromised privileged credentials to conduct relay attacks.
Establish Command-and-Control Communication (C2)
In many instances of Fog ransomware investigated by Darktrace’s Threat Research team, devices were observed making regular connections to the remote access tool AnyDesk. This was exemplified by consistent communication with the endpoint “download[.]anydesk[.]com” via the URI “/AnyDesk.exe”. In other cases, Darktrace identified the use of another remote management tool, namely SplashTop, on customer servers.
In ransomware attacks, threat actors often use such legitimate remote access tools to establish command-and-control (C2) communication. The use of such services not only complicates the identification of malicious activities but also enables attackers to leverage existing infrastructure, rather than having to implement their own.
Internal Reconnaissance
Affected devices were subsequently observed making an unusual number of failed internal connections to other internal locations over ports such as 80 (HTTP), 3389 (RDP), 139 (NetBIOS) and 445 (SMB). This pattern of activity strongly indicated reconnaissance scanning behavior within affected networks. A further investigation into these HTTP connections revealed the URIs “/nice ports”/Trinity.txt.bak”, commonly associated with the use of the Nmap attack and reconnaissance tool.
Simultaneously, some devices were observed engaging in SMB actions targeting the IPC$ share and the named pipe “srvsvc” on internal devices. Such activity aligns with the typical SMB enumeration tactics, whereby attackers query the list of services running on a remote host using a NULL session, a method often employed to gather information on network resources and vulnerabilities.
Lateral Movement
As attackers attempted to move laterally through affected networks, Darktrace observed suspicious RDP activity between infected devices. Multiple RDP connections were established to new clients, using devices as pivots to propagate deeper into the networks, Following this, devices on multiple networks exhibited a high volume of SMB read and write activity, with internal share drive file names being appended with the “.flocked” extension – a clear sign of ransomware encryption. Around the same time, multiple “readme.txt” files were detected being distributed across affected networks, which were later identified as ransom notes.
Further analysis of the ransom note revealed that it contained an introduction to the Fog ransomware group, a summary of the encryption activity that had been carried out, and detailed instructions on how to communicate with the attackers and pay the ransom.
Figure 1: Packet capture (PCAP) of the ransom note file titled “readme.txt”.
Data Exfiltration
In one of the cases of Fog ransomware, Darktrace’s Threat Research team observed potential data exfiltration involving the transfer of internal files to an unusual endpoint associated with the MEGA file storage service, “gfs302n515[.]userstorage[.]mega[.]co[.]nz”.
This exfiltration attempt suggests the use of double extortion tactics, where threat actors not only encrypt victim’s data but also exfiltrate it to threaten public exposure unless a ransom is paid. This often increases pressure on organizations as they face the risk of both data loss and reputational damage caused by the release of sensitive information.
Darktrace’s Cyber AI Analyst autonomously investigated what initially appeared to be unrelated events, linking them together to build a full picture of the Fog ransomware attack for customers’ security teams. Specifically, on affected networks Cyber AI Analyst identified and correlated unusual scanning activities, SMB writes, and file appendages that ultimately suggested file encryption.
Figure 2: Cyber AI Analyst’s analysis of encryption activity on one customer network.
Figure 3: Cyber AI Analysts breakdown of the investigation process between the linked incident events on one customer network.
Safeguarding vulnerable sectors with real-time ransomware mitigation
As novel and fast-moving ransomware variants like Fog persist across the threat landscape, the time taken for from initial compromise to encryption has significantly decreased due to the enhanced skill craft and advanced malware of threat actors. This trend particularly impacts organizations in the education sector, who often have less robust cyber defenses and significant periods of time during which infrastructure is left unmanned, and are therefore more vulnerable to quick-profit attacks.
Traditional security methods may fall short against these sophisticated attacks, where stealthy actors evade detection by human-managed teams and tools. In these scenarios Darktrace’s AI-driven product suite is able to quickly detect and respond to the initial signs of compromise through autonomous analysis of any unusual emerging activity.
When Darktrace’s Autonomous Response capability was active, it swiftly mitigated emerging Fog ransomware threats by quarantining devices exhibiting malicious behavior to contain the attack and blocking the exfiltration of sensitive data, thus preventing customers from falling victim to double extortion attempts.
Darktrace’s First 6: Half-Year Threat Report 2024 highlights the latest attack trends and key threats observed by the Darktrace Threat Research team in the first six months of 2024.
Focuses on anomaly detection and behavioral analysis to identify threats
Maps mitigated cases to known, publicly attributed threats for deeper context
Offers guidance on improving security posture to defend against persistent threats
Appendices
Credit to Qing Hong Kwa (Senior Cyber Analyst and Deputy Analyst Team Lead, Singapore) and Ryan Traill (Threat Content Lead)
Darktrace Model Detections:
- Anomalous Server Activity::Anomalous External Activity from Critical Network Device
- Anomalous Connection::SMB Enumeration
- Anomalous Connection::Suspicious Read Write Ratio and Unusual SMB
- Anomalous Connection::Uncommon 1 GiB Outbound
- Anomalous File::Internal::Additional Extension Appended to SMB File
