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
Max Heinemeyer
Global Field CISO
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14
Mar 2021
What is Remote Desktop Protocol?
With the rise of the dynamic workforce, IT teams have been forced to rely on remote access more than ever before. There are now almost five million Remote Desktop Protocol (RDP) servers exposed to the Internet – around two million more than before the pandemic. Remote desktops are an essential feature for the majority of companies and yet are often exploited by cyber-criminals. Events such as the Florida water plant incident, where an attacker attempted to manipulate the chemical concentration in the water supply of a whole city, show how fatal the consequences of such a cyber-threat can be.
Last month, Darktrace detected a server-side attack at a technology company in the APAC region. The hackers brute-forced an RDP server and attempted to spread throughout the organization. The early detection of this breach was crucial in stopping the cyber-criminals before they could create a botnet and use it to cause serious damage, potentially launching a ransomware or distributed denial-of-service (DDoS) attack.
All it takes is one vulnerable RDP server for a threat actor to gain an initial foothold into an organization and spread laterally to build their botnet army. A bot is simply an infected device which can be controlled by a malicious third party; once a network of these hosts has been accumulated, a hacker can perform a range of actions, including:
Exfiltration of user credentials and payment data
Uploading Trojan malware to the server, which opens a backdoor to the system while masquerading as legitimate software
Deploying ransomware, as seen last year in a Dharma attack
Renting out access to the company’s infrastructure to other threat actors
In fact, there is little an attacker can’t do once they have gained remote access to these devices. Botnet malware tends to contain self-updating functions that allow the owner to add or remove functionality. And because the attackers are using legitimate administrative RDP credentials, it is extremely difficult for traditional security tools to detect this malicious activity until it is far too late.
DDoS for hire: A cyber-criminal enterprise
The commerce of cyber-crime has boomed in recent years, further complicating matters. There are now subscription-based and rental models easily available on the Dark Web for a range of illegal activities from Ransomware-as-a-Service to private data auctions. As a result, it is becoming increasingly common for attackers to infect servers and sell the use of these bots online. DDoS for hire services offer access to botnets for as little as $20 per hour. In fact, some of these kits are even legal and market themselves as ‘IP stressers’ or ‘booters’, which can be used legitimately to test the resilience of a website, but are often exploited and used to take down sites and networks.
These developments have sparked a new wave in DDoS and botnet malware attacks as hackers capitalize on the added financial incentive to create botnets and rent them on the Dark Web. ‘Botnet builder’ tools help low-skilled attackers create bots by providing botnet malware and assisting with the initial infection. Sophisticated RDP attacks have blossomed as a result of these kits, which lower the skill-threshold of such attacks and thus make them widely accessible.
Automated RDP attack under the microscope
Figure 1: A timeline of the attack
An Internet-facing RDP server hosting an online games site was recently compromised at a technology company with around 500 devices on its network. The attacker used brute force to glean the correct password and gain remote access to the desktop. It was at this point that Darktrace’s Cyber AI began to detect unusual administrative RDP connections from rare external locations.
In many ways, this incident is typical of an RDP compromise. Credential brute-forcing is a common initial vector for server-side attacks, alongside credential stuffing and exploiting vulnerabilities. In this case, the threat actor likely planned to utilize the exposed server as a pivot point to infect other internal and external devices, possibly to create a botnet-for-hire or exfiltrate sensitive information.
Figure 2: Cyber AI Analyst highlights unusual connections to internal IP addresses from an example breach device
Approximately 14 hours after this compromise, the attacker downloaded multiple files from rare domains. Over the next 18 hours the attacker made over 4.4 million internal and external connection attempts on port 445 using the vulnerable SMBv1 protocol. The majority of these attempts were SMB Session Failures using the credential “administrator”. The server engaged in successful SMB sessions with over 270 internal and external IP addresses.
Outgoing connections to rare but benign locations on ports normally used internally may not match a specific attack profile, meaning they are missed by signature-based security tools. However, despite a lack of threat intelligence on the multiple file download sources, Darktrace’s AI was able to observe the highly unusual nature of the activity, leading to high-confidence detections.
Figure 3: An example graph from Darktrace’s Threat Visualizer showing a large increase in the number of anomalous external connections
Botnet malware and automation
The speed of movement and lack of data exfiltration in this incident suggest that the attack was automated, likely with the help of botnet builder tools. The use of automation to accelerate and mask the breach could have led to severe consequences had Darktrace not alerted the security team in the initial stages.
Attacks against Internet-facing RDP servers remain one of the most common initial infection vectors. With the rise of automated scanning services and botnet malware tools, the ease of compromise has shot up. It is only matter of time before exposed servers are exploited. Furthermore, heavily automated attacks are constantly running and can spread rapidly across the organization. In such cases, it is vital for security teams to be made aware of malicious activity on devices as quickly as possible.
Darktrace’s AI not only pinpointed by itself that the infection had originated on a specific RDP server, it also detected every step of the attack in real time, despite a lack of clear existing signatures. Self-learning AI detects anomalous activity for users and devices across the digital environment and is therefore crucial in shutting down threats at machine speed. Moreover, the visibility provided by Darktrace DETECT greatly reduces the attack surface and identifies badly maintained shadow IT, providing an extra layer of security over the digital business.
Thanks to Darktrace analyst Tom McHale for his insights on the above threat find.
Darktrace model detections:
Compliance / Internet Facing RDP Server
Anomalous File / Zip or Gzip from Rare External Location
Anomalous File / Incoming RAR File
Anomalous File / EXE from Rare External Location
Anomalous File / Internet Facing System File Download
Experimental / Rare Endpoint with Young Certificate
Anomalous Connection / New User Agent to IP Without Hostname
Device / New User Agent and New IP
Anomalous File / Anomalous Octet Stream
Device / Anomalous SMB Followed By Multiple Model Breaches
Device / Anomalous RDP Followed By Multiple Model Breaches
Compliance / External Windows Communications
Anomalous Server Activity / Outgoing from Server
Device / Increased External Connectivity
Device / SMB Session Bruteforce
Unusual Activity / Unusual Activity from New Device
Device / Network Scan - Low Anomaly Score
Device / Large Number of Connections to New Endpoints
Device / High Volume of Connections from Guest or New Device
Compromise / Suspicious File and C2
Anomalous File / Script from Rare Location
Anomalous File / Multiple EXE from Rare External Locations
Device / Initial Breach Chain Compromise
Anomalous Server Activity / Rare External from Server
Compromise / High Volume of Connections with Beacon Score
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.
SEO Poisoning and Fake PuTTY sites: Darktrace’s Investigation into the Oyster backdoor
What is SEO poisoning?
Search Engine Optimization (SEO) is the legitimate marketing technique of improving the visibility of websites in organic search engine results. Businesses, publishers, and organizations use SEO to ensure their content is easily discoverable by users. Techniques may include optimizing keywords, creating backlinks, or even ensuring mobile compatibility.
SEO poisoning occurs when attackers use these same techniques for malicious purposes. Instead of improving the visibility of legitimate content, threat actors use SEO to push harmful or deceptive websites to the top of search results. This method exploits the common assumption that top-ranking results are trustworthy, leading users to click on URLs without carefully inspecting them.
As part of SEO poisoning, the attacker will first register a typo-squatted domain, slightly misspelled or otherwise deceptive versions of real software sites, such as putty[.]run or puttyy[.]org. These sites are optimized for SEO and often even backed by malicious Google ads, increasing the visibility when users search for download links. To achieve that, threat actors may embed pages with strategically chosen, high-value keywords or replicate content from reputable sources to elevate the domain’s perceived authority in search engine algorithms [4]. In more advanced operations, these tactics are reinforced with paid promotion, such as Google ads, enabling malicious domains to appear above organic search results as sponsored links. This placement not only accelerates visibility but also impacts an unwarranted sense of legitimacy to unsuspected users.
Once a user lands on one of these fake pages, they are presented with what looks like a legitimate software download option. Upon clicking the download indicator, the user will be redirected to another separate domain that actually hosts the payload. This hosting domain is usually unrelated to the nominally referenced software. These third-party sites can involve recently registered domains but may also include legitimate websites that have been recently compromised. By hosting malware on a variety of infrastructure, attackers can prolong the availability of distribution methods for these malicious files before they are taken down.
What is the Oyster backdoor?
Oyster, also known as Broomstick or CleanUpLoader, is a C++ based backdoor malware first identified in July 2023. It enables remote access to infected systems, offering features such as command-line interaction and file transfers.
Oyster has been widely adopted by various threat actors, often as an entry point for ransomware attacks. Notable examples include Vanilla Tempest and Rhysida ransomware groups, both of which have been observed leveraging the Oyster backdoor to enhance their attack capabilities. Vanilla Tempest is known for using Oyster’s stealth persistence to maintain long-term access within targeted networks, often aligning their operations with ransomware deployment [5]. Rhysida has taken this further by deploying Oyster as an initial access tool in ransomware campaigns, using it to conduct reconnaissance and move laterally before executing encryption activities [6].
Once installed, the backdoor gathers basic system information before communicating with a command-and-control (C2) server. The malware largely relies on a ‘cmd.exe’ instance to execute commands and launch other files [1].
In previous SEO poisoning cases, the file downloaded from the fake pages is not just PuTTY, but a trojanized version that includes the stealthy Oyster backdoor. PuTTY is a free and open-source terminal emulator for Windows that allows users to connect to remote servers and devices using protocols like SSH and Telnet. In the recent campaign, once a user visits the fake software download site, ranked highly through SEO poisoning, the malicious payload is downloaded through direct user interaction and subsequently installed on the local device, initiating the compromise. The malware then performs two actions simultaneously: it installs a fully functional version of PuTTY to avoid user suspicion, while silently deploying the Oyster backdoor. Given PuTTY’s nature, it is prominently used by IT administrators with highly privileged account as opposed to standard users in a business, possibly narrowing the scope of the targets.
Oyster’s persistence mechanism involves creating a Windows Scheduled Task that runs every few minutes. Notably, the infection uses Dynamic Link Library (DLL) side loading, where a malicious DLL, often named ‘twain_96.dll’, is executed via the legitimate Windows utility ‘rundll32.exe’, which is commonly used to run DLLs [2]. This technique is frequently used by malicious actors to blend their activity with normal system operations.
Darktrace’s Coverage of the Oyster Backdoor
In June 2025, security analysts at Darktrace identified a campaign leveraging search engine manipulation to deliver malware masquerading as the popular SSH client, PuTTY. Darktrace / NETWORK’s anomaly-based detection identified signs of malicious activity, and when properly configured, its Autonomous Response capability swiftly shut down the threar before it could escalate into a more disruptive attack. Subsequent analysis by Darktrace’s Threat Research team revealed that the payload was a variant of the Oyster backdoor.
The first indicators of an emerging Oyster SEO campaign typically appeared when user devices navigated to a typosquatted domain, such as putty[.]run or putty app[.]naymin[.]com, via a TLS/SSL connection.
Figure 1: Darktrace’s detection of a device connecting to the typosquatted domain putty[.]run.
The device would then initiate a connection to a secondary domain that hosts the malicious installer, likely triggered by user interaction with redirect elements on the landing page. This secondary site may not have any immediate connection to PuTTY itself but is instead a hijacked blog, a file-sharing service, or a legitimate-looking content delivery subdomain.
Figure 2: Darktrace’s detection of the device making subsequent connections to the payload domain.
Following installation, multiple affected devices were observed attempting outbound connectivity to rare external IP addresses, specifically requesting the ‘/secure’ endpoint as noted within the declared URIs. After the initial callback, the malware continued communicating with additional infrastructure, maintaining its foothold and likely waiting for tasking instructions. Communication patterns included:
· Endpoints with URIs /api/kcehc and /api/jgfnsfnuefcnegfnehjbfncejfh
· Endpoints with URI /reg and user agent “WordPressAgent”, “FingerPrint” or “FingerPrintpersistent”
This tactic has been consistently linked to the Oyster backdoor, which has shown similar URI patterns across multiple campaigns [3].
Darktrace analysts also noted the sophisticated use of spoofed user agent strings across multiple investigated customer networks. These headers, which are typically used to identify the application making an HTTP request, are carefully crafted to appear benign or mimic legitimate software. One common example seen in the campaign is the user agent string “WordPressAgent”. While this string references a legitimate web application or plugin, it does not appear to correspond to any known WordPress services or APIs. Its inclusion is most likely designed to mimic background web traffic commonly associated with WordPress-based content management systems.
Figure 3: Cyber AI Analyst investigation linking the HTTP C2 activity.
Case-Specific Observations
While the previous section focused on tactics and techniques common across observed Oyster infections, a closer examination reveals notable variations and unique elements in specific cases. These distinct features offer valuable insights into the diverse operational approaches employed by threat actors. These distinct features, from unusual user agent strings to atypical network behavior, offer valuable insights into the diverse operational approaches employed by the threat actors. Crucially, the divergence in post-exploitation activity reflects a broader trend in the use of widely available malware families like Oyster as flexible entry points, rather than fixed tools with a single purpose. This modular use of the backdoor reflects the growing Malware-as-a-Service (MaaS) ecosystem, where a single initial infection can be repurposed depending on the operator’s goals.
From Infection to Data Egress
In one observed incident, Darktrace observed an infected device downloading a ZIP file named ‘host[.]zip’ via curl from the URI path /333/host[.]zip, following the standard payload delivery chain. This file likely contained additional tools or payloads intended to expand the attacker’s capabilities within the compromised environment. Shortly afterwards, the device exhibited indicators of probable data exfiltration, with outbound HTTP POST requests featuring the URI pattern: /upload?dir=NAME_FOLDER/KEY_KEY_KEY/redacted/c/users/public.
This format suggests the malware was actively engaged in local host data staging and attempting to transmit files from the target machine. The affected device, identified as a laptop, aligns with the expected target profile in SEO poisoning scenarios, where unsuspecting end users download and execute trojanized software.
Irregular RDP Activity and Scanning Behavior
Several instances within the campaign revealed anomalous or unexpected Remote Desktop Protocol (RDP) sessions occurring shortly after DNS requests to fake PuTTY domains. Unusual RDP connections frequently followed communication with Oyster backdoor C2 servers. Additionally, Darktrace detected patterns of RDP scanning, suggesting the attackers were actively probing for accessible systems within the network. This behavior indicates a move beyond initial compromise toward lateral movement and privilege escalation, common objectives once persistence is established.
The presence of unauthorized and administrative RDP sessions following Oyster infections aligns with the malware’s historical role as a gateway for broader impact. In previous campaigns, Oyster has often been leveraged to enable credential theft, lateral movement, and ultimately ransomware deployment. The observed RDP activity in this case suggests a similar progression, where the backdoor is not the final objective but rather a means to expand access and establish control over the target environment.
Cryptic User Agent Strings?
In multiple investigated cases, the user agent string identified in these connections featured formatting that appeared nonsensical or cryptic. One such string containing seemingly random Chinese-language characters translated into an unusual phrase: “Weihe river is where the water and river flow.” Legitimate software would not typically use such wording, suggesting that the string was intended as a symbolic marker rather than a technical necessity. Whether meant as a calling card or deliberately crafted to frame attribution, its presence highlights how subtle linguistic cues can complicate analysis.
Figure 4: Darktrace’s detection of malicious connections using a user agent with randomized Chinese-language formatting.
Strategic Implications
What makes this campaign particularly noteworthy is not simply the use of Oyster, but its delivery mechanism. SEO poisoning has traditionally been associated with cybercriminal operations focused on opportunistic gains, such as credential theft and fraud. Its strength lies in casting a wide net, luring unsuspecting users searching for popular software and tricking them into downloading malicious binaries. Unlike other campaigns, SEO poisoning is inherently indiscriminate, given that the attacker cannot control exactly who lands on their poisoned search results. However, in this case, the use of PuTTY as the luring mechanism possibly indicates a narrowed scope - targeting IT administrators and accounts with high privileges due to the nature of PuTTY’s functionalities.
This raises important implications when considered alongside Oyster. As a backdoor often linked to ransomware operations and persistent access frameworks, Oyster is far more valuable as an entry point into corporate or government networks than small-scale cybercrime. The presence of this malware in an SEO-driven delivery chain suggests a potential convergence between traditional cybercriminal delivery tactics and objectives often associated with more sophisticated attackers. If actors with state-sponsored or strategic objectives are indeed experimenting with SEO poisoning, it could signal a broadening of their targeting approaches. This trend aligns with the growing prominence of MaaS and the role of initial access brokers in today’s cybercrime ecosystem.
Whether the operators seek financial extortion through ransomware or longer-term espionage campaigns, the use of such techniques blurs the traditional distinctions. What looks like a mass-market infection vector might, in practice, be seeding footholds for high-value strategic intrusions.
Credit to Christina Kreza (Cyber Analyst) and Adam Potter (Senior Cyber Analyst)
Appendices
MITRE ATT&CK Mapping
· T1071.001 – Command and Control – Web Protocols
· T1008 – Command and Control – Fallback Channels
· T0885 – Command and Control – Commonly Used Port
· T1571 – Command and Control – Non-Standard Port
· T1176 – Persistence – Browser Extensions
· T1189 – Initial Access – Drive-by Compromise
· T1566.002 – Initial Access – Spearphishing Link
· T1574.001 – Persistence – DLL
Indicators of Compromise (IoCs)
· 85.239.52[.]99 – IP address
· 194.213.18[.]89/reg – IP address / URI
· 185.28.119[.]113/secure – IP address / URI
· 185.196.8[.]217 – IP address
· 185.208.158[.]119 – IP address
· putty[.]run – Endpoint
· putty-app[.]naymin[.]com – Endpoint
· /api/jgfnsfnuefcnegfnehjbfncejfh
· /api/kcehc
Darktrace Model Detections
· Anomalous Connection / New User Agent to IP Without Hostname
· Anomalous Connection / Posting HTTP to IP Without Hostname
· Compromise / HTTP Beaconing to Rare Destination
· Compromise / Large Number of Suspicious Failed Connections
· Compromise / Beaconing Activity to External Rare
· Compromise / Quick and Regular Windows HTTP Beaconing
· Device / Large Number of Model Alerts
· Device / Initial Attack Chain Activity
· Device / Suspicious Domain
· Device / New User Agent
· Antigena / Network / Significant Anomaly / Antigena Breaches Over Time Block
The content provided in this blog is published by Darktrace for general informational purposes only and reflects our understanding of cybersecurity topics, trends, incidents, and developments at the time of publication. While we strive to ensure accuracy and relevance, the information is provided “as is” without any representations or warranties, express or implied. Darktrace makes no guarantees regarding the completeness, accuracy, reliability, or timeliness of any information presented and expressly disclaims all warranties.
Nothing in this blog constitutes legal, technical, or professional advice, and readers should consult qualified professionals before acting on any information contained herein. Any references to third-party organizations, technologies, threat actors, or incidents are for informational purposes only and do not imply affiliation, endorsement, or recommendation.
Darktrace, its affiliates, employees, or agents shall not be held liable for any loss, damage, or harm arising from the use of or reliance on the information in this blog.
The cybersecurity landscape evolves rapidly, and blog content may become outdated or superseded. We reserve the right to update, modify, or remove any content without notice.
The benefits of bringing together network and email security
In many organizations, network and email security operate in isolation. Each solution is tasked with defending its respective environment, even though both are facing the same advanced, multi-domain threats.
This siloed approach overlooks a critical reality: email remains the most common vector for initiating cyber-attacks, while the network is the primary stage on which those attacks progress. Without direct integration between these two domains, organizations risk leaving blind spots that adversaries can exploit.
A modern security strategy needs to unify email and network defenses, not just in name, but in how they share intelligence, conduct investigations, and coordinate response actions. Let’s take a look at how this joined-up approach delivers measurable technical, operational, and commercial benefits.
Technical advantages
Pre-alert intelligence: Gathering data before the threat strikes
Most security tools start working when something goes wrong – an unusual login, a flagged attachment, a confirmed compromise. But by then, attackers may already be a step ahead.
By unifying network and email security under a single AI platform (like the Darktrace Active AI Security Platform), you can analyze patterns across both environments in real time, even when there are no alerts. This ongoing monitoring builds a behavioral understanding of every user, device, and domain in your ecosystem.
That means when an email arrives from a suspicious domain, the system already knows whether that domain has appeared on your network before – and whether its behavior has been unusual. Likewise, when new network activity involves a domain first spotted in an email, it’s instantly placed in the right context.
This intelligence isn’t built on signatures or after-the-fact compromise indicators – it’s built on live behavioral baselines, giving your defenses the ability to flag threats before damage is done.
Alert-related intelligence: Connecting the dots in real time
Once an alert does fire, speed and context matter. The Darktrace Cyber AI Analyst can automatically investigate across both environments, piecing together network and email evidence into a single, cohesive incident.
Instead of leaving analysts to sift through fragmented logs, the AI links events like a phishing email to suspicious lateral movement on the recipient’s device, keeping the full attack chain intact. Investigations that might take hours – or even days – can be completed in minutes, with far fewer false positives to wade through.
This is more than a time-saver. It ensures defenders maintain visibility after the first sign of compromise, following the attacker as they pivot into network infrastructure, cloud services, or other targets. That cross-environment continuity is impossible to achieve with disconnected point solutions or siloed workflows.
Operational advantages
Streamlining SecOps across teams
In many organizations, email security is managed by IT, while network defense belongs to the SOC. The result? Critical information is scattered between tools and teams, creating blind spots just when you need clarity.
When email and network data flow into a single platform, everyone is working from the same source of truth. SOC analysts gain immediate visibility into email threats without opening another console or sending a request to another department. The IT team benefits from the SOC’s deeper investigative context.
The outcome is more than convenience: it’s faster, more informed decision-making across the board.
Reducing time-to-meaning and enabling faster response
A unified platform removes the need to manually correlate alerts between tools, reducing time-to-meaning for every incident. Built-in AI correlation instantly ties together related events, guiding analysts toward coordinated responses with higher confidence.
Instead of relying on manual SIEM rules or pre-built SOAR playbooks, the platform connects the dots in real time, and can even trigger autonomous response actions across both environments simultaneously. This ensures attacks are stopped before they can escalate, regardless of where they begin.
Commercial advantages
While purchasing “best-of-breed" for all your different tools might sound appealing, it often leads to a patchwork of solutions with overlapping costs and gaps in coverage. However good a “best-in-breed" email security solution might be in the email realm, it won't be truly effective without visibility across domains and an AI analyst piecing intelligence together. That’s why we think “best-in-suite" is the only “best-in-breed" approach that works – choosing a high-quality platform ensures that every new capability strengthens the whole system.
On top of that, security budgets are under constant pressure. Managing separate vendors for email and network defense means juggling multiple contracts, negotiating different SLAs, and stitching together different support models.
With a single provider for both, procurement and vendor management become far simpler. You deal with one account team, one support channel, and one unified strategy for both environments. If you choose to layer on managed services, you get consistent expertise across your whole security footprint.
Even more importantly, an integrated AI platform sets the stage for growth. Once email and network are under the same roof, adding coverage for other attack surfaces – like cloud or identity – is straightforward. You’re building on the same architecture, not bolting on new point solutions that create more complexity.
