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Black Kingdom Returns to Exploit Zero-Day Vulnerabilities in Unpatched Microsoft Exchange Servers

Arete Analysis

Summary

Black Kingdom ransomware recently resurfaced to target a zero-day vulnerability in Microsoft Exchange servers.

Earlier this month, Microsoft released a statement notifying the public of a zero-day exploit that affected its on-premises Exchange Servers, versions 2013 through 2019. Within a week, Arete incident responders spoke to approximately 100 affected companies — small- and medium-sized companies that had no reason to be on the target list of the Microsoft-coined hacking group Hafnium.

The worst part about this zero-day announcement was that sometime around March 10, the threat group published the proof-of-concept (PoC) code to GitHub. The irony: Microsoft owns GitHub; GitHub hosted the PoC code.  
 
Since that publication, the security community has been speculating on a second wave of attacks involving this exploit. A wave that would be worse than the first. A wave that would be linked to all sorts of corporate business interruptions from the likes of data exfiltration and ransomware.

How right was the speculation? Basically, the community #nailedit.

Second Wave of Ransomware Attacks Now Hitting

On March 12, DarkReading.com reported that a hacking group was deploying the ransomware variant,  DEARCRY!, which was affecting unpatched Microsoft Exchange Servers.  
 
Now, Arete has identified a second variant — Black Kingdom — that is also exploiting unpatched Microsoft Exchange Servers. The Black Kingdom variant made media headlines when it briefly surfaced and started to exploit the Pulse VPN zero-day in the early summer months of 2020. After, the group quietly disappeared … until this week. 

How the Black Kingdom Ransomware Group Operates

After the Black Kingdom group gains access to a network, they will perform some reconnaissance and start their encryption, leaving behind a fitting ransom note entitled decrypt_file.txt to announce their return.

Black Kingdom’s ransom note is one of the longest notes left behind by ransomware groups. It provides explicit instructions on how to contact them, the ransom amount, the Bitcoin wallet ID, and a message stating that they exfiltrated data from the network. The note also states that refusal to pay would lead them to publish the attack and stolen files on social media.

Steps you can take to help prevent unauthorized access to your network

The Microsoft Exchange vulnerability is widespread and, if like many other previously reported vulnerabilities, may not get patched automatically or quickly enough to prevent ransomware groups and any other malicious actors from gaining unauthorized access to your network.  
 
Follow these recommended steps protect your business and systems from unauthorized access: 

1. Immediately patch the Exchange Server. If the system is not or cannot be patched, disable Outlook for Web Access (OWA). If you cannot disable OWA, remove the Exchange Server from the internet.

  • Prior to running the tool, create a backup of the Exchange Server in its current state.

  • Microsoft released a One-Click Mitigation tool to patch, upgrade, and scan for malicious activity. You can find the tool on the company’s Security Center blog. 

  • Disable OWA according to the Microsoft guidelines. This may cause users to not have access to email.  

  • If you cannot patch or disable OWA, remove the system from the internet by disconnecting the network cable or powering the system down. This action will cause significant disruption and prevent users from receiving emails in certain configurations.  

 2. The U.S. Cybersecurity & Infrastructure Security Agency (CISA) issued a mitigation alert. Please read the article.

  • Block IP addresses at the firewall.

  • Enable geofencing if your organization does not conduct business outside of the United States.

3. Change passwords for all Windows-based user accounts.

  • Start with the most privileged domain administrator and service accounts, then work towards the least privileged user accounts. Also include the local administrator password.

4. Preserve any firewall, web application firewall, or VPN logs.

5. Deploy an Enterprise Detection and Response (EDR) tool throughout the environment to detect any unauthorized activity.

If you suspect any unauthorized access to your Microsoft Exchange Server or network, contact us.  

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Red alert symbols and warning icons spreading across a digital network, representing firewall compromise and widespread cyber intrusion.
Red alert symbols and warning icons spreading across a digital network, representing firewall compromise and widespread cyber intrusion.

Article

FortiGate Exploits Enable Network Breaches and Credential Theft

A recent security report indicates that threat actors are actively exploiting FortiGate Next-Generation Firewall (NGFW) appliances as initial access vectors to compromise enterprise networks. The activity leverages recently disclosed vulnerabilities or weak credentials to gain unauthorized access and extract configuration files, which often contain sensitive information, including service account credentials and detailed network topology data. 

Analysis of these incidents shows significant variation in attacker dwell time, ranging from immediate lateral movement to delays of up to two months post-compromise. Since these appliances often integrate with authentication systems such as Active Directory and Lightweight Directory Access Protocol (LDAP), their compromise can grant attackers extensive access, substantially increasing the risk of widespread network intrusion and data exposure. 

What’s Notable and Unique 

  • The activity involves the exploitation of recently disclosed security vulnerabilities, including CVE-2025-59718, CVE-2025-59719, and CVE-2026-24858, or weak credentials, allowing attackers to gain administrative access, extract configuration files, and obtain service account credentials and network topology information. 


  • In one observed incident, attackers created a FortiGate admin account with unrestricted firewall rules and maintained access over time, consistent with initial access broker activity. After a couple of months, threat actors extracted and decrypted LDAP credentials to compromise Active Directory. 

  • In another case, attackers moved from FortiGate access to deploying remote access tools, including Pulseway and MeshAgent, while also utilizing cloud infrastructure such as Google Cloud Storage and Amazon Web Services (AWS). 

Analyst Comments 

Arete has identified multiple instances of Fortinet device exploitation for initial access, involving various threat actors, with the Qilin ransomware group notably leveraging Fortinet device exploits. Given their integration with systems like Active Directory, NGFW appliances remain high-value targets for both state-aligned and financially motivated actors. In parallel, Arete has observed recent dark web activity involving leaked FortiGate VPN access, further highlighting the expanding risk landscape. This aligns with the recent reporting from Amazon Threat Intelligence, which identified large-scale compromises of FortiGate devices driven by exposed management ports and weak authentication, rather than vulnerability exploitation. Overall, these developments underscore the increasing focus on network edge devices as entry points, reinforcing the need for organizations to strengthen authentication, restrict external exposure, and address fundamental security gaps to mitigate the risk of widespread compromise. 

Sources 

FortiGate Edge Intrusions | Stolen Service Accounts Lead to Rogue Workstations and Deep AD Compromise

Article

Vulnerability Discovered in Anthropic’s Claude Code

Security researchers discovered two critical vulnerabilities in Anthropic's agentic AI coding tool, Claude Code. The vulnerabilities, tracked as CVE-2025-59536 and CVE-2026-21852, allowed attackers to achieve remote code execution and to compromise a victim's API credentials. The vulnerabilities exploit maliciously crafted repository configurations to circumvent control mechanisms. It should be noted that Anthropic worked closely with the security researchers throughout the process, and the bugs were patched before the research was published. 

What’s Notable and Unique 

  • The configuration files .claude/settings.json and .mcp.json were repurposed to execute malicious commands. Because the configurations could be applied immediately upon starting Claude Code, the commands ran before the user could deny permissions via a dialogue prompt, or they bypassed the authentication prompt altogether. 


  • .claude/settings.json also defines the endpoint for all Claude Code API communications. By replacing the default localhost URL with a URL they own, an attacker could redirect traffic to infrastructure they control. Critically, the authentication traffic generated upon starting Claude Code included the user's full Anthropic API key in plain text and was sent before the user could interact with the trust dialogue. 


  • Restrictive permissions on sensitive files could be bypassed by simply prompting Claude Code to create a copy of the file's contents, which did not inherit the original file's permissions. A threat actor using a stolen API key could gain complete read and write access to all files within a workspace. 

Analyst Comments 

The vulnerabilities and attack paths detailed in the research illustrate the double-edged nature of AI tools. The speed, scale, and convenience characteristics that make AI tools attractive to developer teams also benefit threat actors who use them for nefarious purposes. Defenders should expect adversaries to continue seeking ways to exploit configurations and orchestration logic to increase the impact of their attacks. Organizations planning to implement AI development tools should prioritize AI supply-chain hygiene and CI/CD hardening practices. 

Sources 

  • Caught in the Hook: RCE and API Token Exfiltration Through Claude Code Project Files | CVE-2025-59536 | CVE-2026-21852

Article

Ransomware Trends & Data Insights: February 2026

After a slight lull in January, Akira and Qilin returned to dominating ransomware activity in February, collectively accounting for almost half of all engagements that month. The rest of the threat landscape remained relatively diverse, with a mix of persistent threats like INC and PLAY, older groups like Cl0p and LockBit, and newer groups like BravoX and Payouts King. Given current trends, the first quarter of 2026 will likely remain relatively predictable, with the top groups from the second half of 2025 continuing to operate at fairly consistent levels month to month.

Figure 1. Activity from the top 5 threat groups in February 2026

Throughout the month of February, analysts at Arete identified several trends behind the threat actors perpetrating cybercrime activities: 

  • In February, Arete observed Qilin actively targeting WatchGuard Firebox devices, especially those vulnerable to CVE-2025-14733, to gain initial access to victim environments. CVE-2025-14733 is a critical vulnerability in WatchGuard Fireware OS that allows a remote, unauthenticated threat actor to execute arbitrary code. In addition to upgrading WatchGuard devices to the latest Firebox OS version, which patches the bug, administrators are urged to rotate all shared secrets on affected devices that may have been compromised and may be used in future campaigns.


  • Reports from February suggest that threat actors are increasingly exploring AI-enabled tools and services to scale malicious activities, demonstrating how generative AI is being integrated into both espionage and financially motivated threat operations. The Google Threat Intelligence Group indicated that state-backed threat actors are leveraging Google’s Gemini AI as a force multiplier to support all stages of the cyberattack lifecycle, from reconnaissance to post-compromise operations. Separate reporting from Amazon Threat Intelligence identified a threat actor leveraging commercially available generative AI services to conduct a large-scale campaign against FortiGate firewalls, gaining access through weak or reused credentials protected only by single-factor authentication.


  • The Interlock ransomware group recently introduced a custom process-termination utility called “Hotta Killer,” designed to disable endpoint detection and response solutions during active intrusions. This tool exploits a zero-day vulnerability (CVE-2025-61155) in a gaming anti-cheat driver, marking a significant adaptation in the group’s operations against security tools like FortiEDR. Arete is actively monitoring this activity, which highlights the growing trend of Bring Your Own Vulnerable Driver (BYOVD) attacks, in which threat actors exploit legitimate, signed drivers to bypass and disable endpoint security controls.

Sources

  • Arete Internal