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Malware Spotlight: Fog Ransomware

Arete Analysis

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Cybersecurity Trends

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Executive Summary

Since April 2024, Arete’s Incident Response (IR) team has responded to multiple engagements attributed to the Fog ransomware group. Engagements attributed to this group have been trending up since mid-June and through July 2024, accounting for nearly 20% of Arete’s ransomware and extortion engagements in July. The Fog ransomware group is especially noteworthy as it is one of the few threat actors specifically targeting one industry: education. Since April, Arete has observed that 70% of Fog victims have been education organizations. This spotlight explores the ransomware group’s observed behavior, background information on the threat actor, and statistics from Incident Response engagements, along with a technical analysis of Fog’s ransomware executable. Finally, we discuss security recommendations to better defend against this evolving cyber threat and mitigate the risk of financial and reputation losses.

Incident Response Data on the Fog Ransomware Group

The information below is based on Fog ransomware incidents investigated by Arete since April 2024. Our IR, Threat Intelligence, and Data Analytics teams work together to analyze key data points during every ransomware engagement and form real-time threat actor (TA) insights.

  • The median initial demand is $220,000.

  • The median ransom payment facilitated is $100,000.

  • 36% of engagements involved data exfiltration, often using tools like MEGAsync and Filezilla.

  • Common initial access vectors include brute force attacks against remote desktop protocol (RDP) and compromised virtual private network (VPN) credentials.

  • Tools observed during investigations include CobaltStrike, Mimikatz, ngrok, WinRAR, AnyDesk, Advanced Port Scanner, and GMER, among others. The group demonstrates distinct skill in evading common anti-malware defenses.

  • The ransom note file name is commonly “readme.txt” and includes a link to a TOR site used for negotiations.

  • The group operates a data leak site (DLS) self-proclaimed as “The Fog Blog.”

Background

Fog ransomware was a prominent newcomer in the second quarter (Q2) of 2024 and demonstrated a noticeable trend of attacking entities in the education sector. Arete engagements involving Fog ransomware more than doubled during July 2024 compared to previous months.

Technical Analysis

Malware analysis revealed that Fog ransomware:

  • Supports multiple command-line arguments.

  • Encrypts files on the system and mounted shares.

  • Adds the following extensions to encrypted files: .fog, .ffog, or .flocked (e.g., file.docx.flocked).

  • Creates a ransom note with the following filename: readme.txt.

  • Self-identifies the group as Fog in the ransom note.

  • References a data leak site in the ransom note that, when accessed, self-identifies the group as Fog.

  • Kills a list of processes and services.

  • Maintains a list of whitelisted files and directories to make sure it will not render the system unusable, preventing recovery when running a decryptor.

  • Attempts to prevent system recovery by deleting the system’s volume shadow copies.

  • Creates a mutex during execution.

  • Creates a log file named DbgLog.sys.Execution Pattern/Arguments

Fog ransomware needs command line arguments to execute and encrypt files in the system. Command line arguments supported:

Command line arguments

Description

-id

Key to decrypt ransomware configuration.

-nomutex

Skip mutex check.

-target

Specific location to encrypt files.

-console

Creates a new console window for output.

-size

File size threshold to encrypt.

-log

Log the ransomware activity.

-procoff

Presently unknown.

-uncoff

Presently unknown.

The ransomware will not execute in the system without the “-id” argument followed by a 6-character value that is unique in each engagement.

Execution of the ransomware to encrypt files:

Fog.exe -id [6-characters]

Example:
Fog.exe -id A9p3RZ

Figure 1. Code in the ransomware to check command line argument “-ID”


The ransomware uses the “-id” argument followed by a 6-character value to decrypt a JSON-based ransomware configuration information at runtime. Decrypted JSON field name and descriptions:

Name
Description

RSAPubKey

Public key used in the file encryption process.

LockedExt

Extension added to encrypted files.

NoteFileName

Ransom note name.

PathStopList

Excludes listed directories.

FileMaskStopList

Excludes listed file extensions.

ShutdownProcesses

Terminates list of processes.

ShutdownServices

Terminates list of services.


Stop Services and Processes

Before file encryption, the ransomware terminates a pre-determined list of processes and services to encrypt as many files as possible.

Process names:

“notepad.exe”, “calc.exe”, “*sql*”

Service names:

“Dhcp”, “Dnscache”, “*sql*”

Code in the ransomware showing this operation:

Figure 2. Process termination decompiled code

Figure 3. Service termination decompiled code


File and Directory Exclusions

The ransomware excludes system-related files and folders, ransomware-related files, and whitelisted extensions during encryption.

Excluded file extensions:

“*.exe”, “*.dll”, “*.lnk”, “*.sys”

Excluded directories:

“tmp”, “winnt”, “Application Data”, “AppData”, “temp”, “thumb”, “$Recycle.Bin”, “System Volume Information”, “Windows”, “Boot”


Inhibit System Recovery

Windows operating systems contain features that can help fix corrupted system files, including shadow copies, which are backups of files created by the Volume Shadow Copy Service (VSS). By deleting shadow copies, the ransomware can prevent victims from restoring files from backups, making it more difficult for them to recover their data without paying the ransom.

The ransomware deletes volume shadow copies before file encryption by starting the following process:

cmd.exe /c vssadmin delete shadows /all /quiet

System Network Connections Discovery

Fog ransomware can enumerate network-mounted shares by scanning the network interfaces.

Data Encrypted for Impact

The ransomware initially finds available drives and then loads the files one by one using the Windows API FindFirstFileW and FindNextFileW. The ransomware generates random AES keys to encrypt the files, and after encrypting them, the keys are encrypted using a public RSA key. The resulting key is again encrypted and placed at the end of the file.

Figure 4. Data encryption code

Figure 5. Extension added to the encrypted files

Figure 6. Encrypted AES key appended to encrypted file.

Fog ransomware extensions observed:

“.FOG”, “.FLOCKED”, “.FFOG”

During execution, the ransomware creates a file named DbgLog.sys in the same directory and logs the ransomware activity. If the “-log” argument is used during ransomware execution, the ransomware creates and encrypts a lock_log.txt file under the C:\ProgramData directory. Encrypting the lock_log.txt file created might be an oversight in the ransomware code possibly indicating that the ransomware is still under development.

Figure 7. Log file created by the ransomware.

Upon successful execution, the ransomware creates ransom notes with the file name readme.txt.

Figure 8. Fog ransom note

Ransom note content:

If you are reading this, then you have been the victim of a cyber attack. We call ourselves Fog and we take responsibility for this incident. You can check out our blog where we post company data:

xbkv2qey6u3g3dqxcoyjnrt4h5sgrhkar6whuo74wo63hijnn677jnyd[.]onion You might appear there if you opt out of our communication.

We are the ones who encrypted your data and also copied some of it to our internal resource. The sooner you contact us, the sooner we can resolve this incident and get you back to work.

To contact us you need to have Tor browser installed:

  1. Follow this link: <url>.onion

  2. Enter the code: <code>

  3. Now we can communicate safely.

    If you are a decision-maker, you will get all the details when you get in touch. We are waiting for you.


Modify Registry

The ransomware did not perform any registry key modification.

Mutex

The mutex is the fundamental tool for managing shared resources between multiple threads or processes. Typically, ransomware uses a mutex to avoid reinfecting the victim system and causing multiple layers of encryption. The ransomware creates the following mutex value: XDLJgan1TkNkWfrQ78xVrust07YDq45X.

Figure 9. Mutex value created while debugging the ransomware


Network Activity

The ransomware did not try to communicate with a remote server other than encrypting data from mounted shares.

Indicators of Compromise

Indicator

Type

Context

B6360765c786ceee0eb28bee64709172b4e2e066449968e011390be1afd8f36c515edfedadb458be0f569fc2bedb6c4139782516d6faf464b4881739e312e9fabbE67260804526323484f564eebeb6c99ed021b960b899ff788aed85bb7a9d75c3e44c342198e0ad8dd8c0f7bda19d4deb33f0d8355e3e78827505c3b858c82d54

SHA256 hash

Fog ransomware

C:\readme.txt

File path

Fog ransom note

.fog, .flocked, .ffog

Extension

Encrypted file extensions

vssadmin delete shadows /all /quiet

Process

Volume Shadow Copy deletion

XDLJgan1TkNkWfrQ78xVrust07YDq45X
JBgB4ZHxUhNdJL9mz61WFXxl0GUXPAXw
Gxu7w1Hj1ojGy99XUbpyG3JuYV0twle2

Mutex

Mutex value objects created by the Fog ransomware

xbkv2qey6u3g3dqxcoyjnrt4h5sgrhkar6whuo74wo63hijnn677jnyd[.]onion

URL

TA data leak site (DLS)


Data Leak Site

The ransom note contains a data leak site (DLS) that, when accessed, displayed the following page, self-identifying the group as Fog:

Figure 10. TOR DLS: xbkv2qey6u3gd3qxcojynrt4h5sgrhkar6whuo74wo63hijnn677jnyd[.]onion


Detection Mechanisms

Custom Detections and Blocking with Arete’s Arsenal

SentinelOne S1QL 1.0 query syntax (STAR rule):

Fog Ransomware

EndpointOS = ″Windows″ AND
( (ObjectType = ″Process″ AND SrcProcCmdLine RegExp ″\\.exe\\s{1,3}\\-id\\s{1,3}[a-zA-Z0-9]
{6}″) OR (ObjectType = ″File″ AND EventType In (″File Creation″, ″File Scan″) AND
TgtFilePath Contains Anycase ″\\

EndpointOS = ″Windows″ AND
( (ObjectType = ″Process″ AND SrcProcCmdLine RegExp ″\\.exe\\s{1,3}\\-id\\s{1,3}[a-zA-Z0-9]
{6}″) OR (ObjectType = ″File″ AND EventType In (″File Creation″, ″File Scan″) AND
TgtFilePath Contains Anycase ″\\

EndpointOS = ″Windows″ AND
( (ObjectType = ″Process″ AND SrcProcCmdLine RegExp ″\\.exe\\s{1,3}\\-id\\s{1,3}[a-zA-Z0-9]
{6}″) OR (ObjectType = ″File″ AND EventType In (″File Creation″, ″File Scan″) AND
TgtFilePath Contains Anycase ″\\

Akira Ransomware




Note: These threat hunting queries may need to be tuned for your specific network environment.


Yara





Recommended Mitigations

  • Utilize an endpoint detection and response (EDR) solution with the capability to halt detected processes and isolate systems on the network based on identified conditions.

  • Block any known attacker C2s in the firewall.

  • Implement multi-factor authentication on RDP and VPN to restrict access to critical network resources.

  • Eliminate unnecessary RDP ports exposed to the internet.

  • Block a high number of SMB connection attempts from one system to others in the network over a short period of time.

  • Perform periodic dark web monitoring to verify if data is available for sale on the black market.

  • Perform penetration tests.

  • Periodically patch systems and update tools.

  • Monitor connections to the network from suspicious locations.

  • Monitor downloads and uploads of files to file-sharing services outside standard work hours.

  • Monitor file uploads from domain controllers to the internet.

  • Monitor network scans from uncommon servers (e.g., RDP server).

Organizations can find the full list of US government-recommended ransomware prevention and mitigation guidance here: https://www.cisa.gov/stopransomware/ransomware-guide.

Arete provides data-driven cybersecurity solutions to transform your response to emerging cyber threats. Learn more.


References


At Arete, we envision a world without cyber extortion, where people, businesses, and governments can thrive. We are taking all that we know from over 8,000 engagements to inform our solutions and strengthen powerful tools to better prevent, detect, and respond to the cyber extortion threats of tomorrow. Our elite team of experts provides unparalleled capabilities to address the entire cyber threat lifecycle, from incident response and restoration to advisory and managed security services. To learn more about our solutions, visit www.areteir.com.

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

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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 

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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

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What’s Notable and Unique 

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  • .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

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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: 

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  • 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

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Not Your Average RAT 

MIMICRAT displays above-average defense evasion and sophistication, including: 

  • A five-stage PowerShell sequence beginning with Event Tracing for Windows and Anti-Malware Scan Interface bypasses, which are commonly used in red teaming for evading detection by EDR and AV toolsets.  

  • The malware later uses a lightweight scripting language that is scripted into memory, allowing malicious actions without files that could easily be detected by an EDR tool.  

  • MIMICRAT uses malleable Command and Control profiles, allowing for a constantly changing communication infrastructure. 

  • The campaign uses legitimate compromised infrastructure, rather than attacker-owned tools, and is prepped to use 17 different languages, which increases global reach and defense evasion. 

Analyst Comments 

The ClickFix social engineering technique remains an effective means for threat actors to obtain compromised credentials and initial access to victim environments, enabling them to deploy first-stage malware. Coupled with the sophisticated MIMICRAT RAT, the effectiveness of this campaign could increase. Arete will continue monitoring for changes to the ClickFix techniques, the deployment of MIMICRAT in other campaigns, and other pertinent information relating to the ongoing campaign. 

Sources 

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