We use many technologies to detect email spam. They analyze every part of a message: headers, text, images, and links. But the first thing our email products focus on is the “From” field. If the address contains a domain that’s already on our denylist, it’s an easy call. But spammers know the drill, so they register domains in industrial quantities to make emails, even from a single campaign, arrive from completely different addresses. Clearly, they don’t do that by hand. The process has long been automated, and attackers’ algorithms can register huge numbers of domains in a matter of minutes. But every domain needs a name. Coming up with a unique one for each isn’t feasible. So spammers use domain generation algorithms (DGAs), which create similar names with minor variations, based on date and time, pseudo-random numbers, and so on.
In fact, to block mailings such as these, you don’t need every address on your denylist. We’re good at automating processes, too. For some time now, our email solutions have included a technology that detects domains generated by DGAs, and shuts down entire batch with a single rule. Let me explain how this technology works.
How we detect and block generated domain names in bulk
As soon as any of our technologies flags an email as spam, its address becomes subject to further examination. If it meets a number of criteria, its domain is added to our database of domains we consider “bad”. We built a cascade of classifiers that compares these domains by similarity, groups them into clusters, and creates a short but comprehensive rule that blocks the entire cluster. Our email protection solutions receive this rule with an update. From then on, they block all emails sent from addresses on any domain that matches it — even if those domains have never been used in spam campaigns yet.
Here’s a real-world example. Let’s say the classifier flags several lookalike domains in a collection of “bad” ones it has gathered:
sxkjcc.asia
sxkjcd.asia
sxkjcy.asia
Then, one rule is enough to catch them all: @sxkjc[a-z0-9]\.asia\b
How to protect your company from spam
According to our analysis, 44.9% of emails sent last year were spam. Unwanted, unsolicited emails distract employees, waste working time, and clog inboxes. In some cases, they also contain scam, phishing, or malware content. To protect business customers from this deluge of threats, our solutions for corporate email security have anti-spam technologies that we constantly improve. This includes the technology described above for blocking domains generated by DGAs.
Have you ever tried to open an “encrypted” email or an urgent document, only to realize with horror that your usual DOCX attachment is actually a file with the extension .docx.exe? Or maybe you received an email supposedly with an invoice, contract, or internal memo attached — only for the file to prove something entirely different from what it claimed to be? If so, you were likely targeted in a malware infection attempt.
Cybercriminals routinely disguise malicious files as harmless documents and archives, banking on recipients blindly clicking without checking the extension. Kaspersky experts analyzed the file formats most frequently deployed in malicious email blasts to reveal what really lurks behind these extensions — and how attackers weaponize them in their campaigns.
Important note: the extensions we’ll be discussing here are routinely used for completely legitimate files. For example, Windows executables normally end in .exe. However, we’re focusing specifically on scenarios where attackers disguise or spoof a file’s extension to pass it off as a totally different type of file.
What extensions are most commonly found in malware?
Our experts analyzed malicious email blasts from the beginning of 2026 to pinpoint the 15 most common dangerous file extensions.
.exe
.js
.html
.dll
.bat
.vbs
.xls
.pdf
.jse
.au3
.docx
.htm
.wsf
.scr
.lnk
The top 15 file extensions used in malicious email blasts
Let’s take a look at what files with these extensions actually do under the hood.
Executable files
An executable file is a compiled computer program that’s ready to run. Once launched, a malicious executable can download additional payloads, alter system settings, steal user data, connect your device to external attacker-controlled servers, and much more. These are the most common executable extensions found in malicious email campaigns:
.exe
.dll
.com
.scr
.exe
The classic Windows executable extension. It powers every program you use daily, from web browsers and games to office suites and software installers.
By the way, attackers often wrap EXE malware in double extensions: invoice.docx.exe, report.pdf.exe, or even photo.jpg.exe. They take advantage of a default Windows setting: hiding extensions for known file types. Because this setting is turned on by default, users only see the first part of the file name — invoice.docx, report.pdf, or photo.jpg — and assume it’s just a normal document or image. But the second you open this trap file, the malware fires up.
.dll
Another common extension abused in malicious campaigns is .dll (dynamic-link library). These libraries hold functions that Windows programs frequently require while running, such as printing a document. This modular architecture prevents redundant code by letting multiple applications call on the exact same library for specific tasks. However, if an attacker replaces a legitimate library with an infected one, running any normal program that calls on it can trigger malicious code.
.com
While files with the .com extension have absolutely nothing to do with the web domain of the same name, cybercriminals likely count on victims mistaking these files for links in an unusual format. In reality, it’s a legacy Windows executable format.
While modern versions of Windows no longer rely on this file type, the operating system can still run and execute it, which makes opening one a very bad idea.
.scr
SCR files are screensavers — those idle Windows screen animations featuring abstract patterns, the iconic bouncing logo, or whatever else. Despite their harmless reputation, screensavers are essentially executables just like EXE files: once opened, they can install extra components or alter system settings all the same. In malicious campaigns, these files routinely masquerade as images, screenshots, or documents.
Scripts
Scripts are text files containing a sequence of commands that a computer runs automatically in order. They can download files, install and launch programs, modify security settings, and plenty more. Email attackers rely on the following script types most often:
.js and .jse
.bat
.cmd
.vbs
.wsf
.vbe
.au3
.js and .jse
The .js and .jse extensions are used by JavaScript files. Most users associate JavaScript strictly with websites and don’t realize these files can run locally on their computer as standalone programs.
We recently wrote about CrystalX RAT, a remote access Trojan that logs keystrokes, steals clipboard contents, and injects malicious components into web browsers upon installation. Its JS scripts monitored cryptocurrency wallet addresses and silently swapped them with the attacker’s own to hijack transactions in real time.
.cmd and .bat
BAT and CMD files are designed to automate tasks in Windows and can execute virtually any command in the operating system. Launching a file like this can infect your computer with malware.
.vbs
Visual Basic Script files with the .vbs extension are a long-time favorite among cybercriminals. Attackers frequently use double extensions to disguise .vbs files as plain text documents, such as invoices, app installation guides, or emails. For example, in the early 2000s, the iconic ILOVEYOU worm spread via a VBS script, infecting millions of computers worldwide. Victims received an email with the subject line “ILOVEYOU”, but instead of a sweet confession, the attachment contained a malicious script. At its peak, this “romantic” worm had infected up to 10% of all internet-connected computers.
Web files
This category mainly includes .html, .htm, .hta and .svg files. Often these are phishing pages in disguise, designed to mimic login forms for email, cloud services, banking apps, and other platforms. Naturally, any credentials entered on these fake pages go straight to the attackers hunting for your accounts and money. Web files can also trigger malicious scripts and download additional payloads onto your system.
Archives
Cybercriminals routinely package malicious files inside archives (most commonly having .zip, .rar, and .7z extensions). These archives are often password-protected or encrypted to hinder content scanning. Additionally, attackers sometimes create intentionally corrupted archives with damaged data structures: basic security filters often stumble over broken files and skip scanning them altogether, whereas archive managers can automatically repair and unpack the contents anyway.
That’s why we recommend using advanced security solutions that can spot phishing emails and prevent you from launching dangerous files hidden inside archives.
PDF files (.pdf)
To many, PDF feels like a completely harmless file format — after all, what could go wrong with flight tickets, documents, and slide decks? But beyond static text and images, PDFs can host clickable links, forms, and even scripts.
Malicious code in a PDF is usually tucked away behind visual elements or embedded as hidden text. What’s more, the risk comes not just from the file itself, but also from the software you use to view it: for instance, attackers spent months exploiting a vulnerability in Adobe Acrobat that allowed them to execute malicious PDF files remotely. If you regularly work on PDF documents, make sure the JavaScript feature is disabled in your Adobe Acrobat. To do this, go to Preferences → JavaScript in the menu.
By the way, heavy users of AI agents and chatbots should keep in mind that AI frequently falls for prompt injections: hidden commands buried inside images, text, and web pages that the AI interprets as legitimate instructions, and executes without your knowledge.
LNK shortcuts (.lnk)
Files with the .lnk extension are standard Windows shortcuts that we use every day for quick access to apps and folders. By themselves, shortcuts don’t contain any software — they simply tell Windows what file to open or what command to run.
However, an attacker can give a shortcut any icon and any name they want, completely hiding its real destination. The moment a victim clicks the shortcut, the infection chain kicks off. The malicious code usually executes silently in the background, leaving virtually no visual clues that anything went wrong.
Cybercriminals also like to hide LNK files behind double extensions — betting that users won’t look close enough to see what kind of file they’re actually opening. Here’s a classic example:
Windows File Explorer displays only the first part of the extension, .docx. The file type is clearly listed as a shortcut, but very few people notice that detail
Your best bet is to avoid clicking incoming LNK files altogether: no one in their right mind has a reason to email you a desktop shortcut.
By the way, if you haven’t updated Windows in a while, we’ve bad news for you: malicious code can automatically execute without you ever clicking the LNK — simply opening the folder where it lives is enough to trigger it. Microsoft finally released a patch for this long-abused vulnerability in December 2025. So don’t put off system updates — install them regularly to patch up the security holes attackers rely on.
Microsoft Office documents
.xls and .xlsx
The file extension .xls is used by older versions of Microsoft Excel. Millions of people use Excel spreadsheets every day, and many are still running outdated software, which makes XLS files a favorite bait for cybercriminals. Budgets, employee rosters, financial records, urgent reports, and other spreadsheets stop being harmless the moment a user clicks Enable Content, allowing the program to run macros — custom Microsoft Office scripts and commands built into the files. Once allowed, the malicious macro gets free rein to download files, launch programs, harvest system data, and alter settings.
Unlike XLS, XLSX is the modern Excel format, which doesn’t support embedded macros by default. Unfortunately, that doesn’t guarantee complete safety: the spreadsheet can still pull live data from external sources and automatically connect to web destinations in the background.
.docx
Although modern DOCX files don’t support macros by default, attackers have long figured out other ways to weaponize these documents. Typically, malicious DOCX files contain embedded links or instructions that force Word to contact external servers and pull down malicious payloads. Back in 2023, for instance, researchers uncovered a Microsoft Office vulnerability that allowed attackers to trigger built-in Windows system tools and download malware straight from the web.
Macros are sequences of commands that automate complex, repetitive tasks in Excel, Outlook, Word, and PowerPoint. In modern versions of Microsoft Office, documents containing embedded macros are saved in specialized file formats with extensions like .xlsm, .xltm, .xlsb, .docm, and .pptm. While Microsoft Office includes built-in security controls — such as blocking files downloaded from the web and warning you about potentially dangerous macros — if you don’t actually use macros and have no specific reason to run them, treat files with these extensions with caution.
How to check if a file is safe
Use a comprehensive security suite that automatically scans files for malware, prevents infections, blocks visits to suspicious sites, and completely removes malicious files from your devices. You can take advantage of a 30-day trial of Kaspersky Premium if you urgently need to disinfect your computer or scan a file for viruses.
Never open attachments from unknown senders. Curiosity can cost you dearly, since many malware strains specifically hunt for credit card and crypto wallet data. Let the Mail Anti-Virus feature included in our security solutions do the heavy lifting: it automatically filters out malicious or suspicious emails and protects you from phishing attacks.
Examine incoming emails carefully: cybercriminals have become adept at disguising malicious messages. Always verify email addresses against official ones and make sure you know the signs of phishing and spam. Attackers might try to scare you with claims of a hacked account, lure you to shady sites with “unbeatable deals”, or urge you to review a work document attached to the email. Any attempt to manipulate your emotions is a major red flag that someone is trying to trick you.
Never open files with double extensions. It’s almost always a trap.
If you run into a file with an unfamiliar extension, take a moment to look it up online. Remember: a photo or plain text document will never have an .lnk or .vbs extension.
Check out our other posts on email and file security:
When targeting an organization to steal information, maintaining a low profile is critical for attackers. They typically aim for long-term persistence, which requires avoiding security alerts while preserving access in case they’re detected and the organization initiates incident response or routine password resets. Malware such as infostealers or ostensibly legitimate remote monitoring and management (RMM) tools fail the first requirement: their use triggers EDR and generates suspicious events in SIEM consoles. Relying on stolen credentials conflicts with the second requirement: the moment the security team suspects a compromise, passwords can be changed immediately — terminating access. If attackers attempt to steal browser cookies instead of passwords, they face a different challenge: many online services now correlate device characteristics with the expected session cookie and block access if the cookie is used on an unrecognized device. Furthermore, cookie theft protection mechanisms implemented in Chromium-based browsers (such as Chrome, Edge, and Opera) this year have made this approach significantly more difficult.
To address this persistence challenge, the ToddyCat APT — whose main game is spying — developed a novel technique. Kaspersky experts discovered this method during an incident investigation and named it Shadow Token via Remote Debug (STRD). This technique allows the attackers to establish reliable, persistent access to a victim’s mailbox and other resources in Google Workspace. With minor adjustments, the same approach could potentially be adapted to target other services that grant third-party application access via OAuth 2.0 authentication.
How an STRD attack works
First, the attackers must compromise the victim’s system with malware. In past campaigns, ToddyCat gained initial access to organizations by exploiting known vulnerabilities in server software and distributing malicious loaders via messaging apps. The specific employee targeted by the attackers might not notice the intrusion at all. This can occur, for example, if the adversary first obtains privileged administrative credentials and uses them to deploy the malware onto targeted machines remotely. Crucially, the deployment and execution of this malware mustn’t trigger immediate security alerts.
Once active, the malware executes an STRD attack, connecting the attackers’ remote service to the victim’s mailbox using the OAuth 2.0 protocol. This process requires no user interaction and shows no visible activity on the screen. To the cloud environment (Google Workspace, in the case at hand), the activity appears as if the user has legitimately authorized a third-party app for email access or data backup.
After that, the malware can terminate its operations and even delete itself from the system. The adversary retains direct access to the mailbox using the acquired OAuth token — and they need no connection to the victim’s endpoint or to the corporate network for that. Depending on the organization’s Google Workspace configuration, this access can persist for an extended period and survive subsequent password resets.
The core concept of Shadow Token via Remote Debug
At the heart of this attack is a connection to Google Workspace services via OAuth 2.0. This is a legitimate workflow used whenever a third-party application requests access to calendar data, emails, or Google Drive files. For example, to display calendar meetings in Zoom and automatically generate conference links, a user must authorize Zoom to access Google services. Similarly, configuring a third-party email client or calendar app requires granting permission. During this authorization process, the service requesting access opens a new browser window. In this window, Google Workspace first prompts the user to select the appropriate account. Once the account is chosen, the subsequent screen displays the specific permissions requested by the third-party app, allowing the user to either approve or deny access. For this scenario to proceed seamlessly, the user must already be authenticated to Google services in their browser — which is typically the case in organizations using Google Workspace. If the user isn’t authenticated, additional steps for entering credentials and completing multi-factor authentication are introduced into the sequence.
The ToddyCat hackers developed a malicious tool called Umbrij to facilitate a two-step covert authorization process when the user is already authenticated in Google. First, the malware identifies the browsers installed on the system, and locates the specific folder storing the user’s active profile for each. The attackers target Chrome and Edge, as these are the browsers most likely serving as the primary ones within the organization.
Next, Umbrij copies the entire user profile folder to a different directory on the machine. It then launches an instance of the browser, specifying the path to the duplicated profile via the command line. Because this duplicate profile contains the user’s session cookies, websites with saved credentials won’t prompt for re-authentication. Furthermore, since this occurs on the exact same computer where the primary browser is running, online services detect no anomalies. The browsing history for this newly launched instance is isolated within the new folder, keeping it hidden from the user’s main account activity.
Crucially, the browser is launched in a dedicated debugging mode typically reserved for web development. The browser window and user interface don’t appear on the screen at all (headless mode). Instead, the browser can be controlled through a debugging port using the DevTools protocol, allowing the malware to issue commands and read the state of the screen. To orchestrate these actions, Umbrij leverages Puppeteer, a legitimate automation library.
After verifying that the debugging browser instance has launched successfully, Umbrij opens a legitimate Google Workspace OAuth screen within it. The request sent to Google is engineered to bypass additional security checks while requesting maximum access privileges. For the application ID — the identity supposedly requesting these extensive permissions — the malware impersonates one of two legitimate tools: Google Workspace Migration for Microsoft Outlook (GWMMO), or Google Workspace Sync for Microsoft Outlook (GWSMO).
When Google opens the window within the headless browser, Umbrij uses debugging tools to programmatically click on the corporate account name and the confirmation buttons. As a result, Google generates an authorization code for the app. Umbrij extracts and saves this code, subsequently forwarding it to the attackers’ command-and-control server. Finally, operating entirely within their own infrastructure rather than on the victim’s computer, the attackers exchange this authorization code for an OAuth access token. This single token is all they need to maintain long-term unrestricted access to the mailbox.
How to protect against OAuth token theft
If a Google Workspace account is compromised, incident response measures must include the following steps after collecting the necessary logs and other forensic data for investigation:
Resetting the affected user’s password
Terminating all active web sessions for the user
Revoking OAuth tokens and third-party app permissions
Reviewing and removing access granted through legacy App Passwords
In addition, security and IT teams must systematically audit issued OAuth permissions, revoke unjustifiable access rights, and restrict capabilities that allow excessive or unauthorized permission grants. We covered this topic in detail in our article on blocking unwanted AI assistants.
How to prevent exploitation of Shadow Token via Remote Debug
While Kaspersky users are protected against the Umbrij tool, security teams should proactively implement policies that prevent standard users from launching browsers in debugging mode. This functionality is intended exclusively for website and web app developers. This restriction can be enforced through the DeveloperToolsAvailability group policy (available for both Chrome and Edge).
Additionally, configure monitoring within your SIEM/XDR to track the launch of browser instances with an active debugging port. This event serves as a strong indicator of this specific attack technique.
Scammers have found a way to abuse legitimate Apple account notification emails to trick targets into calling fake tech support numbers.
According to a report from BleepingComputer, scammers create an Apple account and insert a phishing message into the personal information fields, then modify the account so that Apple sends a genuine security alert about the change to the target.
BleepingComputer was able to replicate the attack.
The attacker creates an Apple ID they control, then stuffs the phishing message into the personal information fields (first name, last name, possibly address), splitting it across fields because they will not fit into just one.
To launch the phish, the attacker changes something benign on their specially created Apple account, such as shipping information, which causes Apple’s systems to send a “Your Apple account was updated” security email.
While the original alert is addressed to the attacker’s iCloud email, they are then able to redistribute it to a wider victim list, for example through a mailing list.
In the copy the targets receive, the email headers still show a legitimate Apple sender, and the presence of the attacker’s iCloud address can even make it look like “someone else” has gained access to the account.
Because Apple includes those user-supplied fields in the security email, the phishing text is delivered inside a legitimate message sent from Apple’s own infrastructure.
This method, called call-back phishing, filters out suspicious users, so the scammers can focus on the people who fell for the first part.
The emails come from a legitimate source, sail through every security filter because of that, and look convincing enough to scare the receiver into thinking someone spent $899 from their PayPal account.
But the structure of the email does not make sense.
“Dear User” is immediately followed by the scam message where your name should have been. The header says it’s about account information rather than a purchase. And the iCloud account does not belong to the recipient. So, once you know how it’s done, they’re not impossible to spot. Which is why we wrote this blog.
And when in doubt, you can always ask Malwarebytes Scam Guard.
Scam Guard identified the screenshot as a scam and guides users through the next steps.
Scams like these work, because many users still view phone calls as more trustworthy than email, especially if the email itself passed all the usual technical authenticity checks and they initiated the call themselves.
How to stay safe
Tech support scammers will try to convince callers to install some kind of remote desktop application to steal data from your computer, or ask for financial details so they can steal your money.
To stay safe from these scammers:
Be wary of unexpected alerts about high‑value purchases you do not recognize. They are suspicious even if they come from a real domain.
Never call a number sent to you by unsolicited means or even found in sponsored search results.
Carefully read emails and text messages, even if they come form trustworthy addresses. Does the email make sense from a structural and linguistic point of view?
If someone claiming to be support for a legitimate company asks for remote access or payment details during a call, hang up and contact the company through official channels.
Use Malwarebytes Scam Guard to analyze any kind of message that alarms you or urges you to take immediate action.
Something feel off? Check it before you click.
Malwarebytes Scam Guard helps you analyze suspicious links, texts, and screenshots instantly.
Once. Someone named “Vincenzo lozzo” wrote to Epstein in email, in 2016: “I wouldn’t pay too much attention to this, Schneier has a long tradition of dramatizing and misunderstanding things.” The topic of the email is DDoS attacks, and it is unclear what I am dramatizing and misunderstanding.
Rabbi Schneier is also mentioned, also incidentally, also once. As far as either of us know, we are not related.
EDITED TO ADD (2/7): There is more context on the Justice.gov website version.
The LastPass Threat Intelligence, Mitigation, and Escalation (TIME) team has published a warning about an active phishing campaign in which fake “maintenance” emails pressure users to back up their vaults within 24 hours. The emails lead to credential-stealing phishing sites rather than any legitimate LastPass page.
The phishing campaign that started around January 19, 2026, uses emails that falsely claim upcoming infrastructure maintenance and urge users to “backup your vault in the next 24 hours.”
Image courtesy of LastPass
“Scheduled Maintenance: Backup Recommended
As part of our ongoing commitment to security and performance, we will be conducting scheduled infrastructure maintenance on our servers. Why are we asking you to create a backup? While your data remains protected at all times, creating a local backup ensures you have access to your credentials during the maintenance window. In the unlikely event of any unforeseen technical difficulties or data discrepancies, having a recent backup guarantees your information remains secure and recoverable. We recommend this precautionary measure to all users to ensure complete peace of mind and seamless continuity of service.
Create Backup Now (link)
How to create your backup 1 Click the “Create Backup Now” button above 2 Select “Export Vault” from you account settings 3 Download and store your encrypted backup file securely”
The link in the email points to mail-lastpass[.]com, a domain that doesn’t belong to LastPass and has now been taken down.
Note that there are different subject lines in use. Here is a selection:
LastPass Infrastructure Update: Secure Your Vault Now
Your Data, Your Protection: Create a Backup Before Maintenance
Don’t Miss Out: Backup Your Vault Before Maintenance
Important: LastPass Maintenance & Your Vault Security
Protect Your Passwords: Backup Your Vault (24-Hour Window)
It is imperative for users to ignore instructions in emails like these. Giving away the login details for your password manager can be disastrous. For most users, it would provide access to enough information to carry out identity theft.
Stay safe
First and foremost, it’s important to understand that LastPass will never ask for your master password or demand immediate action under a tight deadline. Generally speaking, there are more guidelines that can help you stay safe.
Don’t click on links in unsolicited emails without verifying with the trusted sender that they’re legitimate.
Always log in directly on the platform that you are trying to access, rather than through a link.
Use a real-time, up-to-date anti-malware solution with a web protection module to block malicious sites.
Report phishing emails to the company that’s being impersonated, so they can alert other customers. In this case emails were forwarded to abuse@lastpass.com.
Pro tip: Malwarebytes Scam Guard would have recognized this email as a scam and advised you how to proceed.
We don’t just report on threats—we help safeguard your entire digital identity
Cybersecurity risks should never spread beyond a headline. Protect your, and your family’s, personal information by using identity protection.
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