10 min read
Online advertisements are everywhere, from news websites and social media platforms to streaming services and mobile apps. Most people click on ads without thinking twice, but not every advertisement is safe. Some are designed specifically to spread malware, steal personal information, or redirect users to dangerous websites.
This growing cyber threat is known as malvertising. So, what is malvertising, and why has it become a serious concern for both internet users and website owners? Let’s find out!
What is malvertising?Link to heading
Malvertising is a cyberattack method where criminals secretly inject malicious code into legitimate online advertising networks. Once embedded, these compromised ads are served to everyday users without any warning, quietly redirecting them to dangerous websites and putting their personal data and device security at serious risk.
For cybercriminals, malvertising is an attractive and highly profitable strategy. It is notoriously difficult to trace, rarely triggers immediate suspicion, and spreads through the same advertising infrastructure that powers trusted, high-traffic websites. Because the malicious code hides inside ordinary-looking ads, users encounter it naturally while browsing, with no indication that anything is wrong.
>>> Learn more: What is a malicious website? How do malicious websites work?
How does malvertising work?Link to heading
To understand what is malvertising, it helps to first look at how the online advertising ecosystem works. The modern ad delivery chain is deeply layered and interconnected, involving publisher websites, ad exchanges, ad servers, retargeting networks, and content delivery networks working together in real time.
Every time a user loads a page, a chain of rapid redirects takes place across multiple servers before an ad is finally rendered on screen. This complexity is precisely what malicious actors exploit, they insert harmful content at the points in the chain that publishers and ad networks are least equipped to monitor or defend.
When a visitor encounters a malvertising-infected ad, simply loading the page can be enough to trigger the attack, no click required in many cases. Once the malicious code executes on the device, it behaves like any conventional malware. It can corrupt or delete files, steal sensitive data, create hidden backdoors for remote access, or silently monitor everything the user does.
In more aggressive cases, the malware manipulates, copies, or leaks data that is then held for ransom or sold on dark web marketplaces.
Some malvertising campaigns go further by deploying exploit kits, specialized malware packages engineered to scan a system, identify unpatched software vulnerabilities, and take advantage of those security gaps automatically.
Types of malvertising attacksLink to heading
Understanding what is malvertising also means understanding the different ways attackers use it. Malvertising is not a single technique, but a group of attack methods that target users through ads in different ways.
Drive-by download (no user interaction required)Link to heading
This is one of the most dangerous forms of malvertising because users can become infected without even clicking on an advertisement. Simply visiting a website that displays a malicious ad may allow malware to exploit vulnerabilities in the browser, plugins, or operating system to install ransomware, spyware, or trojans automatically.
These attacks typically target unpatched software vulnerabilities and operate silently in the background. The most effective way to reduce the risk is to keep browsers, plugins, and operating systems fully updated at all times.
Malicious Redirect via Iframe InjectionLink to heading
In this type of attack, cybercriminals inject malicious iframes or JavaScript into online advertisements to automatically redirect users to dangerous websites, including phishing pages, fake technical support sites, or malware distribution platforms.
Because these iframes are often completely hidden from view, they can be extremely difficult to detect. Implementing a strong Content Security Policy (CSP) and carefully controlling third-party scripts can significantly reduce the risk of iframe-based attacks.
Fake software update overlaysLink to heading
This attack method relies heavily on social engineering. Users are shown convincing pop-up messages claiming that Chrome, Flash Player, Windows, or another application requires an urgent update. The interface is designed to closely resemble legitimate system notifications in order to trick users into downloading malware.
A key rule to remember is that legitimate software updates are never delivered through third-party advertising pop-ups on websites.
Steganography-hidden payloadsLink to heading
In this technique, attackers hide malicious code inside image files such as PNG or JPG advertisements using steganography. The image appears completely normal to users, but it secretly contains an encoded payload that can be extracted and executed when the advertisement loads in the browser.
This method is particularly dangerous because it can bypass many traditional ad-scanning systems that primarily inspect JavaScript files or embedded URLs.
Polyglot files in ad creativesLink to heading
Polyglot files are specially crafted files capable of functioning as multiple file formats at the same time. For example, a single file may appear to be a legitimate PNG image while also containing executable JavaScript code.
Advertising security systems may identify the file as harmless image content, while a browser may interpret and execute it as malicious code under certain conditions. This sophisticated technique is commonly used to bypass ad network security checks and evade detection.
>>> Learn more: Scan site for malware: Complete guide to check your website
Malvertising vs. Adware: What is the difference?Link to heading
Those researching what is malvertising often encounter the term adware in the same context, but the two are distinct threats that operate in fundamentally different ways. Adware is a software program that runs directly on a user's device. It is typically bundled with legitimate applications or installed without the user's awareness.
Once active, adware bombards users with unwanted advertisements, hijacks search queries to redirect them toward advertising pages, and harvests user data to enable targeted ad delivery.
The core distinction between the two lies in how and where each threat operates. Malvertising deploys its malicious payload through a publisher's webpage, targeting anyone who happens to visit that page. Adware, by contrast, targets individual users directly on their own machines, it does not need a publisher's website to function.
Additionally, malvertising only affects users at the moment they load an infected page, while adware, once installed, runs persistently in the background, continuing to impact the user regardless of which sites they visit.
In essence, what is malvertising comes down to a web-based, infrastructure-level attack that exploits the advertising supply chain, whereas adware is a device-level infection that operates independently and continuously on the victim’s computer. Understanding this distinction is essential for choosing the right defense strategy against each threat.
How do malvertisements affect users?Link to heading
Malvertising poses a serious threat to users regardless of whether they interact with an ad or not. Simply loading a page that serves a compromised ad can be enough to trigger an attack. The most common ways passive exposure causes harm include:
Drive-by download attacks: These occur when malware or adware is silently installed on a user's device without any deliberate action on their part. The attack exploits unpatched vulnerabilities in the user's browser or its plugins, executing the malicious payload the moment the ad renders on screen.
Forced browser redirects: Users are involuntarily pushed away from the page they intended to visit and sent to a malicious destination. This happens without warning and exposes the user to further threats the moment the redirect completes.
Unauthorized display of malicious content or pop-ups: Some malvertisements execute embedded JavaScript to inject additional ads, overlays, or harmful content beyond what the legitimate ad network ever approved or displayed.
When a user goes further and actually clicks on a malicious ad, the consequences escalate significantly:
Malware or adware installation: A click can directly trigger code execution that installs harmful software on the device, often without displaying any visible prompt or confirmation to the user.
Redirect to a malicious website: Rather than landing on the destination the ad appeared to promote, the user is sent to a hostile site designed to exploit, deceive, or further infect their device.
Phishing attacks: Certain malicious ads redirect users to convincing replicas of legitimate websites, banking portals, login pages, or service platforms, with the sole purpose of tricking them into submitting credentials, financial details, or other sensitive personal information.
Awareness of these attack vectors is the first and most critical step toward safer browsing habits.
The impact of malvertisements on publishersLink to heading
When cybercriminals successfully compromise an ad network, the publishers relying on that network bear a disproportionate share of the damage. Their brand reputation suffers immediately, user trust erodes, site traffic drops, and advertising revenue declines. In more serious cases, publishers may face legal liability for the harm their platform, however unintentionally, inflicted on visitors.
Publishers are generally aware of this risk, but detecting and neutralizing malicious ads before they reach users remains an exceptionally difficult problem to solve. Ad networks operate at enormous scale, serving creatives from thousands of advertisers simultaneously and rendering them dynamically through real-time bidding systems.
The speed and volume at which ads are selected, served, and displayed makes it practically impossible to thoroughly vet every single creative before it appears in front of a real user.
Methods of malware insertion into adsLink to heading
Understanding how malware enters the advertising pipeline is essential for recognizing where the attack surface truly lies. Attackers have developed multiple techniques to compromise ads at different points in the delivery chain:
Malware in ad calls
Every time a webpage loads an ad, the ad exchange routes the request through a series of third-party servers before the creative is delivered. If an attacker gains access to any one of those intermediary servers, they can inject malicious code directly into the ad payload before it ever reaches the user's browser.
Malware injected post-click
Clicking an ad does not send the user directly to a landing page. Instead, the request passes through a sequence of redirect URLs. If an attacker has compromised any single URL in that chain, they can intercept the redirect and silently execute malicious code on the user's device mid-journey.
Malware in ad creatives
Malicious code can be embedded directly inside text or banner ad creatives. HTML5, for example, allows ad creatives to combine images and JavaScript within a single file, an opening that attackers readily exploit. Ad networks that still support the Flash (.swf) format are particularly exposed to this vector.
Malware within a tracking pixel
Pixels are small pieces of code embedded in ad calls or landing pages to collect analytics and tracking data. A legitimate pixel only transmits data outward, but if an attacker intercepts the pixel's delivery path, they can push a response containing malicious code back to the user's browser without any visible indication.
Malware within video ads
Standard video ad formats such as VAST can carry third-party pixels inside them, some of which may contain malicious code. Video players offer no inherent protection against this. Beyond pixels, a video ad can also infect users by surfacing a malicious URL at the point the video ends.
Malware within Flash video
Flash-based video ads carry a particularly dangerous capability, they can inject an invisible inline frame (iframe) directly into the page, which silently downloads malware without requiring the user to click or interact with the video at all. Malicious code can also be planted inside pre-roll banner elements that load in the background while the main video file is still buffering.
Malware on a landing page
Even after a user clicks a legitimate-looking ad and lands on a page hosted by a reputable website, the threat is not necessarily over. Clickable elements on otherwise genuine landing pages can execute malicious code on arrival. This method is especially dangerous because the user has no reason to suspect foul play, the ad appeared legitimate, the landing page looks authentic, yet an infected element on that page quietly compromises their device.
How to avoid and prevent malvertisingLink to heading
Knowing what is malvertising is only the first step, taking concrete action to defend against it is what ultimately matters. Malvertising is difficult to detect and even harder to neutralize after the fact. Effective defense requires deliberate action from both end users and the publishers who serve ads to them.
How users can protect themselvesLink to heading
- Keep all software updated: Browsers, operating systems, and plugins should be updated regularly to ensure the latest security patches are applied. Outdated software is a primary target for malware because known vulnerabilities remain open and exploitable long after patches have been released.
- Use an ad blocker: Ad blockers prevent many malvertising attempts by stopping the ad itself from loading in the browser, eliminating the delivery mechanism entirely before any malicious code can execute.
- Disable JavaScript and Flash where possible: Both technologies are frequently used to deliver malicious payloads. Disabling them reduces the attack surface significantly, though it may limit the functionality of certain websites. The security tradeoff is generally worth it for high-risk browsing environments.
- Exercise caution with pop-ups: Never click on pop-up ads or unexpected windows. Close them using the window's native close button or, if necessary, through the system's task manager, never by interacting with the content inside the pop-up itself.
- Run reputable antivirus software: A well-maintained antivirus solution can detect and neutralize a wide range of malware strains before they cause damage. Keeping virus definitions current is just as important as having the software installed in the first place.
- Enable click-to-play for multimedia content: Click-to-play settings require explicit user action before any multimedia element runs in the browser. This prevents malicious scripts or video payloads from executing automatically on page load.
No single measure provides complete protection. The strongest defense is always a combination of these tools paired with consistent awareness and deliberate online behavior.
How publishers can reduce malvertising riskLink to heading
- Thoroughly vet ad network partners: Before integrating any ad network, investigate its reputation, security track record, and the robustness of its fraud prevention practices. Publishers should specifically ask about ad delivery paths and what safeguards the network has in place to identify and block malicious creatives before they reach end users.
- Implement a rigorous ad creative scanning process: Every ad creative should pass through a thorough scanning process before it is approved for display. This step helps identify embedded malware, obfuscated scripts, or any unauthorized code that could compromise user devices or expose the publisher to liability.
- Restrict allowed file types in ad frames: Enforcing a strict policy on which file formats are accepted in ad placements dramatically reduces exposure. Limiting creatives to static formats such as JPG or PNG, while blocking JavaScript-dependent or Flash-based formats, removes the most commonly exploited delivery mechanisms from the equation entirely.
Applying these practices consistently allows publishers to substantially reduce their malvertising exposure and maintain a safer, more trustworthy environment for every visitor to their platform.
ConclusionLink to heading
After learning what is malvertising, readers will hopefully have a clearer understanding of how malicious advertisements operate and the serious risks they can pose to both users and websites. Although these attacks often hide inside seemingly normal ads, the risks they create are very real.
The best defense against malvertising is a proactive approach to cybersecurity. Keeping software updated, avoiding suspicious pop-ups, using reliable security tools, and carefully managing third-party advertising content can significantly reduce the risk of infection.
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