What Is Malvertising? How Malicious Ads Slip Past Your Ad Blocker
Key Takeaways
You installed an ad blocker. You're careful about the sites you visit. You don't click on anything suspicious. And yet, malicious ads are still reaching you, and some of them don't even need you to click.
Malvertising is one of the quieter threats online. It doesn't announce itself. It hides inside the same ad networks that power legitimate websites, and it exploits a simple truth: most people trust that ads on a familiar site are safe. They're not always.
This is what malvertising is, how it works, and why your ad blocker alone isn't enough to stop it.
What is malvertising?
Malvertising, short for malicious advertising, is when attackers inject harmful code into online ads that appear on otherwise legitimate websites. The ad itself looks normal. It might be a banner, a search result listing, or a pop-up. But underneath, it's carrying a payload built to steal your data, redirect you to a phishing page, or silently download malware onto your device.
The key distinction from other threats: the website you're visiting doesn't have to be malicious. Attackers don't hack the site. They buy ad space through the same programmatic networks that any legitimate advertiser uses, then slip their malicious creative into the rotation.
That's what makes it effective. When you see an ad on a major news site or a trusted search engine, you're not thinking "this might infect me." You're just browsing.
How malvertising actually works
Online ads aren't served by the websites you visit. They come from a chain of third-party ad networks, exchanges, and demand-side platforms working behind the scenes. A single page load can trigger requests to a dozen external servers before an ad appears on your screen.
Attackers insert themselves into that chain. The approach varies, but the most common methods are:
- Drive-by downloads. Malicious code in the ad executes as soon as the ad loads, no click required. Your browser renders the ad, the code runs, and if there's a vulnerability in your browser or a plugin, malware installs silently.
- Forced redirects. You land on a page and get pushed to a different URL before you can do anything. The destination might be a fake tech support page, a phishing site, or an instant-download prompt. In 2024, forced redirects accounted for 81% of all malicious ad attacks in October alone, according to AdMonsters.
- Click-based attacks. The ad looks like a real download button or a search result. You click, you get malware. This is the method used in a high-profile 2024 campaign where threat actors placed fake Google Authenticator ads in Google Search, ads that displayed "google.com" as the URL, and delivered an information-stealing malware called DeerStealer when users clicked through, according to Bleeping Computer.
The DeerStealer case is worth pausing on. The fake ad cleared Google's verified advertiser process. It showed a legitimate Google URL. It looked exactly like the real thing. Google blocked the campaign after it was reported, but the campaign ran.
Why ad blockers don't always catch it
Ad blockers work by matching known ad-serving domains and URLs against a blocklist. If the ad request matches something on the list, it gets blocked. That's useful. It filters out a lot of junk.
But malvertising routinely gets around this for a few reasons.
First, attackers use legitimate ad networks. If the ad is being served through Google's infrastructure or a major programmatic exchange, it won't appear on any blocklist, because that same infrastructure delivers millions of clean ads every day.
Second, they use cloaking. The ad creative shown to security reviewers and automated systems looks different from what a real user sees. The malicious code only activates for real visitors, based on browser fingerprinting, geographic location, or device type. BlackFog describes this as a core evasion tactic: attackers specifically design campaigns to pass through publisher filters.
Third, some attacks don't come from an ad URL at all. The ad loads from a clean source, then pulls in a malicious script from a secondary server once it's already on your page. The blocker never sees the bad request because it was nested inside something it already approved.
What's actually at risk
The payload varies. What attackers do once they've gotten through depends on what they're after.
Common outcomes include:
- Credential theft. Info-stealers like DeerStealer harvest passwords, browser cookies, and saved login sessions. That's enough to take over accounts without needing your password at all.
- Ransomware delivery. Some campaigns drop ransomware that encrypts your files and demands payment. SentinelOne notes that the shift toward ransomware delivery via malvertising accelerated significantly from 2019 onward.
- Browser hijacking. Extensions or scripts get installed that redirect searches, inject ads, or track your activity across sites.
- Fake software downloads. The user downloads what looks like VLC, 7-Zip, or a browser update and gets malware bundled in.
Malvertising surged 10% in 2024 year-over-year, and over 70% of users now perceive at least half of online ads as untrustworthy, per AdMonsters. In the U.S., one in every 160 ads served in 2024 was malicious.
What actually protects you
Ad blockers help. They're not useless. But they're a filter, not a firewall. Here's what actually reduces your risk:
Keep software updated. Drive-by downloads depend on vulnerabilities in outdated browsers and plugins. If your browser is current, most drive-by code has no foothold. This is the single most effective technical countermeasure against no-click malvertising.
Be skeptical of search ads. Paid search results sit above organic results and can be bought by anyone, including attackers. When downloading software, go directly to the official site rather than clicking on a top ad. Bookmark the sites you use regularly.
Scan downloads before running them. Even if something looks legitimate, running an unknown executable is always a risk. Real-time scanning catches many known threats before they execute.
Use a browser-layer security solution. Tools that monitor what actually runs in your browser, not just what URLs you visit, can catch malicious scripts even when they've slipped past blockers. Guardio works at this layer on your browser and your phone, detecting malicious behavior in real time and blocking threats before they reach your device. With 100% phishing detection in independent tests, it's built for exactly the kind of threat that looks legitimate until it isn't.
Don't rely on verified badges. The Google Authenticator campaign showed that attackers can pass platform verification. A checkmark or an official-looking URL isn't proof of safety.
Stay protected from malicious ads
Malvertising is built to look like nothing is wrong. That's what makes protection at the device level worth having. Guardio monitors active scripts and behavior in real time across your browser and your phone, catching threats that ad blockers miss and stopping them before they reach your device.
Get a free security scan with Guardio today and stay protected from malvertising.
Conclusion
Malvertising works because it looks normal. It uses the same pipes as legitimate advertising, clears the same automated checks, and appears on sites you trust. Your ad blocker catches a lot, but it wasn't built for threats that live inside the infrastructure it can't block.
The practical takeaway isn't to stop using the web. It's to stop treating an ad blocker as your only line of defense. Keep your browser updated, be deliberate about what you click and download, and use a security solution that operates at the browser and device level, where these threats actually happen.
FAQs
What is malvertising in simple terms?
Malvertising is when attackers hide malicious code inside online ads. The ads appear on legitimate websites through real ad networks, so they look completely normal, but they can install malware, redirect you to phishing pages, or steal your data.
Can you get malware from an ad without clicking?
Yes. Drive-by download attacks execute malicious code as soon as the ad loads in your browser. No click is needed. These attacks typically exploit vulnerabilities in outdated browsers or browser plugins.
Do ad blockers protect against malvertising?
Partially. Ad blockers filter out known ad-serving domains, but they can't catch malvertising that runs through legitimate ad networks or uses cloaking to hide from automated filters. A browser-level security solution gives you more reliable protection.
How did malvertising bypass Google's ad platform?
In a 2024 campaign, attackers created verified Google advertiser accounts and ran fake Google Authenticator ads that displayed google.com as the URL. Cloaking techniques showed Google's reviewers a clean version of the ad while real visitors saw the malicious one.
What does malvertising install on your device?
It depends on the campaign. Common payloads include info-stealers (which steal passwords and cookies), ransomware, browser hijackers, and fake software installers bundled with malware.
Is malvertising getting worse?
Malvertising increased 10% year-over-year in 2024. Forced redirects were the dominant attack method, and mobile devices are increasingly targeted because security protections there tend to be weaker.
What's the difference between malvertising and phishing?
Phishing typically involves a deceptive message, an email or text, that tricks you into handing over credentials. Malvertising delivers threats through ads, often without any interaction required. They can overlap when a malicious ad leads to a phishing page.
How can I protect myself from malvertising?
Keep your browser and software updated, avoid clicking on search ads to download software, scan files before running them, and use a browser-level security tool that monitors active scripts and behavior, not just URLs.
