Zero-Day Vulnerabilities and Malvertising on Android: A Guide to Protection and Security

Android is the world’s most widely used smartphone operating system. In 2024, roughly 82% of all smartphones sold ran on Android. In German-speaking countries (DACH), Android also leads with a market share of around 60–70%. In the United States, the current market share is about 45–50%. This extensive distribution makes Android devices an attractive target for cyberattacks. At the same time, security experts have observed a sharp increase in newly discovered vulnerabilities in recent years, especially so-called zero-day flaws. In 2021 alone, a record number of zero-day vulnerabilities were exploited for the first time, and 2022 still saw numerous cases—almost twice as many as in 2020. Threats like Pegasus spyware or large-scale Android malware campaigns are making headlines and demonstrate how real the danger has become.

Another often underestimated threat is malvertising—malicious advertising. Cybercriminals are increasingly using online ads as an entry point to distribute malware. Even legitimate websites can unknowingly deliver infected ads. Zero-day exploits and malvertising form a particularly tricky combination: attackers can exploit unknown security holes in Android or browsers to inject malware via manipulated ad banners—often without you doing a thing. This guide will clearly explain what zero-day vulnerabilities and malvertising are all about and provide practical steps you can take to protect your Android device against such attacks.
 

What Is a Zero-Day Vulnerability?

A zero-day vulnerability is a security flaw in software that the vendor is not yet aware of—meaning they’ve had “zero days” to provide a patch. If an attacker discovers such a vulnerability, they can exploit it before an update is available. This makes zero-day exploits extremely dangerous because there is initially no defense in the form of security updates. In the worst case, a zero-day vulnerability remains undiscovered and open for years, traded in underground forums or by hacker groups. Anyone using the affected application or system could potentially become a victim of a zero-day attack—from the general public to specifically targeted individuals like journalists or businesses. Zero-days are especially appealing for state-backed hackers and cybercriminals because they can compromise even well-protected devices.

A zero-day exploit is the actual attack that exploits such an unknown vulnerability. Attackers often try to trick you into opening a booby-trapped link or attachment—perhaps via email or text message, which triggers the security hole. There are also zero-click attacks requiring no user interaction at all. Clever hackers might exploit an unpatched flaw in a messaging app or the system by sending a specially crafted message or call to the target device that automatically executes malicious code. This scenario happened with the notorious Pegasus spyware: this highly sophisticated surveillance tool can fully infect a smartphone without a single tap or click on your part. After a successful infection, the attacker can do anything as though they physically have your device in hand—from reading private data to installing more malware.

In short: zero-day vulnerabilities are unknown and unpatched security holes. When actively exploited, they’re called zero-day exploits. Because no patch exists at first, the risk is high—and immediate. Android’s broad user base and its slow update rollout put the advantage even more in the attackers’ hands. Even if Google fixes a flaw, it can take weeks to months for the update to reach all devices. Attackers exploit this time window. That makes it crucial to quickly identify such vulnerabilities and protect your device as effectively as possible—more on that in the protection measures below.
 

What Are Malvertising and Drive-by Downloads?

Malvertising (short for “malicious advertising”) refers to criminally placed or manipulated online ads deliberately designed to spread malware. Instead of legitimate advertising, malicious code is delivered via ad networks—often so cleverly disguised that neither website operators nor you as a user realize it. To you, it looks like a normal ad, but behind the scenes, it can already be trying to download malware or redirect you to a fraudulent website. Malvertising requires little or no user interaction. Typical examples include:

1. A banner ad that secretly installs a virus when clicked.
2. Pop-ups that claim “Your device is infected!” and prompt you to download a “cleaning app”—which is actually malware itself.

Malvertising attackers often still try to get you to click something. A possible scenario: You see a tempting offer or security alert in an ad on a reputable news site and click on it. Behind the scenes, however, the ad server was hacked and booby-trapped with malicious code. By clicking, you might trigger a drive-by download or get redirected to a phishing site. A drive-by download occurs when malware is downloaded in the background without your active knowledge—so the malware installs itself while you think you’re just clicking on a harmless link or banner.

An even more devious variety requires no click at all. This happens when an ad exploits security flaws in the browser or system. Just loading the webpage containing the infected ad can download malware. These drive-by downloads often rely on known vulnerabilities in browser plug-ins or the operating system. As soon as you visit the affected page, the exploit code runs in the background and attempts to inject malware onto the device—without any further action from you. This has happened before: even renowned websites have fallen victim to malvertising campaigns where their ad space was abused. The sites themselves weren’t compromised, but the ads they showed came from an infected ad network—visitors to those pages could get malware without having done anything “wrong.”

Malvertising can take many forms. Beyond drive-by downloads (automatic infection), there might be forced redirects taking you to a harmful site without warning, or scareware banners that play on fear. But they all share the same trick: using ad content as a Trojan horse. It’s especially perfidious because you don’t even have to visit shady sites—trusted websites can unknowingly carry malicious ads. That’s why you must stay alert, and it’s crucial to use technical safeguards like ad blockers and regularly updated software.

How Are Zero-Day Vulnerabilities and Malvertising Connected?

Zero-day exploits and malvertising can be a dangerous mix. In many cases, malvertising attacks rely on known (but perhaps not yet widely patched) vulnerabilities—for example, a flaw in Android’s WebView component or a media player plugin. But there are scenarios in which malvertising specifically exploits zero-day vulnerabilities to infect devices that are otherwise fully up-to-date. A common tactic: an attacker smuggles an “exploit kit” through an ad network—a bundle of malicious code that scans your device for vulnerabilities when the ad loads in your browser. If the kit finds a suitable flaw, it exploits it immediately to install malware. If that vulnerability was previously unknown, it’s a zero-day attack via malvertising.

A well-known example that illustrates the dangers of zero-day exploits—even if it wasn’t directly spread through ads—is the Pegasus spyware. Developed by the NSO Group, Pegasus used multiple zero-day flaws to infiltrate smartphones. Some variants even supported zero-click attacks: a specially crafted message would install the spyware without any user interaction. Android devices were also targeted by Pegasus in various forms, though often through slightly different methods. The Pegasus case made global headlines and proved that zero-day exploits truly exist and can compromise even the newest, best-secured devices. Once infected, Pegasus could read virtually all data on the phone in real time (calls, chats, camera/mic, location, etc.)—all without you noticing. This demonstrates the vast damage zero-day vulnerabilities can inflict, especially in tandem with advanced malware.

Learn more in our blog post on “iPhone Zero-Click Exploits: How They Work and How to Protect Yourself”: https://www.protectstar.com/en/blog/iphone-zero-click-exploits-how-they-work-and-how-to-protect-yourself

Malvertising-driven attacks on Android have also grown in recent years. They’re often ad-fraud campaigns leveraging social engineering or bypassing security measures rather than focusing on technical flaws. One recent example is the “Vapor” campaign from 2025, where attackers published over 300 malicious apps on the Google Play Store. Masquerading as harmless tools, they were downloaded millions of times before Google took them down. Once installed, they displayed constant fullscreen ads and led you to fake login pages to steal your credentials and credit card details. Ads were thus the central attack method. This shows malware via ads isn’t only a threat when browsing the web but also within seemingly trustworthy apps.

The link between zero-day flaws and malvertising is especially clear with drive-by attacks: a compromised ad banner can contain a zero-day exploit that activates as soon as it displays on a vulnerable device. Particularly on Android—where many users don’t receive updates right away—attackers can use ads to target devices with known flaws before patches are fully rolled out. In the worst-case scenario, an entirely unknown Android vulnerability gets deployed via malvertising, leaving you virtually no chance to protect yourself through mere caution. That’s why a holistic approach is crucial, including technical measures (updates, security apps, ad blockers) and vigilant behavior. The next section offers tips on how to greatly lower your risk.
 

How Can You Protect Yourself?

The good news is that there’s plenty you can do to protect your Android device from zero-day attacks and malvertising. While perfect security doesn’t exist, combining a few key precautions and the right tools can fend off most attacks or at least detect them early.

Even if you’re not a tech expert, you can prevent many attacks by following a few basic rules and adjusting some settings. Here’s a detailed guide that’s also clear for beginners:

1. Keep Your System and Apps Up to Date

• Install Updates Promptly: Download Android updates and security patches as soon as they’re available. Many exploits target known vulnerabilities in older versions that haven’t been patched yet.
• Regular App Updates: Make sure your apps on the Google Play Store (or your device’s official store) are up to date. These updates often close security gaps alongside adding new features.
• Enable Automatic Updates: Wherever possible, switch on automatic updates so your device and apps stay current without you having to remember.

2. Only Install Apps from Trusted Sources

• Google Play Store & Official Stores: Prefer to get your apps from the Google Play Store or the official store for your device (e.g., Samsung Galaxy Store). Direct downloads from developers’ websites (e.g., for Protectstar) can also be safe if they’re reputable.
• Avoid Many Third-Party Stores: Many unofficial app stores or (illegal) websites offering APK files are high-risk for malware.
  Learn more in our blog post on “How to Install APK Files on Android: Everything You Need to Know”: https://www.protectstar.com/en/blog/how-to-install-apk-files-on-android-everything-you-need-to-know
• Watch Out for Suspicious Apps: Check ratings, download counts, and permissions. A simple wallpaper or flashlight app typically shouldn’t need access to SMS or contacts.

3. Configure Secure Device Settings

• Lock Screen Setup: Use a strong password or PIN (preferably longer, e.g. 6+ digits). Biometric unlocking (fingerprint, face recognition) is convenient but should be properly configured.
• Developer Options: Only enable them if you truly need them, and turn them off afterward. USB debugging can be a security risk if left on all the time.
• Check App Permissions: Regularly look in your settings to see which apps can access your camera, microphone, location, contacts, etc. Revoke any permissions that aren’t necessary.

4. Watch Out for Phishing and Suspicious Links

• Don’t Click Links Rashly: Whether in ads, text messages, or emails—be cautious whenever something urges you to click, especially if it uses fear tactics (“Your account is locked!”).
• Don’t Open Unknown Attachments: If you’re unsure whether a sender is trustworthy, it’s safer to ignore attachments.
• When in Doubt, Ask: If someone sends you a link via messenger, check with them to confirm it’s legitimate before tapping it.

5. Use an Ad Blocker

• Block Malvertising: An ad blocker stops most website ads from loading in the first place, drastically reducing the chance of encountering a malicious banner. Unfortunately, Google Play Store policies often prohibit such apps since they affect Google’s own ad revenue.
• Browser Adblock Features: Many modern browsers (e.g., Firefox, Brave, Samsung Internet) offer built-in protections against pop-ups and tracking. Enable them in your settings.

6. Secure Browser and Search Settings

• Enable Safe Browsing: Use “Google Safe Browsing” in Chrome or similar features in Firefox to get warnings about known phishing and malware sites.
• Avoid Outdated/Insecure Plugins: While Flash and similar plugins are mostly gone on smartphones, disable any older plugins if you still have them.
• Caution with Search Ads: Search engines often show ads at the top. Check the URL before you tap it.

7. Device & Data Security (Backups & Encryption)

• Regular Backups: Routinely back up photos, documents, and contacts to external storage (e.g. an external drive) or a reputable cloud service. This prevents data loss if your device is damaged or attacked.
• Check Encryption: Most newer Android phones come with full-device encryption enabled, preventing unauthorized access to your data if the device is lost or stolen (without your password).

8. Beware of Public Wi-Fi: Use a VPN

• Secure Connection: In cafés, hotels, or airports, you should only enter passwords, banking details, etc. via an encrypted connection. A VPN (Virtual Private Network) secures you by encrypting your traffic.
• Choose a Trustworthy VPN: Free VPN services often rely on ads or data sales. Opt for a reputable provider with clear data protection policies.
  Read more in our blog post on “A Guide: Understanding and Effectively Using a VPN”: https://www.protectstar.com/en/blog/a-guide-understanding-and-effectively-using-a-vpn
• VPN Isn’t a Silver Bullet: It protects against eavesdropping over Wi-Fi but doesn’t replace antivirus software or safe user habits.

9. Use Mobile Security Software

• Antivirus & Anti-Spyware: Consider adding an extra security app. Many solutions use AI (artificial intelligence) to detect new, unknown threats.
  Learn more here https://www.protectstar.com/en/blog/anti-spy-vs-antivirus-ai-which-protectstar-security-concept-fits-you-best
• Behavior-Based Detection: Modern apps can spot abnormal activity (e.g., a supposedly benign app suddenly sending large amounts of data) and warn you.
  Find out more about Anti Spy Android here https://www.protectstar.com/en/blog/anti-spy-worlds-first-antispyware-app-dual-certification
• Supplementary Defense: Such security apps don’t replace your caution but can act as a valuable second line of defense.

10. Avoid Outdated Protocols and Plugins

• Stay with Modern Browsers: Always use the newest version of your browser. Older versions may have unpatched vulnerabilities.
• No Unnecessary Add-ons: Steer clear of obscure or outdated browser extensions that rarely get updates—they can be a gateway for malware.

11. Background Data Restrictions

• Monitor App Access: In the settings, you can often decide whether each app is allowed to use background data. This not only saves data usage but also prevents unknown malware from silently downloading or transmitting data.
• Don’t Disable Vital Services: Be careful not to block background data for essential services like messenger notifications.

12. Don’t Forget Older Smartphones

• Security Updates: Many manufacturers only provide software and security updates for a limited time. Check if your phone is still supported.
• Consider Upgrading: If your phone no longer receives updates, it becomes progressively less secure. It may be worth investing in a newer phone or installing alternative firmware (e.g., GrapheneOS), if available.

13. Raise Awareness Among Friends and Family

• Share What You Know: Talk to relatives, friends, and coworkers about current scams, fraud attempts, and security apps.
• Stay Vigilant Together: The more people know about phishing, malvertising, and zero-day vulnerabilities, the harder it is for cybercriminals to succeed.

By following these measures, you significantly reduce your risk. No single tip can guarantee 100% protection, but together they make it much harder for attackers. In particular, the principle “patch before exploit”—installing updates before hackers can pounce—is essential. Prevention is the best defense: if your system has no known vulnerabilities and you block obvious malvertising tactics, attackers will typically look for easier targets.
 

What to Do in an Emergency if Your Smartphone Is Infected

Even if you’re careful, you might suspect that your Android device is infected with malware or spyware. Maybe it’s behaving strangely—pop-ups appear, apps open by themselves, it’s extremely slow, or your data connection is running hot in the background. What should you do if it comes to that? Here are some immediate steps:

1. Stay Calm and Disconnect from the Network
   Turn off Wi-Fi and mobile data as soon as possible. This prevents ongoing malware from causing further damage or transmitting additional data. An infected device should be isolated until it’s cleaned.
2. Start Your Device in Safe Mode (Optional)
   Many Android devices offer a “safe mode,” which only loads essential system services. Suspicious apps are disabled in safe mode, making them easier to uninstall.
3. Identify and Remove Malware
   Scan your device with a security app. If you already have a trusted antivirus or anti-spyware tool installed, run a full scan. Otherwise, consider briefly going online to download a reputable security app like Protectstar Antivirus AI or Anti Spy. Uninstall suspicious apps (unfamiliar names, recently installed, excessive permissions). Some malware tries to gain admin rights to thwart removal—disable those rights in settings, then remove the app.
4. Re-scan Your Smartphone
   After deleting suspicious apps, perform another scan—ideally with a second security solution for double-checking. Update your system to close any exploited vulnerability.
5. Change Passwords and Secure Your Accounts
   If malware was indeed found, immediately change all important passwords—Google, email, online banking, social media, etc. Use another, clean device to do this if possible. Turn on two-factor authentication wherever you can.
6. Consider a Factory Reset
   If malware can’t be removed or you’re worried about hidden remnants (e.g., advanced spyware), a factory reset could help. Back up important photos and files separately first to avoid restoring malware with them.
7. Seek Professional Help
   If you suspect you’re the victim of a targeted spyware attack or if problems persist even after a reset, consult an expert. In most cases, however, the above steps are enough to remove common Android malware.

Reflect on how the infection occurred—did you trust a dubious app or click on a scam link? Learn from the incident to better protect yourself in the future. Ideally, it never gets that far in the first place—specialized security apps can spot suspicious activity early on.

Find out more in our blog post “Detecting, Removing, and Preventing Spyware on Android Devices”: https://www.protectstar.com/en/blog/detecting-removing-and-preventing-spyware-on-android-devices
 

Security Apps for Protection: Protectstar Anti Spy & Antivirus AI

Beyond basic precautions, security apps can make a decisive difference. The German company Protectstar offers two Android-focused solutions that together provide comprehensive all-around protection:

1. Protectstar Anti Spy
   https://www.protectstar.com/en/products/anti-spy
2. Protectstar Antivirus AI
   https://www.protectstar.com/en/products/antivirus-ai

Both apps are ad-free and highly privacy-conscious. They complement each other perfectly to defend against spyware/stalkerware as well as classic malware—including new, unknown threats. Below is a brief overview of how they work and why they stand out from standard security apps.

Protectstar Anti Spy – Detecting and Removing Spy Apps

Protectstar Anti Spy is a specialized anti-spyware tool for Android, built to identify and remove spyware, stalkerware, and hidden surveillance apps. Unlike a basic antivirus scanner, Anti Spy focuses on all forms of stealthy spy software that can lurk in the background—whether it’s government-grade spyware or secretly installed stalker apps. Anti Spy uses a dual-engine scanning method that combines classic signature detection with AI-based algorithms, enabling it to spot both known and brand-new spying tools. Even zero-day spyware is discovered early thanks to the AI component, which analyzes suspicious behavior.

Other strong points are user-friendliness and respect for your privacy: All you have to do is tap “Scan” to check your entire device—no special expertise required. At the same time, the app protects your personal data by not collecting anything unnecessary or showing ads. It requires only essential permissions and works quietly in the background.

Learn more here https://www.protectstar.com/en/blog/anti-spy-worlds-first-antispyware-app-dual-certification
 

Protectstar Antivirus AI – AI-Powered Defense Against Malware & Zero-Day Attacks

Protectstar Antivirus AI is a full-featured antivirus solution for Android that employs advanced AI to stop all kinds of malware—from classic viruses and Trojans to fresh zero-day attacks. The app uses self-learning AI algorithms to continuously monitor the system for suspicious behavior. This way, Antivirus AI can also detect newly emerging malware before it appears in signature databases. It’s backed by a second, classic scan engine to reliably identify known threats. This dual-engine architecture delivers an exceptionally high detection rate along with low false positives.

Antivirus AI provides real-time protection, real-time updates, and runs efficiently without draining your battery. Similar to Anti Spy, it’s ad-free and prioritizes privacy, avoiding the collection of sensitive user data. The straightforward design makes it easy to protect your device without navigating complex menus.

Find out more here https://www.protectstar.com/en/blog/android-antivirus-scanner-antivirus-ai-earns-dekra-masa-l1-certification

Together, Protectstar Anti Spy and Antivirus AI offer layered protection that complements one another and can detect zero-day threats at an early stage. Both apps are developed in Germany, which many users value for quality and trustworthiness, especially concerning data protection. They let you effectively secure your smartphone without handing over your data to third parties or being annoyed by ads.
 

Conclusion

Zero-day vulnerabilities and malvertising pose serious risks for Android users. However, by keeping your software up to date, acting cautiously, and using the right security tools, you can greatly minimize these dangers. Stay informed about emerging threats, follow the tips outlined here, and rely on proven security solutions. That way, you can continue using your Android device with confidence, rather than worrying about every ad or incoming message. Security starts with awareness—and this guide has shown you what it takes to keep zero-day loopholes and malware-laced ads at bay.