How I Found a Zero Day on a Mobile App

When readers imagine finding a zero day vulnerability, Hollywood often comes to mind: flickering screens, dramatic soundtracks, and an elite hacker at work. However, the reality—while less theatrical—is even more fascinating. My discovery of a zero day on a popular mobile app was shaped by tenacity, curiosity, and a series of systematic strategies. This is the detailed journey of how I stumbled upon, confirmed, and responsibly reported a zero day bug lurking in plain sight on millions of devices.

Scoping the Target and Gathering Tools

Every successful hunt begins with reconnaissance. My initial goal was clear: assess the security posture of a rapidly trending mobile application that had yet to win over security researchers’ hearts. As an afficionado of mobile pentesting, I used a mixture of manual analysis and automated tools to dissect the application’s structure and network behavior.

Practical steps I took:

• Downloaded the APK: Using APKPure, I grabbed the latest version to avoid obfuscation changes present in some older releases.
• Reverse Engineered the APK: With tools like JADX and Apktool, I decompiled the app to readable Java code and resources.
• Set Up a Test Environment: My device ran on a custom Android image with root privileges, using Magisk for easy module management. To intercept the app’s traffic, I installed Burp Suite CA certificate on the device, thereby forcing the app’s encrypted connections through my proxy.
• Network Traffic Analysis: With HTTPS interception in place, I started inspecting requests during different workflows, from registration to content uploads.

Takeaway: Building a strong recon stage is like laying the foundation stones for everything that follows. A chain is only as strong as its weakest link—so set up your environment meticulously.

Probing Endpoints: The Hunt Begins

After gaining traffic insights, I began focusing on API endpoints. Mobile applications often communicate with backend services over secured endpoints. However, many companies overlook proper API security, assuming only the mobile app will interact with their server.

While testing, I:

• Catalogued all API endpoints the app used, mapping request structures and responses.
• Switched device languages, locations, and inputs to observe different app behaviors.
• Purposely crafted malformed requests to see if any error was gracefully or poorly handled (looking for stack traces or unhandled exceptions).

Example: One endpoint fetched user profile data when given a UUID. Surprisingly, modifying this UUID to other values returned different user profiles, revealing improper authorization controls—what’s classically known as an Insecure Direct Object Reference (IDOR). While this wasn’t a zero day (since similar issues are widely documented), it gave a taste of the lax mindset with which security had been implemented.

Actionable Tip:

Whenever you spot endpoints returning more data than they should, dig deeper. Corporations often rely on the obscurity of APIs, but Security through Obscurity is no defense at all.

Uncovering the Flaw: Sequence of Precise Moves

After several days of testing, frustration was mounting. Standard issues—IDOR, information leakage—were there but patched quickly. What set apart my hunt was persistence and patient exploration of less-trodden angles, especially around file handling and authentication flows.

The Eureka Moment: A Forgotten Endpoint

While fuzz-testing the endpoints, I discovered something odd—a file upload API that was supposed to accept only images, invoked during profile updates. But by inspecting the client logic, I realized the file type validation occurred exclusively on the mobile side (JavaScript check), with no matching server enforcement.

To test this, I:

• Modified a request to upload a crafted ZIP file with an embedded PHP shell, disguised with a .jpeg extension.
• Sent the request to the API endpoint using Burp Suite Repeater.
• Observed the response and then navigated to the file's likely public path.

The server not only saved the file—but executed it when visited. This classic Unrestricted File Upload vulnerability opened up Remote Code Execution (RCE) on their server.

Verification and Impact Assessment

To validate my findings while minimizing harm, I:

• Uploaded a benign file that echoed server information (for validation, not exploitation).
• Checked the time and permissions under which the command ran.
• Repeated the workflow with different extensions and payloads, confirming that server-side validation was completely absent!

Insight: Zero days often arise not just from advanced exploitation but from overlooking basic principles. Here, a lack of defense-in-depth led the developers to trust the client—a massive oversight.

The Ethical Route: Responsible Disclosure

Having confirmed a catastrophic zero day, my next step was clear: responsible disclosure. Navigating this phase properly protects both users and the reputation of security researchers.

My disclosure process:

• Documentation: Carefully organized my findings. I included proof-of-concept requests, screenshots, and minimal-impact evidence highlighting only what was necessary.
• Report Drafting: Framed my report in a non-accusatory, factual tone. I shared a step-by-step guide for the developers to reproduce the issue.
• Contacting the Vendor: Located a security contact email (sometimes companies publish a dedicated security.txt file on their website or their GitHub repo). In the absence of that, I reached out through public support with a pointer to the gravity of the situation.
• Timeline Agreement: Gave the company 90 days to fix the vulnerability before any public mention, as per industry best practices.
• CVE Assignment: Collaborated with a recognized CNA organization for the formal CVE assignment process.

Key Lesson: Reporting vulnerabilities isn't about one-upmanship; it's about raising the safety bar for everyone involved.

Preventative Measures: Learnings for Developers

The thrill of discovery brings with it the opportunity to educate. Vulnerabilities of this class—remote code execution through file upload—can be devastating, but they're preventable with secure development mindsets.

Developers and companies can:

• Enforce strict server-side file type and content validation: Never trust the client for any security check. Utilize libraries to enforce image format validation and whitelist acceptable media types.
• Sanitize file names and storage paths: Prevent path traversal and other attacks by rigorously normalizing and sanitizing user input.
• Isolate uploaded files: Host uploads in a directory with no execute permissions, and on a different subdomain if possible.
• Regularly scan for insecure APIs: Every release cycle, use automated tools and manual reviews for endpoint vulnerability, especially on all newly exposed and legacy APIs.
• Invest in Security Education: Hold periodic workshops for developers on secure coding and recent exploit trends.

Example in Practice: Facebook and Google employ robust internal and external bounty programs rewarding researchers for such discoveries. These incentives help keep their apps resilient and their users safer.

The Researcher's Toolbox: Techniques and Tips

Modern mobile vulnerabilities require a blend of traditional and mobile-specific testing methods. Below are tools and approaches that proved invaluable during my hunt:

Essential Tools:

• Burp Suite for intercepting, modifying, and replaying requests.
• JADX and Apktool for reverse-engineering Android applications.
• ProxyDroid for setting mobile-wide proxies (when engaging multiple apps).
• Genymotion as an Android emulator, to safely simulate hostile environments.
• Frida or Xposed Framework to instrument apps dynamically and bypass restrictions on the fly.

Workflow Suggestions:

• Automate common tasks using scripts but stay alert for anomalies not easily captured through automation.
• Log everything: When bugs emerge, being able to retrace your steps can mean distinguishing a true finding from a false positive.
• Community engagement: Forums like r/netsec, Twitter infosec crowd, and private Slack/Discord groups are hives of fresh ideas and lasting support.

Pro tip:

Stay up to date with public exploit disclosures via packetstormsecurity.com and CVE feeds. Knowing emerging patterns sharpens your sixth sense for where zero days might lurk.

Real-World Aftermath: Patching and Recognition

The story did not end with disclosure. Over the subsequent weeks:

• The vendor acknowledged my report in less than 36 hours, rapidly escalating the issue internally.
• Their engineering team issued a multi-region rolling update disabling file execution on the affected endpoint, followed by a complete redesign of the file upload logic.
• The vulnerability appeared in the next set of app release notes with a generic attribution to an “anonymous security researcher.”
• I received a generous bug bounty—evidence that goodwill toward security research pays off for both parties.

Industry Reflection: Mobile security is a two-way street. Users push for innovative features, while security researchers and organizations must unite to ensure that innovation doesn’t come at the expense of safety.

Lessons Learned: The Path Forward

Chasing zero days isn’t just about luck or brute force. It’s the accumulation of methodical routines, continuous learning, and a spirit of ethical stewardship.

If you're passionate about digital safety or eager to uncover the next big vulnerability, here’s what I suggest:

• Invest in process, not just tools: Hardware and software are just half the battle; critical thinking, patience, and adaptability matter even more.
• Never stop learning: Exploit techniques and defense mechanisms evolve rapidly. Regularly read research whitepapers, watch conference talks (like Black Hat and DEF CON), and participate in CTF challenges.
• Share responsibly: Fame and fortune may tempt a hasty disclosure, but wielding responsible disclosure is the mark of true professionalism.
• Remain humble: Every vulnerability is a result of human error. Treat developers and vendors as partners in security—as in this journey, shared goals always win.

With every zero day uncovered, the boundaries of digital safety are pushed forward a bit. Whether you’re a seasoned professional or an aspiring hacker, every bug found and reported brings us closer to a safer mobile ecosystem for all.