"My balance increased?"
Hooking-based balance and limit manipulation: The most dangerous attack that can cripple a financial app
"My balance is clearly insufficient, but the payment was made."
One day, the customer service center of fintech app A, which offered both simple payment and microloan features, began receiving a steady stream of strange inquiries.
At first, they thought it was a simple mistake or user misunderstanding. However, the inquiries became increasingly specific.
✔ The payment was approved even though the balance was 0 won.
✔ The loan limit suddenly increased.
✔ All transactions appeared normal on the app screen.
The operations team immediately checked the server logs and payment records.
However, the results were unexpected.
- The server logs were perfectly normal.
- The payment approval/rejection logic was also fine.
- There were no signs of external intrusion.
"This is strange... There's clearly nothing wrong with the server. Why is this happening?"
The Real Problem - The "App," Not the Server, Was Being Deceived
As the investigation continued, the security team discovered a common thread. Most of the affected accounts were accessed from a specific Android environment.
A close analysis revealed that the attackers did not compromise the server.
Instead, they were manipulating the app itself running on the user's smartphone.
The actual attack method was as follows:
• Using a hooking tool to access memory while the app was running.
• Forcibly changing the return value of the balance/limit query function. o Return false → Return true
• The app screen displayed "Insufficient balance."
• Payment/transfer requests were sent to the server as if they were legitimate users.
In other words,
the server was not fooled, but the app had already been manipulated.
The server received a "normal request," the user confirmed "normal approval," and in the meantime, a financial incident occurred.
Why is this attack particularly virulent in financial apps?
The reason hooking-based attacks are so dangerous is because, on the surface, everything appears normal.
✔ Users are reassured by the confirmation screen.
✔ Server logs show no abnormalities.
✔ Small payments can be repeated without notice.
✔ Damage accumulates, and identifying the cause is delayed.
Especially in financial and fintech apps, these attacks aren't simply bugs; they directly lead to a breakdown in trust.
• Customers think, "I can't trust this app."
• Financial institutions are exposed to internal audit and regulatory risks.
• Trust, once lost, is difficult to regain.
At this point, the operations team realized a crucial truth:
"Server security alone is not enough to protect financial apps."
A Shift in Direction – Protect the App Itself
Company A completely changed its strategy:
"If an attack originates from the app, the app must be protected."
This is how they implemented a mobile security system centered on LIAPP, LIKEY, and LISS.
LIAPP – Making the App Execution Environment Trustworthy
First and foremost was LIAPP.
LIAPP Implementation Details
✔ Runtime Integrity Check
→ Real-time verification of app code and memory modifications
✔ Hooking and Debugging Detection
→ App Termination Upon Detection
✔ Memory Manipulation Blocking
→ Balance and Limit Inquiry Function Modification Prevented
✔ Abnormal Environment Access Restrictions
→ Blocking Rooting, Emulators, and Hacking Tool Environments
• ✔ Automated Input and Macro Pattern Detection
• ✔ Fake Screen Overlay Attack Neutralization
In particular, an additional integrity check was applied immediately before payment and remittance, protecting the entire payment flow.
LIKEY – Secure from Authentication to Input
The operations team went one step further. "What if an attacker steals login information?" So, they also implemented **LIKEY (Mobile Security Keypad**.
LIKEY's Role
✔ Key input encryption
✔ Blocks keylogger-based theft
✔ Protects login, password, and payment authentication sections
This blocked the entire path from account theft to hooking attacks.
LISS – Prevents information exposure through the screen
Finally, LISS was added.
LISS Application Effects
✔ Blocks screen capture and screen recording
✔ Blocks external overlay apps
This eliminated the possibility of externally leaking balance, limit, and payment information.
Results – "No More Hacks"
After implementing the security measures, the results were clear.
✔ Completely blocked attempts to manipulate balances and limits
✔ Maintained zero payment anomalies
✔ Passed internal security audits by financial institutions
✔ Restored user trust
This response was even confirmed in the illegal community:
"This app can no longer be hooked."
Only then did the operations team breathe a sigh of relief.
Lessons from Financial App Security
The lesson Company A learned from this incident was clear:
Financial app security isn't about "server protection," but about "trust preservation."
• Securing servers alone isn't enough.
• Protecting the network alone isn't enough.
• The apps in users' hands must be secure.
Minimum Security System for Financial and Fintech Apps
The following are no longer optional for financial apps, but rather fundamental:
• LIAPP → App integrity, hooking, and forgery prevention
• LIKEY → Authentication and input information protection
• LISS → Screen and information leak prevention
These three are not separate functions, but a single security system for operating financial services.
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