€54,000 in Charges in 13 Hours: Unrestricted Firebase Browser Key Triggers Gemini API Calls

In a project that had only used Firebase Authentication, after Firebase AI Logic was turned on, automated Gemini API requests from outside began pouring in.

As a result, over €54,000 was billed in 13 hours, unrelated to actual user traffic. The budget alert was set at €80, but the alarm went off several hours late, and by the time action was taken, costs had already accumulated to around €28,000.

The abnormal traffic stopped after the API was disabled and credentials were rotated, but Google Cloud support classified this as valid usage and refused a refund or fee adjustment.

The key issues are as follows.

• An API-unrestricted Firebase browser key, if exposed, can be exploited client-side
• Because of the delayed cost alert, the damage was only discovered after it had already grown large
• Support may treat this as normal billing on a system level, even if it isn't technically abuse
• Google explained that it is introducing spending limits, authentication keys, automatic key blocking, and prepaid billing to reduce such problems

According to Google's response:

• The Gemini API has billing account caps, and Tier 1 users can be blocked after a $250/month limit
• project spend caps can also be set
• All billing reports have an approximately 10-minute delay
• unrestricted keys will soon be disabled for the Gemini API
• Keys leaked on the public web can be automatically detected and disabled
• Keys created in Google AI Studio are restricted to Gemini API only by default
• Keys created in Google Cloud Console can access a broader scope, requiring per-service restrictions

The author additionally asks the following.

• Whether there are more similar cases after enabling Firebase AI Logic or Gemini
• Whether there are stronger protections beyond App Check, quotas, and server-side calls
• Whether there is an additional dispute channel in such situations

The conclusion of the case is clear. Never put keys in client-side code, and API key restrictions and budget limits must be set — cost defense is a matter of upfront design, not after-the-fact response.