Your Samsung device comes pre-installed with a weather app that seems harmless enough – providing you with current conditions and forecasts for your saved locations. However, what's happening behind the scenes is far more intriguing. In this article, we'll delve into the fascinating world of mobile marketing and explore how a seemingly innocuous feature can create a persistent cross-session tracking identifier.

The Samsung Weather app periodically sends HTTP requests to The Weather Company's API (api.weather.com) at fixed intervals, including a placeid parameter – a 64-character hexadecimal string that maps to a saved location in the user's weather configuration. What's remarkable is that this combination of placeid values across a user's saved locations creates a fingerprint that persists across IP address changes and is trivially observable by the API provider.

But how exactly does this work? Let's take a closer look at the mechanism behind it. The Samsung Weather app polls api.weather.com on a recurring schedule, requesting forecast data, air quality indices, and location metadata for each of the user's saved locations. Every request includes a placeid URL parameter that's used to identify specific locations.

Here's what happens when you save multiple locations: the combination of placeid values becomes unique to your device. In fact, our analysis of 9,211 weather API requests from 42 Samsung device owners over five days reveals that these combinations produce unique user identifiers in an astonishing 96.4% of cases – with stability confirmed across the full observation window.

Now, let's examine the placeid mechanism in more detail. The API responds with a JSON payload that includes the resolved location metadata. The placeid is not just a device-level hash but rather a location-level hash, which means that multiple users who save the same city will transmit the same placeid value.

Of the 143 distinct placeid values observed in our dataset, six appeared in traffic from two or more distinct users – and where location metadata was available, the same placeid resolved to the same city and coordinates regardless of which device transmitted it. The remaining 137 values (95.8%) were unique to a single user.

The fingerprint: combination uniqueness

So, what happens when we combine these placeid values? A single placeid identifies a location, not a user. The fingerprint emerges from the full set of placeid values associated with a device's saved locations.

When we aggregated each user's distinct placeid values into a sorted array across 29 users who transmitted at least one placeid (excluding 12 devices whose requests contained no placeid parameter and one undefined session), the results were striking:

  • 29 users produced 28 distinct fingerprints
  • 27 of 28 fingerprints (96.4%) were unique to a single user
  • The only collision: two users who each tracked a single, identical location

Every user with two or more saved locations had a fingerprint shared by no one else in the dataset. Users ranged from 1 saved location to 17, with the distribution suggesting that even modest use of the weather app's location list creates a highly distinctive identifier.

Persistence across time and network changes

But what happens when users switch networks or devices? Can the same fingerprint be recreated? The answer is yes – as long as the user has saved locations. This persistence is crucial for mobile marketing, enabling targeted advertising that follows users across different devices and networks.

In conclusion, the Samsung Weather app's placeid mechanism creates a powerful identifier that can be used to track user behavior and preferences. By combining these identifiers with other data points, such as browsing history and search queries, marketers can create highly effective campaigns that reach their target audience across multiple platforms. The implications are far-reaching – and it's essential for users to understand the power of their own location data in shaping their mobile marketing experience.