Sensitive data leaks into places developers do not think of as storage: the image cache, the network response cache, a WebView's cache, temporary files, thumbnails, log files. Your app may carefully keep secrets in the Keychain while a cached API response full of personal data sits in plain files in the cache directory. On a lost, compromised, or backed-up device, those caches are readable, and they often persist longer than expected. Here is where sensitive data ends up in caches and temp files, why it matters, and how to handle caching without leaking.
Short answer
Apps cache and buffer data in many places, image and network response caches, WebView caches, temporary files, thumbnails, and logs, and sensitive data can end up in those caches unencrypted, where it is readable on a compromised or backed-up device. Per OWASP MASVS, the fix is to avoid caching sensitive data, or to protect it with secure storage and clear it when it is no longer needed, such as on logout. Do not let authenticated responses, personal data, or downloaded sensitive documents sit in plain cache or temp files, disable caching for sensitive WebView content, and treat caches as storage that needs the same care as anything else holding sensitive data.
What you should know
- Caches are storage: sensitive data in them is exposed like any stored data.
- Many places cache data: image, network, WebView, temp files, thumbnails, logs.
- Caches are often unencrypted: and can persist longer than expected.
- Clear them when done: especially on logout.
- Avoid caching sensitive data: or protect it explicitly.
Where does sensitive data leak into caches?
In the buffering and caching your app and its libraries do automatically. The table lists the common spots.
| Cache or temp location | What can leak |
|---|---|
| Network response cache | Authenticated responses with personal data |
| Image cache | Profile photos, scanned documents, sensitive images |
| WebView cache | Pages and data from authenticated web content |
| Temporary files | Downloaded documents or data being processed |
| Thumbnails and previews | Snapshots of sensitive content |
| Log files | Sensitive values written during debugging |
The recurring theme is that caching is convenient and often automatic, an HTTP client caches responses, an image library caches downloads, a WebView caches pages, so sensitive data flows into these caches without an explicit decision to store it. A developer who is careful to keep a token in the Keychain can still have a cached API response containing the user's personal data sitting in plain files, because the caching happened quietly underneath.
Why does it matter?
Because cached data is stored data, with the same exposure. The contents of your app's cache and temporary directories can be read on a device that is compromised or, depending on the data, included in backups, so sensitive data there is exposed the same way it would be in any insecure storage. Caches also tend to outlive the moment they were useful, persisting after the user logs out or the data is no longer relevant, which widens the window of exposure. And because caches are not where developers expect sensitive data to be, the leak is easy to miss in a review that focuses on the obvious storage. So a cached authenticated response or a temporary copy of a sensitive document is a real data-exposure risk, not a harmless performance detail.
How do you handle caching of sensitive data?
Avoid caching it, protect it if you must, and clear it promptly. The simplest approach is to not cache sensitive data: configure your network client not to cache authenticated or sensitive responses, avoid caching sensitive images or documents, and do not write sensitive data to temporary files. Where caching sensitive data is unavoidable for the app to work, protect it, store it in secure storage or apply the platform's data protection rather than leaving it in plain cache files, and scope it tightly. Clear caches when the data is no longer needed, especially on logout, so a session's cached data does not linger. For WebViews, disable caching of authenticated or sensitive content. The principle is to treat caches and temp files as storage subject to the same rules as the rest of your sensitive data, rather than as invisible scratch space.
What to watch out for
The first trap is an HTTP or image library caching authenticated responses or sensitive media in plain files, which you did not explicitly decide to store; configure caching off for sensitive content. The second is caches persisting after logout, leaving a previous user's data on the device; clear them. The third is a WebView caching authenticated pages, or temp files holding sensitive downloads. A pre-submission scan such as PTKD.com (https://ptkd.com) reads the compiled APK, AAB, or IPA against OWASP MASVS and assesses how your app stores data, which helps you catch sensitive data handling that could leak into caches before you ship. The caching behavior you configure in the app and its libraries.
What to take away
- Sensitive data can leak into caches and temp files, network, image, and WebView caches, temporary files, thumbnails, and logs, often automatically and unencrypted.
- Cached data is stored data with the same exposure on a compromised or backed-up device, and it tends to persist longer than expected.
- Avoid caching sensitive data, protect it with secure storage or data protection if caching is unavoidable, disable caching of sensitive WebView content, and clear caches on logout.
- Use a pre-submission scan such as PTKD.com to assess how your app stores data and catch sensitive data that could leak into caches.
