Investigation of iOS App Startup Crash Caused by UIPasteboard Access and OpenUDID

Since version 9.0.0 of the 58.com iOS client, users reported that the app sometimes timed out during launch, was killed by the system, and could only be recovered by restarting the phone. Similar launch failures were observed in other apps such as JD.com, Dianping, and Tencent Video.

Crash logs extracted via Xcode revealed that the termination occurred on the main thread with an exception type SIGKILL and code 0x8badf00d, indicating the iOS watchdog killed the process because it exceeded the allowed launch time.

The SIGKILL signal cannot be intercepted by the app, which explains why third‑party crash collectors like Bugly did not capture the crash.

Further investigation showed that when the crash was triggered, any clipboard (UIPasteboard) operation in any app caused the app to freeze. The OpenUDID library, which obtains a device identifier by reading 100 private pasteboard slots, was identified as the root cause. The library performs a for‑loop that accesses the clipboard 100 times on the main thread, and many SDKs embed a private copy of this logic.

To verify the impact, a series of experiments were performed:

Running the 100‑iteration UIPasteboard test on the main thread took about 50 seconds, confirming that such a workload would block app launch.

Moving the same test to a background thread allowed the app to start normally, showing that the clipboard API is thread‑synchronised but still blocks the calling thread.

Running both main‑thread and background‑thread tests together showed that total execution time remained unchanged, indicating a global lock on UIPasteboard.

Creating 1 000 concurrent background threads each performing 100 clipboard accesses caused the system clipboard to hang, after which all apps (including JD.com, Tencent Video, etc.) became unresponsive.

The experiments proved that high‑concurrency clipboard access can freeze the entire system, and that OpenUDID’s repeated clipboard reads dramatically increase this risk, especially when multiple SDKs each instantiate their own OpenUDID implementation.

A survey of popular SDKs (Didi, iFlytek, Weibo, Alipay, WeChat, Toutiao, Baidu Map, etc.) showed that most still contain the original OpenUDID logic that accesses the clipboard 100 times, often without modification.

To mitigate the issue, the authors recommend abandoning the clipboard‑based storage of OpenUDID and instead persisting the identifier in the iOS keychain, which is not subject to the same process‑wide synchronization constraints.

In summary, the crash was caused by a systemic interaction between the iOS UIPasteboard API and the OpenUDID implementation; avoiding clipboard access during app startup and replacing OpenUDID’s clipboard logic with keychain storage can prevent the watchdog‑triggered termination and improve overall app stability.