Unexpected crash on App Foreground (iOS 18)

[tareksaeed224]

Describe the bug

Random crashes occurring in the Bugsnag iOS SDK when app returns to foreground on iOS 18, inside session upload code path.

Steps to reproduce

1. Launch app on iOS 18+ (simulator or real device).
2. Background → foreground (or just observe over time).
3. Crash happens, no specific user flow triggers it reliably.

Environment

• Bugsnag version: 6.34.0
• CocoaPods / Carthage version: 1.16.2 (CocoaPods)
• iOS version: iOS 18+
• Device: Physical Device
• Xcode version: Xcode 16.4
• Swift version: 5.9

Stack trace / crash report

EXC_BREAKPOINT: BUG IN CLIENT OF LIBMALLOC: memory corruption of free block

0  libsystem_malloc.dylib                __xzm_xzone_malloc_tiny_outlined
1  CoreFoundation                        ___CFStringAppendFormatCore
2  CoreFoundation                        __CFStringCreateWithFormatAndArgumentsReturningMetadata
3  CoreFoundation                        __CFStringCreateWithFormatAndArgumentsAux2
4  Foundation                            +[NSString stringWithFormat:]
5  <APP_NAME>                            BSGIntegrityHeaderValue
6  <APP_NAME>                            BSGPostJSONData
7  <APP_NAME>                            -[BSGSessionUploader sendSession:completionHandler:]
8  <APP_NAME>                            -[BSGSessionUploader uploadSession:]
9  <APP_NAME>                            -[BugsnagSessionTracker startNewSession]
10 <APP_NAME>                            -[BugsnagSessionTracker startNewSessionIfAutoCaptureEnabled]
11 <APP_NAME>                            -[BugsnagSessionTracker handleAppForegroundEvent]
12 CoreFoundation                        ___CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__
13 CoreFoundation                        ____CFXRegistrationPost_block_invoke
14 CoreFoundation                        __CFXRegistrationPost
15 CoreFoundation                        __CFXNotificationPost
16 Foundation                            -[NSNotificationCenter postNotificationName:object:userInfo:]
17 UIKitCore                             -[UIApplication _sendWillEnterForegroundCallbacks]
18 UIKitCore                             ___101-[_UISceneLifecycleMultiplexer _evalTransitionToSettings:...]_block_invoke_2
19 UIKitCore                             __UIScenePerformActionsWithLifecycleActionMask
20 UIKitCore                             ___101-[_UISceneLifecycleMultiplexer _evalTransitionToSettings:...]_block_invoke
21 UIKitCore                             -[_UISceneLifecycleMultiplexer _performBlock:withApplicationOfDeactivationReasons:fromReasons:]
22 UIKitCore                             -[_UISceneLifecycleMultiplexer _evalTransitionToSettings:fromSettings:forceExit:withTransitionStore:]
23 UIKitCore                             -[_UISceneLifecycleMultiplexer uiScene:transitionedFromState:withTransitionContext:]
24 UIKitCore                             ___186-[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction ...]_block_invoke
25 UIKitCore                             +[BSAnimationSettings tryAnimatingWithSettings:fromCurrentState:actions:completion:]
26 UIKitCore                             __UISceneSettingsDiffActionPerformChangesWithTransitionContextAndCompletion
27 UIKitCore                             -[_UIWindowSceneFBSSceneTransitionContextDrivenLifecycleSettingsDiffAction _performActionsForUIScene:...]
28 UIKitCore                             ___64-[UIScene scene:didUpdateWithDiff:transitionContext:completion:]_block_invoke.229
29 UIKitCore                             -[UIScene _emitSceneSettingsUpdateResponseForCompletion:afterSceneUpdateWork:]
30 UIKitCore                             -[UIScene scene:didUpdateWithDiff:transitionContext:completion:]
31 UIKitCore                             -[UIApplicationSceneClientAgent scene:handleEvent:withCompletion:]
32 FrontBoardServices                    ___76-[FBSScene updater:didUpdateSettings:withDiff:transitionContext:completion:]_block_invoke.146
33 FrontBoardServices                    -[FBSScene _callOutQueue_coalesceClientSettingsUpdates:]
34 FrontBoardServices                    -[FBSScene updater:didUpdateSettings:withDiff:transitionContext:completion:]
35 FrontBoardServices                    ___94-[FBSWorkspaceScenesClient _queue_updateScene:...]_block_invoke_2
36 FrontBoardServices                    -[FBSWorkspace _calloutQueue_executeCalloutFromSource:withBlock:]
37 FrontBoardServices                    ___94-[FBSWorkspaceScenesClient _queue_updateScene:...]_block_invoke.cold.1
38 FrontBoardServices                    ___94-[FBSWorkspaceScenesClient _queue_updateScene:...]_block_invoke
39 libdispatch.dylib                     __dispatch_client_callout
40 libdispatch.dylib                     __dispatch_block_invoke_direct
41 FrontBoardServices                    __FBSSERIALQUEUE_IS_CALLING_OUT_TO_A_BLOCK__
42 FrontBoardServices                    -[FBSMainRunLoopSerialQueue _targetQueue_performNextIfPossible]
43 FrontBoardServices                    -[FBSMainRunLoopSerialQueue _performNextFromRunLoopSource]
44 CoreFoundation                        ___CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__
45 CoreFoundation                        ___CFRunLoopDoSource0
46 CoreFoundation                        ___CFRunLoopDoSources0
47 CoreFoundation                        ___CFRunLoopRun
48 CoreFoundation                        _CFRunLoopRunSpecific
49 GraphicsServices                      _GSEventRunModal
50 UIKitCore                             -[UIApplication _run]
51 UIKitCore                             _UIApplicationMain
52 <APP_NAME>                            main (AppDelegate.swift:70)
53 dyld                                  start

Additional context

• Crash appears to be random, no reproducible steps beyond returning to foreground on iOS 18.
• Similar problems have been reported in other iOS crash-reporting SDKs.
• SQ - MessageBus Crash L:97 DataDog/dd-sdk-ios#2134
• Crashed: com.apple.main-thread getsentry/sentry-cocoa#4926

[agrzegorzewski]

Hi @tareksaeed224,

Thanks for reporting the issue. We suspect this is caused by memory corruption. We're actively investigating the issue and we'll get back to you as soon as we have an update.