Frequent standby-park churn (duplicate logins sharing one clientID) was
crashing the server with 0xC0000409 (STACK_BUFFER_OVERRUN). The heartbeat
handler posts the raw CONTEXT_OBJECT pointer via PostMessageA
(WM_UPDATE_ACTIVEWND) without holding a reference, so RemoveStaleContext
could recycle the context back to the pool and reuse it before the UI
thread consumed the queued message. UpdateActiveWindow then read a
stale/recycled object, and the unguarded Authorization memcpy overflowed
the 200-byte HeartbeatACK field and clobbered the stack cookie.
Hold the object's IoRefCount across the deferred message: fetch_add
before PostMessageA (with rollback on failure) and a paired fetch_sub in
UpdateUserEvent after UpdateActiveWindow returns. RemoveStaleContext
already waits for IoRefCount == 0 before MoveContextToFreePoolList, so
the object is now guaranteed to stay alive for the whole deferred call.
Also bound the three memcpy sites (two Signature[64], one
Authorization[200]) with explicit truncation so an unexpectedly long
signature or license can no longer overflow its fixed buffer in Release
builds, where ASSERT is a no-op.
Co-Authored-By: deepseek-v4-pro
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