The QSharedMemory object, created in Windows service context does not apply the Security Descriptor to the HANDLE object, which prevents accessing the same shared memory object from the user context. 
Similar classes, like QLocalServer, create a security context in case of  QLocalServer::WorldAccessOption access option.

I propose to add a similar flag/option to the QSharedMemory object in order to access the shared memory from other security context, if required by design, at least with flag PAGE_READONLY