Developers from the Replicant project (a free Android offshoot) have documented a serious software back-door in Samsung's Android phones, which "provides remote access to the data stored on the device." They believe it is "likely" that the backdoor could provide "over-the-air remote control" to "access the phone's file system."
At issue is Samsung's proprietary IPC protocol, used in its modems. This protocol implements a set of commands called "RFS commands." The Replicant team says that it can't find "any particular legitimacy nor relevant use-case" for adding these commands, but adds that "it is possible that these were added for legitimate purposes, without the intent of doing harm by providing a back-door. Nevertheless, the result is the same and it allows the modem to access the phone's storage."
The Replicant site includes proof-of-concept sourcecode for a program that will access the file-system over the modem. Replicant has created a replacement for the relevant Samsung software that does not allow for back-door access.
Samsung Galaxy devices running proprietary Android versions come with a back-door that provides remote access to the data stored on the device. In particular, the proprietary software that is in charge of handling the communications with the modem, using the Samsung IPC protocol, implements a class of requests known as RFS commands, that allows the modem to perform remote I/O operations on the phone's storage. As the modem is running proprietary software, it is likely that it offers over-the-air remote control, that could then be used to issue the incriminated RFS messages and access the phone's file system.
...The incriminated RFS messages of the Samsung IPC protocol were not found to have any particular legitimacy nor relevant use-case. However, it is possible that these were added for legitimate purposes, without the intent of doing harm by providing a back-door. Nevertheless, the result is the same and it allows the modem to access the phone's storage.
However, some RFS messages of the Samsung IPC protocol are legitimate (IPC_RFS_NV_READ_ITEM and IPC_RFS_NV_WRITE_ITEM) as they target a very precise file, known as the modem's NV data. There should be no particular security concern about these as both the proprietary implementation and its free software replacement strictly limit actions to that particular file.