Replicant

h1. Samsung Galaxy Back-door

This page contains a technical description of the back-door found in Samsung Galaxy devices.

For a general description of the issue, please refer to the following statement: 

*This back-door is present in most proprietary Android systems running on the affected Samsung Galaxy devices, including the ones that are shipped with the devices. However, when Replicant is installed on the device, this back-door is not effective: Replicant does not cooperate with back-doors.*

h2. Abstract

Samsung Galaxy devices running proprietary Android versions come with a back-door that gives 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.

*Disclaimer: We are not security researchers and cannot establish the absolute certainty that the incriminated software actually contains the back-door we describe. However, everything apparently points in that direction and we believe the information at our disposal is relevant enough to constitute a solid-enough basis for us to issue a statement about this presumed back-door. If you have any indication that would prove our statement wrong, do not hesitate contacting us.*

h2. Analysis

The following analysis was conducted using the @libsec-ril.so@ binary file (the incriminated proprietary software) as extracted from the CyanogenMod 10.1.3 system zip for the Galaxy S 3 (I9300), from location @system/lib/libsec-ril.so@.

*The developers involved in the present analysis did not ever agree to any sort of End User License Agreement that explicitly prohibited the reverse engineering and decompiling operations of the incriminated binary. The reverse engineering operations that lead to these findings originally took place during the development of [[Samsung-RIL]], the free software replacement for the incriminated program. Hence, we believe these operations were conducted for the sole purpose of interoperability and not with the intent of creating a competing product. As the involved developers were based in Europe, we believe the legality of these operations is granted by article 6 of the 1991 EU Computer Programs Directive.*

As a first approach, using the @strings@ tool against the incriminated program reveals numerous suspicious command names that appear to be Samsung IPC protocol definitions:

<pre>

IPC_RFS_READ_FILE

IPC_RFS_WRITE_FILE

IPC_RFS_LSEEK_FILE

IPC_RFS_CLOSE_FILE

IPC_RFS_PUT_FILE

IPC_RFS_GET_FILE

IPC_RFS_RENAME_FILE

IPC_RFS_GET_FILE_INFO

IPC_RFS_UNLINK_FILE

IPC_RFS_MAKE_DIR

IPC_RFS_REMOVE_DIR

IPC_RFS_OPEN_DIR

IPC_RFS_READ_DIR

IPC_RFS_CLOSE_DIR

IPC_RFS_OPEN_FILE

IPC_RFS_FTRUNCATE_FILE

IPC_RFS_GET_HANDLE_INFO

IPC_RFS_CREATE_FILE

</pre>

The names of these commands make it obvious that they let the modem perform I/O operations.

The @strings@ utility also reveals matching function names that seem to implement the handling of these commands:

<pre>

RxRFS_GetFile

RxRFS_CreateFile

RxRFS_ReadDirectory

RxRFS_OpenDirectory

RxRFS_RenameFile

RxRFS_Default

RxRFS_OpenFile

RxRFS_ReadFile

RxRFS_FtruncateFile

RxRFS_WriteFile

RxRFS_GetFileInfoByHandle

RxRFS_GetFileInfo

RxRFS_PutFile

RxRFS_LseekFile

RxRFS_CloseFile

RxRFS_DeleteFile

RxRFS_MakeDirectory

RxRFS_CloseDirectory

RxRFS_RemoveDirectory

TxRFS_CfrmCreateFile

TxRFS_CfrmPutFile

TxRFS_CfrmOpenDirectory

TxRFS_CfrmGetFileInfo

TxRFS_CfrmReadDirectory

TxRFS_CfrmRenameFile

TxRFS_CfrmCloseFile

TxRFS_CfrmFtruncateFile

TxRFS_CfrmGetFileInfoByHandle

TxRFS_CfrmDeleteFile

TxRFS_CfrmCloseDirectory

TxRFS_CfrmRemoveDirectory

TxRFS_CfrmMakeDirectory

TxRFS_CfrmGetFile

TxRFS_CfrmReadFile

TxRFS_CfrmWriteFile

TxRFS_CfrmLseekFile

TxRFS_CfrmOpenFile

</pre>

h2. Notes

Our free software replacement for the incriminated binary is [[Samsung-RIL]] which relies on [[Libsamsung-ipc|libsamsung-ipc]] and it is used in Replicant.

The affected devices have modems that use the Samsung IPC protocol, mostly Intel XMM6160 and Intel XMM6260 modems. Note that despite this back-door, the devices using these modems are most likely to have good modem isolation, compared to other devices using Qualcomm platforms. Bear in mind that this back-door is implemented in software and can easily be removed by installing a free replacement for the incriminated software, for instance by installing Replicant. Hence, we don't consider the incriminated devices as bad targets for Replicant because of this back-door.