A buffer overflows exists in the psnotifyd application of the Pharos PopUp printer client version 9.0. A specially crafted packet can be sent to the victim’s computer and can lead to a heap based buffer overflow resulting in potential remote code execution. This client is always listening, has root privileges, and requires no user interaction to exploit.
Pharos PopUp Printer Client 9.0
9.0 - CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
CWE-122 - Heap-based Buffer Overflow
Pharos PopUp Printer client is printing software that is widely used in Universities all over the United States. This client is a way to manage multiple connections to a single printing point and is constantly listening in the background for a packet from the printer. It is also running with root privilege for easy access to any privileged drivers. These all make this an excellent target where a vulnerability could have a high impact.
The vulnerability is located inside of the BlobData function. Blob data is encoded data returned to the client upon making the connection. It is partially controlled by the data passed in from the attacker to start. The packet is continually parsed and used by this blob data function until the packet’s end is reached. This code is shown below.
__text:0000000100008698 loc_100008698: ; CODE XREF: doBlobDataStuff+B5j __text:0000000100008698 mov rdi, r14 ; a1 __text:000000010000869B call get_blob_var __text:00000001000086A0 cmp byte ptr [rax], 0 __text:00000001000086A3 jnz short loc_1000086F2  __text:00000001000086A5 mov rdi, r14 ; a1 __text:00000001000086A8 call get_blob_var __text:00000001000086AD mov rbx, rax __text:00000001000086B0 mov rdi, r14 __text:00000001000086B3 call getPacketLength  __text:00000001000086B8 inc rbx __text:00000001000086BB dec eax  __text:00000001000086BD mov rdi, r15 ; a1 __text:00000001000086C0 mov rsi, rbx ; a2 __text:00000001000086C3 mov edx, eax ; length  __text:00000001000086C5 call doBlobData_ __text:00000001000086CA mov rdi, r14 ; a1 __text:00000001000086CD mov rsi, r15 ; a2 __text:00000001000086D0 call assign_new_blob  __text:00000001000086D5 mov rdi, r15 __text:00000001000086D8 call delObj __text:00000001000086DD jmp short loc_100008698
The length of the packet is calculated,  and we see RBX is incremented by one. RBX in this instance points to the data in the structure that was sent via the packet. From here we see EAX, , is decremented and is not checked for validity. Finally at , EAX is passed into BlobData as the length. Inside of the BlobData function a new Blob structure is created and the length passed in is EAX, one decremented from the length of the previous packet. Then subsequently at , the new Blob data pointer is swapped from R15 into R14, the register used for all of our checks.
To recap what we know so far, the blob structure’s length is calculated and a new blob structure is created with a length decremented by one. This new blob structure is then used to replace the original blob structure. The vulnerability arises at , because the check is incorrect and should be jump if zero not JNZ. This makes the program fall into an infinite loop where the length is ever decrementing. Looking one time into the doBlobData function we can see the crashing point.
__text:0000000100007805 add rdi, 4 ; unsigned __int64 __text:0000000100007809 call __Znam ; operator new(ulong) __text:000000010000780E mov [r13+PSComDecodePacket.blob_var], rax __text:0000000100007812 mov rdi, r13 __text:0000000100007815 mov esi, r12d __text:0000000100007818 call SetSize __text:000000010000781D mov rdi, [r13+PSComDecodePacket.blob_var] ; __text:0000000100007821 mov edx, r12d ; size_t  __text:0000000100007824 mov rsi, r15 ; void * __text:0000000100007827 call _memcpy __text:000000010000782C add rsp, 8 __text:0000000100007830 pop rbx
The size shown at  is the passed in EAX from the previous function. Without a way to stop this number gets decremented down until it passes a negative value into the memcpy causing an out of bounds write. Due to the fact that multiple concurrent connections can be made there may be a way to use threads to stop the memcpy and cause an exploitable condition to arise.
./exc_handler ./psnotifyd 2017-01-24 14:29:58.030 psnotifyd[38902:8830625] Notify listening thread started 2017-01-24 14:29:58.031 psnotifyd[38902:8830625] Listening on socket 4 2017-01-24 14:29:58.033 psnotifyd[38902:8830621] CFSocketSetAddress bind failure: 48 2017-01-24 14:29:58.034 psnotifyd[38902:8830621] Telling any existing Notify processes that psnotifyd has started up. 2017-01-24 14:30:10.415 psnotifyd[38902:8830625] New notify connection incoming 2017-01-24 14:30:10.415 psnotifyd[38902:8830625] Spawning a new notify request handler thread 2017-01-24 14:30:10.416 psnotifyd[38902:8830625] Listening on socket 4 2017-01-24 14:30:10.416 psnotifyd[38902:8831088] New request handler thread started 2017-01-24 14:30:10.416 psnotifyd[38902:8831088] I got some stuff goin' on Crashed thread log = 0 libsystem_platform.dylib 0x00007fff8db24130 _platform_memmove$VARIANT$Haswell + 528 1 psnotifyd 0x000000010000782c 0x100000000 + 30764 2 psnotifyd 0x0000000100007580 0x100000000 + 30080 3 psnotifyd 0x00000001000086ca 0x100000000 + 34506 4 psnotifyd 0x0000000100002a95 0x100000000 + 10901 5 psnotifyd 0x0000000100002392 0x100000000 + 9106 6 com.apple.Foundation 0x00007fff89e3de64 __NSThread__start__ + 1351 7 libsystem_pthread.dylib 0x00007fff997ec99d _pthread_body + 131 8 libsystem_pthread.dylib 0x00007fff997ec91a _pthread_start + 168 9 libsystem_pthread.dylib 0x00007fff997ea351 thread_start + 13 log name is: ./crashlogs/crashlog.txt --- exception=EXC_BAD_ACCESS:signal=11:is_exploitable=yes:instruction_disassembly=.byte 0xc5 #bad opcode:instruction_address=0x00007fff8db24130:access_type=unknown:access_address=0x00000001057fffe1: Crash accessing invalid address
2017-02-07 - Vendor Disclosure
2017-03-07 - Public Release
Discovered by Tyler Bohan of Cisco Talos. Talos would also like to thank NYU Osiris Lab for helping out with some of the reversing.