An exploitable buffer overflow 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 remote code execution. This client is always listening, has root privileges, and requires no user interaction to exploit.
Pharos PopUp Printer Client 9.0
10.0 - CVSS:3.0/AV:N/AC:L/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 DecodeBinary function. The packet encodes binary data and adds the length of the data to the beginning. By passing in an overly large length the application can be caused to overflow the buffer with controlled data and an exploitable condition arises. The relevant code is shown below.
__text:00000001000063F5 mov rsi, [rbx+PSComDecodePacket.end_of_data]  __text:00000001000063FC movzx ecx, byte ptr [rsi] __text:00000001000063FF mov eax, ecx __text:0000000100006401 and eax, 3Fh __text:0000000100006404 cmp eax, 0Bh __text:0000000100006407 jnz short loc_100006475 __text:0000000100006409 lea rax, [rsi+1] __text:000000010000640D mov [rbx+PSComDecodePacket.end_of_data], rax __text:0000000100006414 mov al, 1 __text:0000000100006416 test cl, cl __text:0000000100006418 js short loc_10000644B __text:000000010000641A mov edx, [rsi+1]  __text:000000010000641D mov [r15], edx __text:0000000100006420 add rsi, 5 __text:0000000100006424 mov [rbx+PSComDecodePacket.end_of_data], rsi __text:000000010000642B mov [r14], rsi __text:000000010000642E add [rbx+PSComDecodePacket.end_of_data], rdx __text:0000000100006435 test cl, 40h __text:0000000100006438 jz short loc_100006449 __text:000000010000643A add rbx, 20h __text:000000010000643E mov rsi, [r14] __text:0000000100006441 mov rdi, rbx __text:0000000100006444 call xor_decode 
Starting at , we see the data location being loaded from a struct and moved into RSI. A few checks are made on the data which is controlled by the attacker and can be easily bypassed. Then at , we see some data being moved directly from the attacker controlled packet and into EDX. Further down, , we see a call to xor_decode which is of interest because the third argument to that function is EDX. The relevant function code for xor decode is shown below.
__text:0000000100006CC0 push rbp __text:0000000100006CC1 mov rbp, rsp __text:0000000100006CC4 push r14 __text:0000000100006CC6 push rbx __text:0000000100006CC7 mov cl, [rdi+(PSComDecodePacket.length+0E0h)] __text:0000000100006CCD mov r9b, [rdi+(PSComDecodePacket.length+0E1h)] __text:0000000100006CD4 test edx, edx  __text:0000000100006CD6 jz short loc_100006D2A __text:0000000100006CD8 lea r8d, [rdx+0FFh] __text:0000000100006CDF add r8b, cl __text:0000000100006CE2 __text:0000000100006CE2 loc_100006CE2: ; CODE XREF: xor_decode+62j __text:0000000100006CE2 movzx ecx, cl __text:0000000100006CE5 lea eax, [rcx+1] __text:0000000100006CE8 movzx r14d, al __text:0000000100006CEC movzx r10d, byte ptr [rdi+r14] __text:0000000100006CF1 movzx r11d, r10b __text:0000000100006CF5 movzx r9d, r9b __text:0000000100006CF9 add r9d, r11d __text:0000000100006CFC movzx eax, r9b __text:0000000100006D00 mov bl, [rdi+rax] __text:0000000100006D03 mov [rdi+r14], bl __text:0000000100006D07 mov [rdi+rax], r10b __text:0000000100006D0B movzx eax, byte ptr [rdi+r14] __text:0000000100006D10 add eax, r10d __text:0000000100006D13 movzx eax, al __text:0000000100006D16 mov al, [rdi+rax] __text:0000000100006D19 xor [rsi], al  __text:0000000100006D1B inc rsi __text:0000000100006D1E inc cl __text:0000000100006D20 dec edx __text:0000000100006D22 jnz short loc_100006CE2 
At the beginning, EDX is being checked against zero, , then we subsequently fall into a loop. Some data is then grabbed from the packet structure and used at location . This is XOR’d against user data in an attempt to deobfuscate the packet received. Continuing down a touch further we see an increment to RSI, CL and a decrement to EDX. EDX is then tested against zero and it is clear the loop will continue until EDX reaches zero. This means that the loop will be executed however many times the attacker passed in from the previous function, directly taken from the packet. The code will continue incrementing the packet pointer in RSI, causing a buffer overflow of attacker controlled data and ultimately leading to remote code execution.
TYBOHAN-M-F0VF% ./exc_handler ./psnotifyd GuardMalloc[exc_handler-95307]: Allocations will be placed on 16 byte boundaries. GuardMalloc[exc_handler-95307]: - Some buffer overruns may not be noticed. GuardMalloc[exc_handler-95307]: - Applications using vector instructions (e.g., SSE) should work. GuardMalloc[exc_handler-95307]: version 108 GuardMalloc[psnotifyd-95308]: Allocations will be placed on 16 byte boundaries. GuardMalloc[psnotifyd-95308]: - Some buffer overruns may not be noticed. GuardMalloc[psnotifyd-95308]: - Applications using vector instructions (e.g., SSE) should work. GuardMalloc[psnotifyd-95308]: version 108 2017-01-25 10:48:59.209 psnotifyd[95308:9320558] Notify listening thread started 2017-01-25 10:48:59.210 psnotifyd[95308:9320558] Listening on socket 4 2017-01-25 10:48:59.224 psnotifyd[95308:9320552] CFSocketSetAddress bind failure: 48 2017-01-25 10:48:59.225 psnotifyd[95308:9320552] Telling any existing Notify processes that psnotifyd has started up. 2017-01-25 10:49:06.770 psnotifyd[95308:9320558] New notify connection incoming 2017-01-25 10:49:06.770 psnotifyd[95308:9320558] Spawning a new notify request handler thread 2017-01-25 10:49:06.771 psnotifyd[95308:9320558] Listening on socket 4 2017-01-25 10:49:06.771 psnotifyd[95308:9321282] New request handler thread started 2017-01-25 10:49:06.771 psnotifyd[95308:9321282] I got some stuff goin' on Crashed thread log = 0 psnotifyd 0x0000000100006d19 0x100000000 + 27929 1 psnotifyd 0x0000000100006449 0x100000000 + 25673 2 psnotifyd 0x00000001000063a3 0x100000000 + 25507 3 psnotifyd 0x0000000100002a4f 0x100000000 + 10831 4 psnotifyd 0x0000000100002392 0x100000000 + 9106 5 com.apple.Foundation 0x00007fff89e3de64 __NSThread__start__ + 1351 6 libsystem_pthread.dylib 0x00007fff997ec99d _pthread_body + 131 7 libsystem_pthread.dylib 0x00007fff997ec91a _pthread_start + 168 8 libsystem_pthread.dylib 0x00007fff997ea351 thread_start + 13 log name is: ./crashlogs/crashlog.txt --- exception=EXC_BAD_ACCESS:signal=11:is_exploitable=yes:instruction_disassembly=xorb %al,(%rsi):instruction_address=0x0000000100006d19:access_type=write:access_address=0x00000001058d5000: Crash accessing invalid address.
Talos would also like to thank NYU Osiris Lab for helping out with some of the reversing.
2017-02-07 - Vendor Disclosure
2017-03-07 - Public Release