CVE-2017-2785
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
https://pharos.com/products-services/
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 DecodeString function. The packet strings are sent to the program encoded in an encoding format. This function parses the packet and decodes the string. First off the function locates the end of the string, reads in some data from the packet and finally passes it into a decode function. This code is shown below.
__text:0000000100005E21 mov rsi, [rbx+PSComDecodePacket.end_of_str] [1]
__text:0000000100005E28 movzx ecx, byte ptr [rsi]
__text:0000000100005E2B mov eax, ecx
__text:0000000100005E2D and eax, 3Fh
__text:0000000100005E30 cmp eax, 10
__text:0000000100005E33 jnz short loc_100005E98
__text:0000000100005E35 lea rax, [rsi+1]
__text:0000000100005E39 mov [rbx+PSComDecodePacket.end_of_str], rax
__text:0000000100005E40 mov al, 1
__text:0000000100005E42 test cl, cl
__text:0000000100005E44 js short loc_100005E74
__text:0000000100005E46 mov edx, [rsi+1] [2]
__text:0000000100005E49 add rsi, 5
__text:0000000100005E4D mov [rbx+PSComDecodePacket.end_of_str], rsi
__text:0000000100005E54 mov [r14], rsi
__text:0000000100005E57 add [rbx+PSComDecodePacket.end_of_str], rdx ;
__text:0000000100005E5E test cl, 40h
__text:0000000100005E61 jz short loc_100005E72
__text:0000000100005E63 add rbx, 20h
__text:0000000100005E67 mov rsi, [r14]
__text:0000000100005E6A mov rdi, rbx
__text:0000000100005E6D call xor_decode [3]
Starting at [1], we see the string 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 [2], we see some data being moved directly from the attacker controlled packet and into EDX. Further down, [3], 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 [1]
__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 [2]
__text:0000000100006D1B inc rsi
__text:0000000100006D1E inc cl
__text:0000000100006D20 dec edx
__text:0000000100006D22 jnz short loc_100006CE2 [3]
At the beginning, EDX is being checked against zero, [1], then we subsequently fall into a loop. Some data is then grabbed from the packet structure and used at location [2]. 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.
./exc_handler ./psnotifyd
2017-01-24 13:18:26.374 psnotifyd[24597:8732293] Notify listening thread started
2017-01-24 13:18:26.375 psnotifyd[24597:8732293] Listening on socket 4
2017-01-24 13:18:26.377 psnotifyd[24597:8732289] CFSocketSetAddress bind failure: 48
2017-01-24 13:18:26.377 psnotifyd[24597:8732289] Telling any existing Notify processes that psnotifyd has started up.
2017-01-24 13:18:36.153 psnotifyd[24597:8732293] New notify connection incoming
2017-01-24 13:18:36.153 psnotifyd[24597:8732293] Spawning a new notify request handler thread
2017-01-24 13:18:36.153 psnotifyd[24597:8732293] Listening on socket 4
2017-01-24 13:18:36.153 psnotifyd[24597:8732519] New request handler thread started
2017-01-24 13:18:36.153 psnotifyd[24597:8732519] I got some stuff goin' on
2017-01-24 13:18:36.157 psnotifyd[24597:8732293] New notify connection incoming
2017-01-24 13:18:36.157 psnotifyd[24597:8732293] Spawning a new notify request handler thread
2017-01-24 13:18:36.157 psnotifyd[24597:8732293] Listening on socket 4
2017-01-24 13:18:36.157 psnotifyd[24597:8732522] New request handler thread started
2017-01-24 13:18:36.157 psnotifyd[24597:8732522] I got some stuff goin' on
2017-01-24 13:18:36.218 psnotifyd[24597:8732519] Exception: *** -[_NSZeroData getBytes:range:]: range {5, 4} exceeds data length 0
Crashed thread log =
0 psnotifyd 0x0000000100006d19 0x100000000 + 27929
1 psnotifyd 0x0000000100005e72 0x100000000 + 24178
2 psnotifyd 0x0000000100005c81 0x100000000 + 23681
3 psnotifyd 0x0000000100002071 0x100000000 + 8305
4 psnotifyd 0x0000000100002937 0x100000000 + 10551
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/overflow.crashlog.txt
---
exception=EXC_BAD_ACCESS:signal=10:is_exploitable=yes:instruction_disassembly=xorb %al,(%rsi):instruction_address=0x0000000100006d19:access_type=write:access_address=0x0000000104000000:
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.