Pharos PopUp Printer Client DecodeString Code Execution Vulnerability(CVE-2017-2785)

2017-09-2000:00:00

Root

www.seebug.org

EPSS

0.039

Percentile

92.0%

JSON

Summary

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.

Tested Versions

Pharos PopUp Printer Client 9.0

Product URLs

https://pharos.com/products-services/

CVSSv3 Score

10.0 - CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H

CWE

CWE-122 - Heap-based Buffer Overflow

Details

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]

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.

Crash Information

./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.