Airscanner Mobile Security Advisory: Remote Password Compromise of 
Microsoft Active Sync 3.7.1

Product:
Microsoft Active Sync 3.7.1

Platform:
Tested on Windows XP Professional SP-2 and Windows Mobile Pocket PC 2003

Requirements:
Windows XP Professional with Active Sync 3.7.1

Credits:
Seth Fogie
Airscanner Mobile Security
www.airscanner.com
July 22, 2005

Risk Level:
Low for denial of service attacks. Medium for password collection attack.

Summary:
”Active Sync” is Microsoft’s default connectivity program that keeps a 
desktop PC and a handheld Pocket PC synchronized. It also includes 
various other features, such as debugging ability, file transfer, etc.

Details:
When a Pocket PC device attempts to sync to a PC, it will send three 
initial packets to the Active Sync program on port 5679. The following 
outlines the contents of the packets:

packet1[] = "\x00\x00\x00\x00";
packet2[] = "\x98\x00\x00\x00"; //SIZE OF NEXT PACKET
packet3[] =
"\x28\x00\x00\x00"
"\x04\x15\x40\x04"
"\x11\x0a\x00\x00" //2577 (AUTORUN?)
"\x05\x00\x00\x00"
"\x59\x29\x6d\x46" //EQUIP ID
"\x00\x00\x00\x00"
"\x28\x00\x00\x00" //LINK TO POCKET_PC1 TEXT
"\x3e\x00\x00\x00" //LINK TO POCKETPC TEXT
"\x5c\x00\x00\x00" //LINK TO SSKD TEXT
"\x78\x00\x00\x00" //LINK TO AXIM X50 TEXT
"\x50\x00\x6f\x00" //TEXT IN UNICODE
"\x63\x00\x6b\x00\x65\x00\x74\x00\x5f\x00\x50\x00\x43\x00\x31\x00\x00\x00\x50\x00" 

"\x6f\x00\x63\x00\x6b\x00\x65\x00\x74\x00\x50\x00\x43\x00\x00\x00"
"\x53\x00\x53\x00\x44\x00\x4b\x00\x00\x00\x00\x00\x44\x00\x65\x00"
"\x6c\x00\x6c\x00\x20\x00\x41\x00\x78\x00\x69\x00\x6d\x00\x20\x00"
"\x58\x00\x35\x00\x30\x00\x00\x00\x03\x00\x00\x00\x00\x00\x00\x00"
"\x04\x00\x00\x00\x02\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
"\x00\x00\x00\x00\x00\x00\x00\x00";

If the equipment ID value is valid, the PC will respond with a 
x12345678. If the equipment ID is not correct, the response will be x03. 
With this static response, it is trivial to brute force the valid 
equipment ID value. The reason this is important is because if you 
change the value in packet1 to x00000001 to the correct corresponding 
PID, a prompt will appear on the PC asking for a PIN value (figure 1). 
If a target enters a password, the information will be passed back to 
the remote, requesting client. If a value other than x01 is sent, that 
value will be XORed with the response to pseudo-'encrypt' the password. 
This method of information gathering is possible from over a network and 
does work over the Internet. From a quick nmap scan, we found about 
roughly 10 computers with this port open per 50 class C subnets.

http://www.airscanner.com/security/activesync371.htm for image
Figure 1: Active Sync Password Dialog

Finally, we discovered that if numerous attempts were made to initialize 
with a PC running Active Sync, after about four attempts the Active Sync 
process freezes. In addition, if a user attempts to sync while a brute 
force equipment ID attempt is underway, the sync will usually fail.

Workaround:
Block Internet and LAN access to port 5679 using a firewall until this 
issue is patched.

Vendor Response
Awaiting response.