Vulnerabilities in Kerio Personal Firewall (Buffer Overflow, Replay)
30 Apr. 2003
Summary
Kerio Personal Firewall (KPF) is "a firewall for workstations designed to protect them against attacks from the Internet and the local network. We found two security vulnerabilities in KPF's remote administration system". Two security vulnerabilities have been found in the product, one allowing remote attackers to replay previous sessions, the other allows overflowing a buffer used by the Kerio firewall, allowing execution of arbitrary code.
Vulnerable systems:
* Kerio Personal Firewall version 2.1.4 and previous versions.
Replay attack:
A replay attack is possible against the authenticated/encrypted channel for remote administration. A design problem in the authentication mechanism for remote administration allows an attacker to replay captured packets from a valid remote administration session in order to reproduce the administrator's directives to the personal firewall.
For example if the attacker is able to sniff a valid session in which the administrator disabled the firewall capabilities, then the attacker will gain the ability to disable the personal firewall at will at any time in the future.
Buffer overflow:
A remotely exploitable buffer overflow exists in the administrator authentication process.
Solution/Vendor Information/Workaround:
Contact the vendor for a fix.
Workaround:
Disable the remote administration feature.
Technical Description - Exploit/Concept Code:
CORE found two security vulnerabilities in Kerio PF's remote administration system.
Replay attack:
A replay attack is possible against the authenticated/encrypted remote administration channel. Because of a design problem in the authentication mechanism for remote administration, it is possible to replay a previously captured administration session.
If 'S' is the workstation running Kerio personal firewall and 'C' is the administrator workstation, the following scheme shows the initial key exchange and authentication packets for a remote administration session:
S C
<--- connect
---> 10 bytes (0f 00 0a 00 01 00 00 00 02 00)
[*] ---> 128 bytes (the initial 64 bytes are 0 and the last
64 bytes are the 'public' key)
---> 128 bytes (Everything is 0ed except the last 4 bytes
[01 00 01 00])
[0] <--- 4 bytes (00 00 00 40)
[1] <--- 64 bytes (This 64 bytes change from session to session)
[2] <--- 32 bytes (From now on, everything is encrypted and differs
from session to session)
---> 4 bytes ()
[3] <--- 64 bytes (user authentication)
........
........ (The session continues with commands and responses)
........
[*] The last 64 bytes of this packet are read from the file 'persfw.key' on the Kerio installation directory.
It was noted from analyzing these sessions that the first differences between different sessions come from the administrator's workstation 'C'. This led us to try replaying an administration session as a whole, with the unexpected result that it was deemed valid by 'S'. This shows that in fact no randomization or serialization is used on the 'server' side 'S', and thus there is no way for Kerio to ensure that the session is new and not a replay of an old one.
As a result, an attacker with access to an encrypted administration session can record the session and replay it to the server later to reissue the administration commands to the personal firewall.
The commands replayed can include enabling/disabling the firewall, adding firewall rules, etc.
Buffer overflow:
A remotely exploitable buffer overflow exists in the administrator authentication process. When Administrator connects to the firewall, a handshake occurs in order to establish an encrypted session. The 4th packet of the handshake (the first packet sent by the administrator) is a 4 byte packet data, with a fixed number of 0x40 (64) indicating the size of the following packet expected to contain the administrator's key.
No boundary checks exist at the firewall side for processing this data, and the recv() reads the 4 bytes and then attempts to read the amount of data indicated by the 4 bytes to a buffer on the stack. As a result, an attacker connecting to the administration port on the personal firewall can construct a packet sequence that will overflow the buffer on the stack, allowing her to execute arbitrary code on the machine running the personal firewall.
It is important to note that these packets are accepted by the personal firewall before authentication of the administrator takes place.
The following proof of concept Python script will make the program jump to address 0x41414141. Note that there is enough space in the buffer (approx. 1800 bytes) to insert a shell code.
---------------------
import os
import socket
import struct
import string
