Vendor Description
“Protect your network from viruses, malware and malicious activity with Kerio Control, the easy-to-administer yet powerful all-in-one security solution.
“Kerio Control brings together next-generation firewall capabilities — including a network firewall and router, intrusion detection and prevention (IPS), gateway anti-virus, VPN, and web content and application filtering. These comprehensive capabilities and unmatched deployment flexibility make Kerio Control the ideal choice for small and mid-sized businesses.”
“Link headquarters to remote users and branch offices securely and easily. Kerio’s own VPN tunneling with dead-simple setup requires minimal configuration, and provides a high performance network connection. Or, use industry-standard IPsec/L2TP for connectivity from mobile devices or third-party firewalls. Enable 2-step verification for an extra layer of security on all forms of remote access.”
Source: http://www.kerio.com/products/kerio-control
Business Recommendation
During a quick evaluation of the Kerio Control VPN protocol, it was apparent, that the cryptographic protocol employed exhibited severe design issues.
Generally, SEC Consult strongly recommends to prefer well-established standard cryptographic protocols rather than proprietary protocols wherever possible (e.g. DTLS, IPsec). Due to their widespread use, they generally receive much greater attention by experts. Therefore, many design issues with these protocols have already been detected and mitigated since.
We therefore recommend businesses to switch from Kerio’s proprietary VPN protocol to a standard protocol (Kerio Control e.g. supports IPsec).
Note that no full audit of Kerio Control, Kerio VPN or the cryptographic protocol has been conducted. In addition to the vulnerabilities described here, we already identified critical vulnerabilities in Kerio Control in 2016. Hence we suspect there are more major security deficiencies in the product. We therefore recommend GFI software to greatly increase the efforts towards product security in order to keep customers secure.
We want to explicitly thank GFI for the professional handling of the communication during this whole process.
Vulnerability Overview / Description
After a TLS connection is established between the Kerio VPN client and the Kerio Control appliance and cryptographic keys have been securely transferred over this connection, the data sent through the VPN is transmitted in UDP packets. Each of these packets is encrypted using Blowfish in CTR mode.
As this mode does not provide data authenticity, encrypted data that is modified by an attacker results in predictable modification of the plaintext. More precisely, bits that are flipped in the ciphertext result in the same bits being flipped in the plaintext after decryption.
Each encrypted UDP datagram contains a simple checksum (the same checksum used by IPv4). Assuming an attacker knows the plaintext data of a datagram and is able to modify its ciphertext, it is trivial to change parts of the message, e.g. inject content into the encrypted stream, while keeping the resulting checksum identical.
Proof Of Concept
SEC Consult provided a proof of concept exploit script to GFI but it has been removed from this advisory in order to give customers more time to upgrade the infrastructure.
Vulnerable / Tested Versions
The version 9.2.7 build 2921 was found to be vulnerable. This version was the latest at the time of discovery and older versions are affected as well.