While VPNs are supposed to protect users, the same tools can pose a threat to user security if found vulnerable. Recently, researchers have found a security flaw in Fortinet VPN that exposes around 200,000 businesses globally to MiTM attacks.
Fortinet VPN Flaw Allowing MiTM Attacks
Researchers from the cybersecurity firm SAM Seamless Network have a serious vulnerability in Fortigate – Fortinet’s security solution. As elaborated in their blog post, this Fortinet VPN flaw exposes around 200,000 businesses to the risk of MiTM attacks.
Briefly, the vulnerability exists in the way the VPN solution validates the SSL certificate. While ideally, the VPN should check for several parameters to validate a certificate. However, Fortigate merely looks for the issuance by Fortigate or another trusted CA.
As explained by the researchers,
The Fortigate router comes with a default SSL certificate that is signed by Fortinet (Self-signed, that is, it was not issued by a trusted CA). Each Fortigate has its own certificate that uses the router’s serial number as the server name for the certificate…
This leaves Fortinet with enough information to verify the certificate was issued to the same server the client is trying to connect to, if it were to verify the serial number. However, Fortinet’s client does not verify the Server Name at all. In fact, any certificate will be accepted.
Hence, an attacker can easily present its own certificate to pass the check and perform man-in-the-middle attacks. The attacker can reroute the traffic to own servers, but can also decrypt the data, steal information, or inject own traffic.
The following video demonstrates the PoC.
Fortinet Refused A Patch
Upon finding this problem, the researchers reached out to Fortinet. However, the vendors refused a fix for it, putting the onus of ensuring security on the users. As the researchers state,
They claim that since the user has the ability to manually replace the certificate, it is the user’s responsibility to make sure the connection is protected.
The researchers believe that small businesses may not be capable enough to handle this issue well. Especially, when Fortinet doesn’t issue any explanatory warning to the users.
This is also evident from the fact that a quick Shodan.io search made the researchers detect over 200,000 businesses vulnerable to MiTM out of the ~ 230,000 (roughly 88%).
While Fortinet has clearly denied a fix, and that the matter is disclosed publicly, all FortiGate users must take care of their security on their own.