Security researchers from Fidelis identified a new method to establish a covert channel communication by abusing widely implemented X.509 public key certificates. Certificates remain as a critical component of secure connection to a website.
When a browser establishes a secure connection to a site, it checks that the server has the private key for the certificate it’s trying to use, this is no one can use our certificate.
A TLS X.509 certificate would have many fields Version, signature algorithm, Validity, subject and so on. Data transferred within X.509 extension can bypass the detection methods, attackers could take it as an advantage to hide data in any one of these fields.
The covert channel is not a new one, history reveals various publications, there are methods to exfiltrate data by using ICMP shell, yes PING as great convert channel to get victim shell access using ICMP Shell.
Jason from Fidelis describes a system that could be used to send or receive data from both a client and a server perspective utilizing research into X.509 certificates specifically in areas where you can place arbitrary binary data into the certificate or utilizing them as a covert channel. The research demonstrates that a sufficiently motivated attacker can utilize technologies outside of their intended purposes to not only accomplish their goals but also end up bypassing common security measures in the process.
Researchers published a PoC explaining the malicious binary could be transferred over TLS traffic that stimulates the malware that already installed in the system.
Researchers suggested mitigations and the detection are possible by creating YARA and Suricata rule by pivoting SSL handshake to detect unexpected extensions and large octet lengths.
Due to widespread of X.509 certificates, most of the organizations are open to attack, disabling the self-signed as like in the PoC could be an effective security measure.