TLS-Abusing Covert Data Channel Bypasses Network Defenses
6.2.2018 securityweek Krypto
Researchers from Fidelis Cybersecurity have discovered a new method of abusing the X.509 public key certificates standard for covert channel data exchange following initial system compromise.

The standard is used in both Transport Layer Security (TLS) and Secure Sockets Layer (SSL) cryptographic Internet protocol implementations, but the manner in which the certificates are exchanged can be abused to hijack them for command and control (C&C) communication, the researchers say.

The X.509 extensions can be used for covert channel data transfer to bypass network protection methods that do not inspect certificate values, the researchers say. To date, no confirmed cases of this technique being abused have been observed, but the widespread use of certificates could put many organizations at risk, Fidelis researchers argue.

To demonstrate their theory, Fidelis Cybersecurity revealed a custom-built framework that serves as proof of concept. However, the researchers point out that detection is possible and that the community can implement protections to identify possible abuse of the covert channel data transfer mechanism.

The use of covert channels for data transfer across the network is not new, and the possible abuse of X.509 certificates for covert network communication was demonstrated before. In fact, the use of the TLS protocol to establish hidden communication channels was detailed a decade ago.

The new research (PDF) by Fidelis’ Jason Reaves into the use of X.509 extensions for covert channel purposes expands on the previous findings to describe a system that could be used to send or receive data from both a client and a server perspective.

Using previous demonstrations that arbitrary data can be placed into X.509 certificates and that these certs can be used as a covert channel, the researcher argues that a sufficiently motivated attacker could “utilize technologies outside of their intended purposes to not only accomplish their goals but also end up bypassing common security measures in the process.”

Reaves analyzed X.509 certificate extensions, which “provide methods for associating additional attributes with users or public keys and for managing relationships between CAs,” but which can be abused for malicious purposes due to ambiguity in the language, which led to relaxed implementations.

Because TLS X.509 certificates have a large number of fields where strings can be stored, actors can take advantage of this to hide data transfer inside one of these fields. The certificates are exchanged before the TLS session is established, meaning that the data transfer doesn’t show up, although it was performed within the certificate exchange itself.

“Testing shows that using this methodology for communication and control in malware will not result in anything beyond an SSL negotiation which could bypass common security mechanisms that are not looking for abnormal data being passed in X.509 certificates,” Reaves says.

Fidelis also came up with a proof of concept to show that file transfer using the X.509 covert channel would be possible. For their demonstration, they chose to simulate a threat actor transferring the password stealing tool Mimikatz to a compromised system.