The vulnerability was found by a group of researchers from the Vrije Universiteit Amsterdam and ETH Zurich. The researchers have presented a detailed security analysis of the attack in their paper, NetCAT: Practical Cache Attacks from the Network. The analysis has been implemented by reverse engineering the behavior of Data-Direct I/O (DDIO), also called as Direct Cache Access (DCA) on recent Intel processors.
The security analysis resulted in the discovery of the first network-based PRIME+PROBE Cache attack, named NetCAT. The NetCAT attack enables attacks in cooperative and general adversarial settings. The cooperative setting can enable an attacker to build a covert channel between a network client and a sandboxed server process without network. In the general adversarial settings, an attacker can enable disclosure of network timing-based sensitive information.
On June 23, 2019, the researchers coordinated the disclosure process with Intel and NCSC (the Dutch national CERT). Intel acknowledged the vulnerability with a bounty and have assigned CVE-2019-11184 to track the issue.
What is a NetCAT attack?
The threat model implemented in the paper targets victim servers with DDIO equipped Intel processors, which are mostly enabled in all Intel server-grade processors, by default since 2012. The launched cache attack is conducted over a network to a target server, such that secret information can be leaked from the connection between the server and a different client.
The researchers say that there are many potential ways to exploit DDIO. The paper states, “For instance, an attacker with physical access to the victim machine could install a malicious PCIe device to directly access the LLC’s DDIO region. Our aim in this paper is to show that a similar attack is feasible even for an attacker with only remote (unprivileged) network access to the victim machine, without the need for any malicious PCIe devices.”
The threat model uses the RDMA in modern NICs to bypass the operating system at the data plane. This provides the remote machines with direct read and write access to a previously specified memory region. The below figure illustrates the model’s target topology, which is also common in data centers.
Image Source: NetCAT: Practical Cache Attacks from the Network
In order to launch the remote PRIME+PROBE attack, the researchers have used the remote read/write primitives provided by the PCIe device’s DDIO capabilities to remotely measure the cache activity. The paper explains two cooperative DDIO-based attacks. In the first scenario, a covert channel between two clients that are not on the same network is used and in the second scenario a covert channel between a client and a sandboxed process on a server is used.
In both scenarios, it was found that the transmission rounds are loosely synchronized with a predefined time window. An attacker can control the machine with an RDMA link to an application server by using the remote PRIME+PROBE to detect network activity in the LLC as shown in the above figure. The user then opens an interactive SSH session to the application server from a different machine. In an interactive SSH session, each keystroke is sent in a separate packet. The attacker is able to recover the inter-packet times from the cache using the ring buffer location and map them to keystrokes.
The security analysis successfully explored the implications of the NetCAT attack, and proved that the DDIO feature on modern Intel CPUs does exposes the system to cache attacks over the network.
The researchers believe that “We have merely scratched the surface of possibilities for network-based cache attacks, and we expect similar attacks based on NetCAT in the future. We hope that our efforts caution processor vendors against exposing microarchitectural elements to peripherals without a thorough security design to prevent abuse.”
A video demonstrating the NetCAT attack is shown below:
In the paper, various other NetCAT-like attacks like the PCIe to CPU attacks have been discussed which may be generalized beyond the given proof-of-concept scenarios.
The researchers have also explained various possible mitigations like disabling DDIO, LLC partitioning, and DDIO improvement against these last-level cache side-channel attacks from PCIe devices.
With repeated vulnerabilities being found in Intel, many are beginning to distrust Intel. Some are even considering moving away to other alternatives.
A Redditor comments, “Another one? Come on man, my i7 2600k already works like crap, and now another vulnerability that surely will affect performance via patches appeared? It is settled, next month I’m ditching Intel.”
Another comment read, “Soooo the moral of the story is, never buy Intel chips.”
For more information about the attack, interested readers can head over to the NetCAT: Practical Cache Attacks from the Network paper for more information.