Kasper Rasmussen

Matthew Rogers, Kasper Rasmussen

The CAN Bus is crucial to the efficiency, and safety of modern vehicle infrastructure. Electronic Control Units (ECUs) exchange data across a shared bus, dropping messages whenever errors occur. If an ECU generates enough errors, their transmitter is put in a bus-off state, turning it off. Previous work abuses this process to disable ECUs, but is trivial to detect through the multiple errors transmitted over the bus. We propose a novel attack, undetectable by prior intrusion detection systems, which disables ECUs within a single message without generating any errors on the bus. Performing this attack requires the ability to flip bits on the bus, but not with any level of sophistication. We show that an attacker who can only flip bits 40% of the time can execute our stealthy attack 100% of the time. But this attack, and all prior CAN attacks, rely on the ability to read the bus. We propose a new technique which synchronizes the bus, such that even a blind attacker, incapable of reading the bus, can know when to transmit. Taking a limited attacker's chance of success from the percentage of dead bus time, to 100%. Finally, we propose a small modification to the CAN error process to ensure an ECU cannot fail without being detected, no matter how advanced the attacker is. Taken together we advance the state of the art for CAN attacks and blind attackers, while proposing a detection system against stealthy attacks, and the larger problem of CAN's abusable error frames.

Munir Geden, Kasper Rasmussen

CPU registers are small discrete storage units, used to hold temporary data and instructions within the CPU. Registers are not addressable in the same way memory is, which makes them immune from memory attacks and manipulation by other means. In this paper, we take advantage of this to provide a protection mechanism for critical program data; both active local variables and control objects on the stack. This protection effectively eliminates the threat of control- and data-oriented attacks, even by adversaries with full knowledge of the active stack. Our solution RegGuard, is a compiler register allocation strategy that utilises the available CPU registers to hold critical variables during execution. Unlike conventional allocations schemes, RegGuard prioritises the security significance of a program variable over its expected performance gain. Our scheme can deal effectively with saved registers to the stack, i.e., when the compiler needs to free up registers to make room for the variables of a new function call. With RegGuard, critical data objects anywhere on the entire stack are effectively protected from corruption, even by adversaries with arbitrary read and write access. While our primary design focus is on security, performance is very important for a scheme to be adopted in practice. RegGuard is still benefiting from the performance gain normally associated with register allocations, and the overhead is within a few percent of other unsecured register allocation schemes for most cases. We present detailed experiments that showcase the performance of RegGuard using different benchmark programs and the C library on ARM64 platform.

Daniele Antonioli, Nils Ole Tippenhauer, Kasper Rasmussen et al.

The Bluetooth standard specifies two transports: Bluetooth Classic (BT) for high-throughput wireless services and Bluetooth Low Energy (BLE) for very low-power scenarios. BT and BLE have dedicated pairing protocols and devices have to pair over BT and BLE to use both securely. In 2014, the Bluetooth standard (v4.2) addressed this usability issue by introducing Cross-Transport Key Derivation (CTKD). CTKD allows establishing BT and BLE pairing keys just by pairing over one of the two transports. While CTKD crosses the security boundary between BT and BLE, little is known about the internals of CTKD and its security implications. In this work, we present the first complete description of CTKD obtained by merging the scattered information from the Bluetooth standard with the results from our reverse-engineering experiments. Then, we perform a security evaluation of CTKD and uncover four cross-transport issues in its specification. We leverage these issues to design four standard-compliant attacks on CTKD enabling new ways to exploit Bluetooth (e.g., exploiting BT and BLE by targeting only one of the two). Our attacks work even if the strongest security mechanism for BT and BLE are in place, including Numeric Comparison and Secure Connections. They allow to impersonate, man-in-the-middle, and establish unintended sessions with arbitrary devices. We refer to our attacks as BLUR attacks, as they blur the security boundary between BT and BLE. We provide a low-cost implementation of the BLUR attacks and we successfully evaluate them on 14 devices with 16 unique Bluetooth chips from popular vendors. We discuss the attacks' root causes and present effective countermeasures to fix them. We disclosed our findings and countermeasures to the Bluetooth SIG in May 2020 (CVE-2020-15802), and we reported additional unmitigated issues in May 2021.

Eman Salem Alashwali, Kasper Rasmussen

Most modern web browsers today sacrifice optimal TLS security for backward compatibility. They apply coarse-grained TLS configurations that support (by default) legacy versions of the protocol that have known design weaknesses, and weak ciphersuites that provide fewer security guarantees (e.g. non Forward Secrecy), and silently fall back to them if the server selects to. This introduces various risks including downgrade attacks such as the POODLE attack [15] that exploits the browsers silent fallback mechanism to downgrade the protocol version in order to exploit the legacy version flaws. To achieve a better balance between security and backward compatibility, we propose a mechanism for fine-grained TLS configurations in web browsers based on the sensitivity of the domain name in the HTTPS request using a whitelisting technique. That is, the browser enforces optimal TLS configurations for connections going to sensitive domains while enforcing default configurations for the rest of the connections. We demonstrate the feasibility of our proposal by implementing a proof-of-concept as a Firefox browser extension. We envision this mechanism as a built-in security feature in web browsers, e.g. a button similar to the \quotes{Bookmark} button in Firefox browsers and as a standardised HTTP header, to augment browsers security.

Eman Salem Alashwali, Kasper Rasmussen

A number of important real-world protocols including the Transport Layer Security (TLS) protocol have the ability to negotiate various security-related choices such as the protocol version and the cryptographic algorithms to be used in a particular session. Furthermore, some insecure application-layer protocols such as the Simple Mail Transfer Protocol (SMTP) negotiate the use of TLS itself on top of the application protocol to secure the communication channel. These protocols are often vulnerable to a class of attacks known as downgrade attacks which targets this negotiation mechanism. In this paper we create the first taxonomy of TLS downgrade attacks. Our taxonomy classifies possible attacks with respect to four different vectors: the protocol element that is targeted, the type of vulnerability that enables the attack, the attack method, and the level of damage that the attack causes. We base our taxonomy on a thorough analysis of fifteen notable published attacks. Our taxonomy highlights clear and concrete aspects that many downgrade attacks have in common, and allows for a common language, classification, and comparison of downgrade attacks. We demonstrate the application of our taxonomy by classifying the surveyed attacks.