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'CacheOut': Researchers unveil new attack on Intel chips

Researchers unveiled a new speculative execution attack that leaks data from most Intel microprocessors and gives attackers greater control over what data is leaked.

Intel is facing yet another speculative execution attack, and this one can bypass previous fixes for past threats such as Meltdown, Spectre and ZombieLoad.

The attack, dubbed 'CacheOut,' leaks data from an assortment of Intel processors that were released before the fourth quarter of 2018. Previous speculative execution threats, which Intel refers to as microarchitectural data sampling (MDS), allow attackers to obtain data in transit as it moves through processor's microarchitectural buffers, though attackers have little control over what data is leaked. Part of Intel's mitigation for such threats were microcode updates that overwrite the buffers when the CPU changes security domains, which include Intel's Software Guard Extension (SGX) enclaves.

However, a team of computer science researchers discovered CacheOut allows attackers to not only bypass the buffer overwrite mitigations but also better control what data is leaked from the chips. The attack also violates "nearly all" of Intel's hardware-backed security domains like SGX, which was designed specifically to prevent data leaks.

"We observe that as data is being evicted from the CPU L1 cache, it is often transferred back to the leaky CPU buffers where it can be recovered by the attacker," the team wrote in its research paper. "CacheOut improves over previous MDS attacks by allowing the attacker to choose which data to leak from the CPU's L1 cache, as well as which part of a cache line to leak."

The research paper demonstrates several attacks scenarios for CacheOut, such as breaking process isolation by recovering AES encryption keys from an OpenSSL-based system and violating isolation between two virtual machines running on the same physical core. The paper's authors include Stephan van Schaik, Marina Minkin, Andrew Kwong, Daniel Genkin of the University of Michigan, and Yuval Yarom of University of Adelaide and Data61, a technology research organization based in Sydney.

In addition, the research team said that Intel's "ad hoc buffer overwrite countermeasures are not sufficient to completely mitigate MDS-type attacks," according to the paper. Furthermore, the researchers claimed that "some of the latest Meltdown-resistant Intel CPUs are still vulnerable, despite all of the most recent patches and mitigations."

An Intel security advisory posted Monday said the chipmaker will release microcode updates to address CacheOut, but it didn't specify when those updates will arrive. Jerry Bryant, director of security communication in Intel's platform assurance and security group, said in a blog post Monday that mitigations would be released "through our normal Intel Platform Update (IPU) process in the near future."

The new vulnerability, listed as CVE-2020-0549, is described by Intel as "L1D Eviction Sampling." The chipmaker assigned CacheOut a CVSS score of 6.5, which is medium on the severity scale.

The research team said CacheOut cannot be exploited through a web browser like other speculative execution attacks. However, the team also noted it's "very unlikely" for organizations to determine whether the vulnerability has been exploited in their networks because, like other speculative execution attacks, CacheOut leaves no traditional log activity behind.

For mitigations, the researchers suggested disabling hyper-threading, flushing the L1D cache and turning off Transactional Synchronization Extensions (TSX) in Intel chips.

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