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Covert Cloud Communication with Lambdas

Reviewed by Greg Wilson / 2023-03-13
Keywords: Cloud Computing, Security

The biggest change in practical computing in the last decade has been the nearly-universal return to shared hardware. As with the mainframes of my youth, companies rent time on other people's computers rather than buying and managing their own. And as with any form of sharing, that opens up new security holes and new avenues of attack.

This paper looks at something I would once have dismisssed as Hollywood word salad: covert communication between lambdas co-resident on the same physical server. The authors leverage the hardware's memory bus to detect what's running, and show that they can send a couple of hundred bits per second between supposedly-isolated processes. That might not seem like a lot, but it's more than enough to share a password or a private key, and like most of computer security it's both fascinating and frightening.

Anil Yelam, Shibani Subbareddy, Keerthana Ganesan, Stefan Savage, and Ariana Mirian. CoResident evil: covert communication in the cloud with lambdas. In Proceedings of the Web Conference 2021. ACM, Apr 2021. doi:10.1145/3442381.3450100.

"Serverless" cloud services, such as AWS lambdas, are one of the fastest growing segments of the cloud services market. These services are popular in part due to their light-weight nature and flexibility in scheduling and cost, however the security issues associated with serverless computing are not well understood. In this work, we explore the feasibility of constructing a practical covert channel from lambdas. We establish that a fast co-residence detection for lambdas is key to enabling such a covert channel, and proceed to develop a reliable and scalable co-residence detector based on the memory bus hardware. Our technique enables dynamic discovery for co-resident lambdas and is incredibly fast, executing in a matter of seconds. We evaluate our approach for correctness and scalability, and use it to establish covert channels and perform data transfer on AWS lambdas. We show that we can establish hundreds of individual covert channels for every 1000 lambdas deployed, and each of those channels can send data at a rate of ~200 bits per second, thus demonstrating that covert communication via lambdas is entirely feasible.