Flipping Bits: Your Credentials Are Certainly Mine

The talk explains bitsquatting, where one bit changes in a domain name held in memory or in transit and turns it into a different name that an attacker can register. The speakers walk through how DNS lookups and TLS handshakes behave when a client ends up at a lookalike host, and why wildcard certificates and long hostnames make the problem harder. They open with a story about a lookalike domain that began receiving email meant for a bank, which led them to look at the technique more closely.

They then describe Certainly, a custom DNS, HTTP and certificate server built to answer for flipped domains. It issues certificates on the fly by completing ACME validation, so clients finish the TLS handshake, and it can log or alter requests and responses, including injecting content into scripts or JSON. Across a test period of about four months, with roughly 25 registered lookalikes for high-traffic services, they recorded tens of millions of DNS requests, a large volume of email, and about two thousand authentication tokens. A hypothesis that solar and cosmic-ray events drive the bit flips was not borne out by their data, so they treated the root cause as an open research question. They also looked at flips in top-level domains.

For defenders, the suggested measures were to register the most likely lookalike domains for priority names, and to restrict outbound connections to expected destinations. They warned that certificate pinning can fail open when it does not account for unknown hostnames, so pinning should be designed to fail closed. The tool was released as open source under the MIT licence.