Is Coinbase holding MicroStrategy’s bitcoin? Despite Michael Saylor’s repeated refusals to publish a proof of reserves for the 597,325 bitcoin (BTC) at MicroStrategy (MSTR), Coinbase CEO Brian Armstrong decided to publicly doxx their most probable location. Disregarding Saylor’s concerns that disclosing the location of the coins could create the risk of social engineering and wrench attacks, Armstrong tweeted about their location while bragging about Coinbase’s “institutional team.” During the thinly veiled disclosure, he admitted that eight of the top 10 publicly traded companies with BTC on their balance sheets use Coinbase Prime. That makes it highly likely that MicroStrategy holds BTC at Coinbase Prime. Five good reasons to suspect Michael Saylor uses Coinbase Prime First of all, MicroStrategy holds the most BTC of any publicly traded company. That already gives it an 80% chance of using Coinbase Prime for its BTC, according to Armstrong’s tweet this week. Second, MicroStrategy has been the leading educator of Bitcoin for Corporations, providing a suite of educational materials, training sessions, and even hosting an annual conference under the same name. If Saylor has been consistently teaching executives how to safely buy BTC and eight of the 10 largest choose Coinbase Prime, it doesn’t take much imagination to guess where MicroStrategy has been buying BTC.Third, on-chain tracking of MicroStrategy’s BTC indicates a high likelihood that it’s paying Coinbase Prime Custody fees. Arkham claims to have traced $50 billion of MicroStrategy’s $65 billion BTC — and most of it is in Coinbase Prime Custody or has paid Coinbase Prime Custody fees during on-chain movements. Fourth, MicroStrategy uses the BTC price on Coinbase — not other exchanges — to determine the value of its holdings for Securities and Exchange Commission (SEC) reporting purposes. Fifth, it also calls Coinbase, unlike other exchanges or custodians, “our principal market for bitcoin.” Read more: MicroStrategy makes ‘weird’ 7,382-BTC withdrawal hours before new bitcoin buy In other words, Armstrong’s tweet this week only solidifies an already compelling suite of data that MicroStrategy uses Coinbase Prime to store a substantial quantity of its BTC. Socially engineering Armstrong’s or Saylor’s employees Social engineering is one of the most serious cybersecurity risks, though the exact percentage of successful attacks that involve social engineering can vary depending on who you ask. A Verizon study tallied pure social engineering attacks at 17% of data breaches from November 2023 to October 2024. That figure rose as high as 68% when they added “human error”-style variables like clicking on a malicious link. Another report from IBM estimated that phishing attacks rose by 84% in its most recent year-over-year tracking period. Coinbase itself has been the target of attacks in which its own employees were a potential weak point. In May 2025, it admitted that cybercriminals bribed customer service representatives to steal customer data and facilitate social engineering attacks against Coinbase’s customers. In February 2023, for example, Armstrong’s team published a postmortem of an attack that targeted unnamed employees with an SMS-based phishing attack. Another successful multi-factor authentication attack drained 6,000 Coinbase accounts of their digital assets. Despite these previous attacks, Armstrong considered it more important to brag about Coinbase’s institutional services like Coinbase Prime.