Man-in-the-browser attacks involve the use of malware like a Trojan horse to intercept and manipulate legitimate user transactions within a web browser. The malware operates at the application layer, which allows it to manipulate data and transactions before encryption occurs. Despite the secure connection indicated by HTTPS, the malware can alter transaction details, add unauthorized transactions, or steal sensitive information without alerting the user to its presence. This is consistent with the symptoms reported by the financial analyst, where unexpected prompts and transaction confirmations occur, suggesting that the web sessions are being tampered with after being initiated and in a manner invisible to both the end-user and the web application.