MAGMI versions prior to 0.7.24 are vulnerable to a remote authentication bypass due to allowing default credentials in the event there is a database connection failure. A remote attacker can trigger this connection failure if the Mysql setting max_connections (default 151) is lower than Apache (or another web server) setting MaxRequestWorkers (formerly MaxClients) (default 256). This can be done by sending at least 151 simultaneous requests to the Magento website to trigger a "Too many connections" error, then use default magmi:magmi basic authentication to remotely bypass authentication.
CWE-287 - Improper Authentication
Improper (or broken) authentication attacks are widespread, and have accounted for many of the worst data breaches in recent years. Improper authentication attacks are a class of vulnerabilities where an attacker impersonates a legitimate user by exploiting weaknesses in either session management or credential management to gain access to the user’s account. This can result in disclosure of sensitive information, and can lead to system compromise, theft, identity theft, and fraud.