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Security3 January 2025·3 min read

AuthQuake: Microsoft's MFA system vulnerable to TOTP brute force attack

Security work is mostly the business of removing footguns before someone finds them.

Paycux engineering

Security work is mostly the business of removing footguns before someone finds them. The interesting question is rarely whether an attack is possible — it is whether the ordinary path through your system makes it easy.

This piece walks through how we think about it at Paycux, what we have changed our minds about, and where the sharp edges are.

Understanding the AuthQuake vulnerability

Understanding the AuthQuake vulnerability is where this gets concrete. Detection beats prevention when prevention is uncertain. If you cannot stop a class of abuse outright, make sure it produces a signal loud enough that someone sees it the same day.

Rotate on a schedule, not on an incident. A rotation path that has been exercised is a routine change; one that has never been used is an outage waiting for the worst possible moment.

The technical breakdown

Consider the technical breakdown. Blast radius is the metric that actually matters. A credential that can read one organization's data is an incident; the same credential scoped to every organization is a company-ending event, and the difference is a design decision made months earlier.

Blast radius is the metric that actually matters. A credential that can read one organization's data is an incident; the same credential scoped to every organization is a company-ending event, and the difference is a design decision made months earlier.

  • Scope credentials to the smallest useful blast radius
  • Alert on the abuse you cannot prevent
  • Exercise the rotation path before you need it
  • Log the decision, not just the outcome

Protecting your organization

Protecting your organization deserves its own treatment. Blast radius is the metric that actually matters. A credential that can read one organization's data is an incident; the same credential scoped to every organization is a company-ending event, and the difference is a design decision made months earlier.

Blast radius is the metric that actually matters. A credential that can read one organization's data is an incident; the same credential scoped to every organization is a company-ending event, and the difference is a design decision made months earlier.

Move beyond traditional TOTPs

Move beyond traditional TOTPs deserves its own treatment. Detection beats prevention when prevention is uncertain. If you cannot stop a class of abuse outright, make sure it produces a signal loud enough that someone sees it the same day.

Blast radius is the metric that actually matters. A credential that can read one organization's data is an incident; the same credential scoped to every organization is a company-ending event, and the difference is a design decision made months earlier.

Detection beats prevention when prevention is uncertain.

Implement proper monitoring and alerts

Consider implement proper monitoring and alerts. Rotate on a schedule, not on an incident. A rotation path that has been exercised is a routine change; one that has never been used is an outage waiting for the worst possible moment.

Detection beats prevention when prevention is uncertain. If you cannot stop a class of abuse outright, make sure it produces a signal loud enough that someone sees it the same day.

Consider third-party authentication providers

Consider third-party authentication providers deserves its own treatment. Detection beats prevention when prevention is uncertain. If you cannot stop a class of abuse outright, make sure it produces a signal loud enough that someone sees it the same day.

Blast radius is the metric that actually matters. A credential that can read one organization's data is an incident; the same credential scoped to every organization is a company-ending event, and the difference is a design decision made months earlier.

The responsible disclosure process

The responsible disclosure process is where this gets concrete. Rotate on a schedule, not on an incident. A rotation path that has been exercised is a routine change; one that has never been used is an outage waiting for the worst possible moment.

Detection beats prevention when prevention is uncertain. If you cannot stop a class of abuse outright, make sure it produces a signal loud enough that someone sees it the same day.

Where this leaves us

If there is one thing worth taking away, it is that the expensive decisions are the ones made implicitly. Making them on purpose costs an afternoon.

If you are working through the same problem and want to compare notes, the docs cover the mechanics and the console shows the behaviour on your own data.

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