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Security9 July 2025·3 min read

Why SMS is not a secure Multi-Factor Authentication (MFA) method

The useful frame is not "is this secure" but "what does an attacker get if this one thing goes wrong, and how quickly would we know".

Paycux engineering

The useful frame is not "is this secure" but "what does an attacker get if this one thing goes wrong, and how quickly would we know".

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

Vulnerable to SIM swap attacks

Vulnerable to SIM swap attacks 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.

Lack of end-to-end encryption

That brings us to lack of end-to-end encryption. 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.

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

Susceptibe to phishing and Man-in-the-Middle attacks

Consider susceptibe to phishing and man-in-the-middle attacks. 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.

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.

Phone numbers are not stable identifiers

Phone numbers are not stable identifiers 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.

Rotate on a schedule, not on an incident.

Reliance on mobile carrier infrastructure

Reliance on mobile carrier infrastructure 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.

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.

Compliance and industry recommendations

Compliance and industry recommendations 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.

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.

Better alternatives

Better alternatives 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.

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.

Where this leaves us

The pattern repeats across every system we have looked at: the hard part is not the mechanism, it is keeping the mechanism honest as the surrounding assumptions change.

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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