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Security19 May 2025·3 min read

Security threats in SPAs and how to defend against them

Almost every incident worth writing about starts with something that was working exactly as designed.

Paycux engineering

Almost every incident worth writing about starts with something that was working exactly as designed. The design was just describing a different world than the one the system ended up in.

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

Threat 1: Cross-Site Scripting (XSS)

Threat 1: Cross-Site Scripting (XSS) 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.

Threat 2: Token storage and theft

That brings us to threat 2: token storage and theft. 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.

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.

  • 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

Threat 3: Broken Object-Level Authorization (BOLA)

Threat 3: Broken Object-Level Authorization (BOLA) 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.

Threat 4: Insecure routing and fake protection

Consider threat 4: insecure routing and fake protection. 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.

Detection beats prevention when prevention is uncertain.

Threat 5: Session and token expiry problems

That brings us to threat 5: session and token expiry problems. 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.

Bonus tips for hardening SPA security

Consider bonus tips for hardening spa security. 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.

How Paycux can help

How Paycux can help is where this gets concrete. 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. 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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