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# Security - First Steps { #security-first-steps }

Let's imagine that you have your **backend** API in some domain.

And you have a **frontend** in another domain or in a different path of the same domain (or in a mobile application).

And you want to have a way for the frontend to authenticate with the backend, using a **username** and **password**.

We can use **OAuth2** to build that with **FastAPI**.

Sebastián Ramírez <******@****.***> 1731896744 +0100

Sebastián Ramírez <******@****.***> 1731170360 +0100

Sebastián Ramírez <******@****.***> 1731896744 +0100

Sebastián Ramírez <******@****.***> 1731896744 +0100

Sebastián Ramírez <******@****.***> 1735583217 +0000

Sebastián Ramírez <******@****.***> 1731896744 +0100

  @Override
  public UnmodifiableIterator<C> descendingIterator() {
    return new AbstractSequentialIterator<C>(last()) {
      final C first = first();

      @Override
      protected @Nullable C computeNext(C previous) {
        return equalsOrThrow(previous, first) ? null : domain.previous(previous);
      }
    };
  }

  val android5 = sslLabsClients.first { it.userAgent == "Android" && it.version == "5.0.0" }
  val android9 = sslLabsClients.first { it.userAgent == "Android" && it.version == "9.0" }
  val chrome33 = sslLabsClients.first { it.userAgent == "Chrome" && it.version == "33" }
  val chrome57 = sslLabsClients.first { it.userAgent == "Chrome" && it.version == "57" }
  val chrome80 = sslLabsClients.first { it.userAgent == "Chrome" && it.version == "80" }