22.5 Insecure encryption activation
Systems that use older plaintext protocols require backward compatibility. In this case, Alice and Bob start the communication without encryption and must explicitly upgrade it to use TLS. As an example, if they use Simple Mail Transfer Protocol (SMTP), published by American computer scientists Jon Postel and Suzanne Sluizer in 1981 (RFC 788), they have to use the STARTTLS command to start a TLS session.
The need to explicitly activate secure communication creates additional attack vectors if application code running on Alice’s or Bob’s machine contains implementation flaws – not programming bugs, but logical mistakes in the implementation affecting its security [150].
One such flaw is missing STARTTLS enforcement. When a legacy protocol can be used without encryption, Bob’s software is responsible for enforcing the desired security level. A common flaw in such software is to request encryption but proceed without it...
