How is Mbed TLS (formerly PolarSSL) protected legally?
Mbed TLS (formerly known as PolarSSL) is, like most software, protected by copyright. It is licensed for use under an open-source Apache 2.0 license. The project is owned by TrustedFirmware.org, and the code is owned by all the people and companies who have contributed to it, collectively known as The Mbed TLS Contributors (except for some files in the 3rdparty directory which have copyright as noted in the file headers).
When contributing code to us, the author is required to sign off all commits with a Developer Certificate of Origin line, confirming that the code submitted can (legally) become part of the project, and be subject to the same Apache 2.0 license.
The PolarSSL brand name is trademarked in the US. You can see the trademark at USPTO for yourself.
You can also find more information about using the Mbed trademark.