Mbed TLS does not have a constant-time/constant-trace arithmetic
library and uses blinding to protect against side channel
attacks.
In the ECDSA signature routine previous Mbed TLS versions used the
same RNG object for generating the ephemeral key pair and for
generating the blinding values. The deterministic ECDSA function
reused this by passing the RNG object created from the private key
and the message to be signed as prescribed by RFC 6979. This meant
that the same RNG object was used whenever the same message was
signed, rendering the blinding ineffective.
If the victim can be tricked to sign the same message repeatedly,
the private key may be recoverable through side channels.