When the client creates its ClientHello message, due to insufficient bounds checking it can overflow the heap-based buffer containing the message while writing some extensions. Two extensions in particular could be used by a remote attacker to trigger the overflow: the session ticket extension and the server name indication (SNI) extension.
Starting with PolarSSL 1.3.0 which added support for session tickets, any server the client connects to can send an overlong session ticket which will cause a buffer overflow if and when the client attempts to resume the connection with the server. Clients that disabled session tickets or never attempt to reconnect to a server using a saved session are not vulnerable to this attack vector.
Starting with PolarSSL 1.0.0, this overflow could also be triggered by an attacker convincing a client to use an overlong hostname for the SNI extension. The hostname needs to be almost as long at SSL_MAX_CONTENT_LEN, which as 16KB by default, but could be smaller if a custom configuration is used. Clients that do not accept hostnames from untrusted parties are not vulnerable to this attack vector.