CVE-2017-3737

OpenSSL 1.0.2 (starting from version 1.0.2b) introduced an “error state”
mechanism. The intent was that if a fatal error occurred during a handshake
then OpenSSL would move into the error state and would immediately fail if
you attempted to continue the handshake. This works as designed for the
explicit handshake functions (SSL_do_handshake(), SSL_accept() and
SSL_connect()), however due to a bug it does not work correctly if
SSL_read() or SSL_write() is called directly. In that scenario, if the
handshake fails then a fatal error will be returned in the initial function
call. If SSL_read()/SSL_write() is subsequently called by the application
for the same SSL object then it will succeed and the data is passed without
being decrypted/encrypted directly from the SSL/TLS record layer. In order
to exploit this issue an application bug would have to be present that
resulted in a call to SSL_read()/SSL_write() being issued after having
already received a fatal error. OpenSSL version 1.0.2b-1.0.2m are affected.
Fixed in OpenSSL 1.0.2n. OpenSSL 1.1.0 is not affected.

Notes