Add ChangeLog entry

ChangeLog

@@ -2,6 +2,21 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+Security
+   * Fix a vulnerability in TLS ciphersuites based on CBC and using SHA-384,
+     in (D)TLS 1.0 to 1.2, that allowed an active network attacker to
+     partially recover the plaintext of messages under some conditions by
+     exploiting timing measurements. With DTLS, the attacker could perform
+     this recovery by sending many messages in the same connection. With TLS
+     or if mbedtls_ssl_conf_dtls_badmac_limit() was used, the attack only
+     worked if the same secret (for example a HTTP Cookie) has been repeatedly
+     sent over connections manipulated by the attacker. Connections using GCM
+     or CCM instead of CBC, using hash sizes other than SHA-384, or using
+     Encrypt-then-Mac (RFC 7366) were not affected. The vulnerability was
+     caused by a miscalculation (for SHA-384) in a countermeasure to the
+     original Lucky 13 attack. Found by Kenny Paterson, Eyal Ronen and Adi
+     Shamir.
+
 API Changes
    * Extend the platform module with a util component that contains
      functionality shared by multiple Mbed TLS modules. At this stage