diff --git a/ChangeLog b/ChangeLog
index b4d2d44..9e0abd7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,15 @@
 
 = mbed TLS x.x.x branch released xxxx-xx-xx
 
+Security
+   * Fix bug in DTLS handling of new associations with the same parameters
+     (RFC 6347 section 4.2.8): after sending its HelloVerifyRequest, the
+     server would end up with corrupted state and only send invalid records to
+     the client. An attacker able to send forged UDP packets to the server
+     could use that to obtain a Denial of Service. This could only happen when
+     MBEDTLS_SSL_DTLS_CLIENT_PORT_REUSE was enabled in config.h (which it is
+     by default).
+
 Bugfix
    * Fix compilation failure when both MBEDTLS_SSL_PROTO_DTLS and
      MBEDTLS_SSL_HW_RECORD_ACCEL are enabled.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index b82e24f..5f6ae62 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3598,13 +3598,16 @@
     int ret;
     size_t len;
 
+    /* Use out_msg as temporary buffer for writing out HelloVerifyRequest,
+     * because the output buffer's already around. Don't use out_buf though,
+     * as we don't want to overwrite out_ctr. */
     ret = ssl_check_dtls_clihlo_cookie(
             ssl->conf->f_cookie_write,
             ssl->conf->f_cookie_check,
             ssl->conf->p_cookie,
             ssl->cli_id, ssl->cli_id_len,
             ssl->in_buf, ssl->in_left,
-            ssl->out_buf, MBEDTLS_SSL_MAX_CONTENT_LEN, &len );
+            ssl->out_msg, MBEDTLS_SSL_MAX_CONTENT_LEN, &len );
 
     MBEDTLS_SSL_DEBUG_RET( 2, "ssl_check_dtls_clihlo_cookie", ret );
 
@@ -3613,11 +3616,11 @@
         int send_ret;
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "sending HelloVerifyRequest" ) );
         MBEDTLS_SSL_DEBUG_BUF( 4, "output record sent to network",
-                                  ssl->out_buf, len );
+                                  ssl->out_msg, len );
         /* Don't check write errors as we can't do anything here.
          * If the error is permanent we'll catch it later,
          * if it's not, then hopefully it'll work next time. */
-        send_ret = ssl->f_send( ssl->p_bio, ssl->out_buf, len );
+        send_ret = ssl->f_send( ssl->p_bio, ssl->out_msg, len );
         MBEDTLS_SSL_DEBUG_RET( 2, "ssl->f_send", send_ret );
         (void) send_ret;