fix for https://github.com/RIOT-OS/RIOT/pull/12907#discussion_r441983334

session_fix.patch

diff --git a/pkg/tinydtls/contrib/sock_dtls.c b/pkg/tinydtls/contrib/sock_dtls.c
index f688cdcf3..4a8d45945 100644
--- a/pkg/tinydtls/contrib/sock_dtls.c
+++ b/pkg/tinydtls/contrib/sock_dtls.c
@@ -82,6 +82,7 @@ static int _read(struct dtls_context_t *ctx, session_t *session, uint8_t *buf,
     DEBUG("sock_dtls: decrypted message arrived\n");
     sock->buf = buf;
     sock->buflen = len;
+    sock->buf_session = session;
 #ifdef SOCK_HAS_ASYNC
     if (sock->async_cb != NULL) {
         sock->async_cb(sock, SOCK_ASYNC_MSG_RECV, sock->async_cb_arg);
@@ -437,6 +438,8 @@ static ssize_t _copy_buffer(sock_dtls_t *sock, sock_dtls_session_t *remote,
 {
     uint8_t *buf = sock->buf;
     size_t buflen = sock->buflen;
+    memcpy(&remote->dtls_session, sock->buf_session, sizeof(remote->dtls_session));
+    _session_to_ep(&remote->dtls_session, &remote->ep);

     sock->buf = NULL;
     if (buflen > max_len) {
diff --git a/pkg/tinydtls/include/sock_dtls_types.h b/pkg/tinydtls/include/sock_dtls_types.h
index 0eb046ebc..0faa3d526 100644
--- a/pkg/tinydtls/include/sock_dtls_types.h
+++ b/pkg/tinydtls/include/sock_dtls_types.h
@@ -70,6 +70,7 @@ struct sock_dtls {
     uint8_t *buf;                           /**< Buffer to pass decrypted data
                                                 back to user */
     size_t buflen;                          /**< Size of buffer */
+    session_t *buf_session;
     credman_tag_t tag;                      /**< Credential tag of a registered
                                                 (D)TLS credential */
     dtls_peer_type role;                    /**< DTLS role of the socket */