diff -ur apache2-2.4.27/modules/proxy/mod_proxy_connect.c apache2-2.4.27.new/modules/proxy/mod_proxy_connect.c
--- apache2-2.4.27/modules/proxy/mod_proxy_connect.c	2017-10-17 10:04:29.244310247 +0200
+++ apache2-2.4.27.new/modules/proxy/mod_proxy_connect.c	2017-10-17 09:00:32.304658856 +0200
@@ -422,6 +422,13 @@
                     ap_log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r, APLOGNO(01025)
                                   "backend was readable");
 #endif
+                    /* now we know, we can read, but we can't be sure that
+                     * there's enogh room in the outgoing socket to write all
+                     * we read.  Reduce timeout on outgoing socket to not risk
+                     * blocking for three minutes in poll(), called by
+                     * send_brigade_blocking(), when the socket send buffer
+                     * fills up */
+                    apr_socket_timeout_set(client.pfd.desc.s, 10);
                     done |= ap_proxy_transfer_between_connections(r,
                                                                   backend.c,
                                                                   client.c,
@@ -431,6 +438,8 @@
                                                                   NULL,
                                                                   CONN_BLKSZ,
                                                                   1);
+                    /* reset timeout */
+                    apr_socket_timeout_set(client.pfd.desc.s, client.timeout);
                     if (!done && client.c->data_in_output_filters) {
                         apr_pollset_remove(pollset, &client.pfd);
                         client.pfd.reqevents = APR_POLLOUT;
@@ -465,6 +474,13 @@
                     ap_log_rerror(APLOG_MARK, APLOG_DEBUG, 0, r, APLOGNO(01027)
                                   "client was readable");
 #endif
+                    /* now we know, we can read, but we can't be sure that
+                     * there's enogh room in the outgoing socket to write all
+                     * we read.  Reduce timeout on outgoing socket to not risk
+                     * blocking for three minutes in poll(), called by
+                     * send_brigade_blocking(), when the socket send buffer
+                     * fills up */
+                    apr_socket_timeout_set(backend.pfd.desc.s, 10);
                     done |= ap_proxy_transfer_between_connections(r,
                                                                   client.c,
                                                                   backend.c,
@@ -474,6 +490,8 @@
                                                                   NULL,
                                                                   CONN_BLKSZ,
                                                                   1);
+                    /* reset timeout */
+                    apr_socket_timeout_set(backend.pfd.desc.s, backend.timeout);
                     if (!done && backend.c->data_in_output_filters) {
                         apr_pollset_remove(pollset, &backend.pfd);
                         backend.pfd.reqevents = APR_POLLOUT;