diff --git a/src/mod_fastcgi.c b/src/mod_fastcgi.c
index e42a6ff0..93d7d9fc 100644
--- a/src/mod_fastcgi.c
+++ b/src/mod_fastcgi.c
@@ -242,6 +242,10 @@ static handler_t fcgi_create_env(server *srv, handler_ctx *hctx) {
 	beginRecord.body.flags = 0;
 	memset(beginRecord.body.reserved, 0, sizeof(beginRecord.body.reserved));
 
+	if ((off_t)buffer_string_space(b) < con->read_queue->bytes_out - hctx->wb->bytes_in) {
+		buffer_string_prepare_copy(b, ((size_t)(con->read_queue->bytes_out - hctx->wb->bytes_in + 4095) & ~4095uL)-1);
+	}
+
 	buffer_copy_string_len(b, (const char *)&beginRecord, sizeof(beginRecord));
 	fcgi_header(&header, FCGI_PARAMS, request_id, 0, 0); /*(set aside space to fill in later)*/
 	buffer_append_string_len(b, (const char *)&header, sizeof(header));
diff --git a/src/mod_proxy.c b/src/mod_proxy.c
index 0da05a4e..44fefa7f 100644
--- a/src/mod_proxy.c
+++ b/src/mod_proxy.c
@@ -714,6 +714,10 @@ static handler_t proxy_create_env(server *srv, gw_handler_ctx *gwhctx) {
 	const int upgrade = hctx->conf.header.upgrade
 	    && (NULL != http_header_request_get(con, HTTP_HEADER_UPGRADE, CONST_STR_LEN("Upgrade")));
 
+	if ((off_t)buffer_string_space(b) < con->read_queue->bytes_out - hctx->gw.wb->bytes_in) {
+		buffer_string_prepare_copy(b, ((size_t)(con->read_queue->bytes_out - hctx->gw.wb->bytes_in + 4095) & ~4095uL)-1);
+	}
+
 	/* build header */
 
 	/* request line */
diff --git a/src/mod_scgi.c b/src/mod_scgi.c
index da4033b4..d98a9676 100644
--- a/src/mod_scgi.c
+++ b/src/mod_scgi.c
@@ -132,7 +132,11 @@ static int scgi_env_add_uwsgi(void *venv, const char *key, size_t key_len, const
 
 	len = 2 + key_len + 2 + val_len;
 
-	buffer_string_prepare_append(env, len);
+	if (buffer_string_space(env) < len) {
+		size_t extend = env->size * 2 - buffer_string_length(env);
+		extend = extend > len ? extend : len + 4095;
+		buffer_string_prepare_append(env, extend);
+	}
 
 	uwlen = uwsgi_htole16((uint16_t)key_len);
 	buffer_append_string_len(env, (char *)&uwlen, 2);
@@ -155,6 +159,10 @@ static handler_t scgi_create_env(server *srv, handler_ctx *hctx) {
 	  : scgi_env_add_uwsgi;
 	size_t offset;
 
+	if ((off_t)buffer_string_space(b) < con->read_queue->bytes_out - hctx->wb->bytes_in) {
+		buffer_string_prepare_copy(b, ((size_t)(con->read_queue->bytes_out - hctx->wb->bytes_in + 4095) & ~4095uL)-1);
+	}
+
         /* save space for 9 digits (plus ':'), though incoming HTTP request
 	 * currently limited to 64k (65535, so 5 chars) */
 	buffer_copy_string_len(b, CONST_STR_LEN("          "));