Index: curl-7.65.3/lib/tftp.c
===================================================================
--- curl-7.65.3.orig/lib/tftp.c	2019-07-16 18:14:37.000000000 +0800
+++ curl-7.65.3/lib/tftp.c	2021-01-07 16:18:29.639261207 +0800
@@ -973,6 +973,7 @@
 {
   tftp_state_data_t *state;
   int blksize;
+  int need_blksize;
 
   blksize = TFTP_BLKSIZE_DEFAULT;
 
@@ -987,15 +988,20 @@
       return CURLE_TFTP_ILLEGAL;
   }
 
+  need_blksize = blksize;
+  /* default size is the fallback when no OACK is received */
+  if(need_blksize < TFTP_BLKSIZE_DEFAULT)
+    need_blksize = TFTP_BLKSIZE_DEFAULT;
+
   if(!state->rpacket.data) {
-    state->rpacket.data = calloc(1, blksize + 2 + 2);
+    state->rpacket.data = calloc(1, need_blksize + 2 + 2);
 
     if(!state->rpacket.data)
       return CURLE_OUT_OF_MEMORY;
   }
 
   if(!state->spacket.data) {
-    state->spacket.data = calloc(1, blksize + 2 + 2);
+    state->spacket.data = calloc(1, need_blksize + 2 + 2);
 
     if(!state->spacket.data)
       return CURLE_OUT_OF_MEMORY;
@@ -1009,7 +1015,7 @@
   state->sockfd = state->conn->sock[FIRSTSOCKET];
   state->state = TFTP_STATE_START;
   state->error = TFTP_ERR_NONE;
-  state->blksize = blksize;
+  state->blksize = need_blksize;
   state->requested_blksize = blksize;
 
   ((struct sockaddr *)&state->local_addr)->sa_family =