diff --git a/CHANGELOG b/CHANGELOG
index 578263bd..66db593c 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -321,6 +321,9 @@ HISTORY
 
   ++ Bugfixes:
 
+  2016-06-30: Simon Goldschmidt (original patch by Fabian Koch)
+  * tcp_in.c: fixed bug #48170 (Vulnerable to TCP RST spoofing)
+
   2016-04-05: Simon Goldschmidt (patch by Philip Gladstone)
   * udp.c: patch #8358: allow more combinations of listening PCB for IPv6
 
diff --git a/src/core/tcp_in.c b/src/core/tcp_in.c
index b411fc41..f291887f 100644
--- a/src/core/tcp_in.c
+++ b/src/core/tcp_in.c
@@ -706,13 +706,23 @@ tcp_process(struct tcp_pcb *pcb)
   if (flags & TCP_RST) {
     /* First, determine if the reset is acceptable. */
     if (pcb->state == SYN_SENT) {
+      /* "In the SYN-SENT state (a RST received in response to an initial SYN),
+          the RST is acceptable if the ACK field acknowledges the SYN." */
       if (ackno == pcb->snd_nxt) {
         acceptable = 1;
       }
     } else {
-      if (TCP_SEQ_BETWEEN(seqno, pcb->rcv_nxt,
-                          pcb->rcv_nxt + pcb->rcv_wnd)) {
+      /* "In all states except SYN-SENT, all reset (RST) segments are validated
+          by checking their SEQ-fields." */
+      if (seqno == pcb->rcv_nxt) {
         acceptable = 1;
+      } else  if (TCP_SEQ_BETWEEN(seqno, pcb->rcv_nxt,
+                                  pcb->rcv_nxt + pcb->rcv_wnd)) {
+        /* If the sequence number is inside the window, we only send an ACK
+           and wait for a re-send with matching sequence number.
+           This violates RFC 793, but is required to protection against
+           CVE-2004-0230 (RST spoofing attack). */
+        tcp_ack_now(pcb);
       }
     }