diff --git a/tests/sys/netpfil/common/Makefile b/tests/sys/netpfil/common/Makefile
index f51b89d5eab4..0003aac28779 100644
--- a/tests/sys/netpfil/common/Makefile
+++ b/tests/sys/netpfil/common/Makefile
@@ -1,31 +1,33 @@
 
 PACKAGE=	tests
 
 TESTSDIR=	${TESTSBASE}/sys/netpfil/common
 
 
 ATF_TESTS_SH+=	\
 		dummynet \
 		pass_block \
 		nat \
 		rdr \
 		tos \
 		fragments \
 		forward
 
 # Tests reuse jail names and so cannot run in parallel.
 TEST_METADATA+=	is_exclusive=true
 
 ${PACKAGE}FILES+=	\
 		utils.subr \
 		runner.subr \
 		pft_icmp_check.py \
 		pft_ping.py \
+		pft_rst.py \
 		pft_synflood.py \
 		sniffer.py
 
 ${PACKAGE}FILESMODE_pft_icmp_check.py=	0555
 ${PACKAGE}FILESMODE_pft_ping.py=	0555
+${PACKAGE}FILESMODE_pft_rst.py=	0555
 ${PACKAGE}FILESMODE_pft_synflood.py=	0555
 
 .include <bsd.test.mk>
diff --git a/tests/sys/netpfil/common/pft_rst.py b/tests/sys/netpfil/common/pft_rst.py
new file mode 100644
index 000000000000..28d951e8a047
--- /dev/null
+++ b/tests/sys/netpfil/common/pft_rst.py
@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2023 Rubicon Communications, LLC (Netgate)
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+#
+
+import logging
+logging.getLogger("scapy").setLevel(logging.CRITICAL)
+import math
+import scapy.all as sp
+import sys
+
+def send_rst(src_ip, src_port, dst_ip, dst_port):
+	sp.send(sp.IP(src=src_ip, dst=dst_ip) /
+	   sp.TCP(sport=src_port, dport=dst_port, seq=1, flags="R"))
+
+send_rst(sys.argv[1], int(sys.argv[2]), sys.argv[3], int(sys.argv[4]))
diff --git a/tests/sys/netpfil/pf/Makefile b/tests/sys/netpfil/pf/Makefile
index 9337b95baf4e..1083f89a5502 100644
--- a/tests/sys/netpfil/pf/Makefile
+++ b/tests/sys/netpfil/pf/Makefile
@@ -1,74 +1,75 @@
 
 PACKAGE=	tests
 
 TESTSDIR=       ${TESTSBASE}/sys/netpfil/pf
 BINDIR=		${TESTSDIR}
 TESTS_SUBDIRS+=	ioctl
 
 ATF_TESTS_SH+=	altq \
 		anchor \
 		divert-to \
 		dup \
 		ether \
 		forward \
 		fragmentation_compat \
 		fragmentation_pass \
 		fragmentation_no_reassembly \
 		get_state \
 		icmp \
 		killstate \
 		macro \
 		map_e \
 		match \
 		modulate \
 		names \
 		nat \
 		pass_block \
 		pfsync	\
 		prio \
 		proxy \
 		ridentifier \
 		route_to \
 		rtable \
 		rules_counter \
 		scrub_compat \
 		scrub_pass \
 		sctp \
 		set_skip \
 		set_tos \
 		src_track \
 		syncookie \
 		synproxy \
 		table \
+		tcp \
 		tos
 
 ATF_TESTS_PYTEST+=	frag6.py
 ATF_TESTS_PYTEST+=	sctp.py
 
 # Tests reuse jail names and so cannot run in parallel.
 TEST_METADATA+=	is_exclusive=true
 
 PROGS=	divapp
 
 ${PACKAGE}FILES+=	CVE-2019-5597.py \
 			CVE-2019-5598.py \
 			daytime_inetd.conf \
 			echo_inetd.conf \
 			fragcommon.py \
 			frag-overindex.py \
 			frag-overlimit.py \
 			frag-overreplace.py \
 			pfsync_defer.py \
 			pft_ether.py \
 			utils.subr
 
 ${PACKAGE}FILESMODE_CVE-2019-5597.py=	0555
 ${PACKAGE}FILESMODE_CVE-2019-5598.py=	0555
 ${PACKAGE}FILESMODE_fragcommon.py=	0555
 ${PACKAGE}FILESMODE_frag-overindex.py=	0555
 ${PACKAGE}FILESMODE_frag-overlimit.py=	0555
 ${PACKAGE}FILESMODE_frag-overreplace.py=	0555
 ${PACKAGE}FILESMODE_pfsync_defer.py=	0555
 ${PACKAGE}FILESMODE_pft_ether.py=	0555
 
 .include <bsd.test.mk>
diff --git a/tests/sys/netpfil/pf/tcp.sh b/tests/sys/netpfil/pf/tcp.sh
new file mode 100644
index 000000000000..84536480b44e
--- /dev/null
+++ b/tests/sys/netpfil/pf/tcp.sh
@@ -0,0 +1,109 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2023 Rubicon Communications, LLC (Netgate)
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1. Redistributions of source code must retain the above copyright
+#    notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+#    notice, this list of conditions and the following disclaimer in the
+#    documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+
+. $(atf_get_srcdir)/utils.subr
+
+common_dir=$(atf_get_srcdir)/../common
+
+atf_test_case "rst" "cleanup"
+rst_head()
+{
+	atf_set descr 'Check sequence number validation in RST packets'
+	atf_set require.user root
+	atf_set require.progs scapy
+}
+
+rst_body()
+{
+	pft_init
+
+	epair_srv=$(vnet_mkepair)
+	epair_cl=$(vnet_mkepair)
+	epair_attack=$(vnet_mkepair)
+
+	br=$(vnet_mkbridge)
+	ifconfig ${br} addm ${epair_srv}a
+	ifconfig ${epair_srv}a up
+	ifconfig ${br} addm ${epair_cl}a
+	ifconfig ${epair_cl}a up
+	ifconfig ${br} addm ${epair_attack}a
+	ifconfig ${epair_attack}a up
+	ifconfig ${br} up
+
+	vnet_mkjail srv ${epair_srv}b
+	jexec srv ifconfig ${epair_srv}b 192.0.2.1/24 up
+	jexec srv ifconfig lo0 inet 127.0.0.1/8 up
+
+	vnet_mkjail cl ${epair_cl}b
+	jexec cl ifconfig ${epair_cl}b 192.0.2.2/24 up
+	jexec cl ifconfig lo0 inet 127.0.0.1/8 up
+
+	jexec cl pfctl -e
+	pft_set_rules cl \
+		"pass keep state"
+
+	# Not required, but pf should log the bad RST packet with this set.
+	jexec cl pfctl -x loud
+
+	vnet_mkjail attack ${epair_attack}b
+	jexec attack ifconfig ${epair_attack}b 192.0.2.3/24 up
+
+	# Sanity check
+	atf_check -s exit:0 -o ignore \
+	    jexec cl ping -c 1 192.0.2.1
+
+	echo "bar" | jexec srv nc -l 1234 &
+	# Allow server time to start
+	sleep 1
+
+	echo "foo" | jexec cl nc -p 4321 192.0.2.1 1234 &
+	# Allow connection time to set up
+	sleep 1
+
+	# Connection should be established now
+	atf_check -s exit:0 -e ignore \
+	    -o match:"ESTABLISHED:ESTABLISHED" \
+	    jexec cl pfctl -ss -v
+
+	# Now insert a fake RST
+	atf_check -s exit:0 -o ignore \
+	    jexec attack ${common_dir}/pft_rst.py 192.0.2.1 1234 192.0.2.2 4321
+
+	# Connection should remain established
+	atf_check -s exit:0 -e ignore \
+	    -o match:"ESTABLISHED:ESTABLISHED" \
+	    jexec cl pfctl -ss -v
+}
+
+rst_cleanup()
+{
+	pft_cleanup
+}
+
+atf_init_test_cases()
+{
+	atf_add_test_case "rst"
+}