diff --git a/sbin/fsck_msdosfs/tests/Makefile b/sbin/fsck_msdosfs/tests/Makefile
index a36439f91bce..c8963837b7b0 100644
--- a/sbin/fsck_msdosfs/tests/Makefile
+++ b/sbin/fsck_msdosfs/tests/Makefile
@@ -1,5 +1,6 @@
 PACKAGE=	tests
 
-ATF_TESTS_SH=	fsck_msdosfs_test
+ATF_TESTS_SH=	fsck_msdosfs_test \
+		fsck_msdosfs_boot_test
 
 .include <bsd.test.mk>
diff --git a/sbin/fsck_msdosfs/tests/fsck_msdosfs_boot_test.sh b/sbin/fsck_msdosfs/tests/fsck_msdosfs_boot_test.sh
new file mode 100644
index 000000000000..51333982a96b
--- /dev/null
+++ b/sbin/fsck_msdosfs/tests/fsck_msdosfs_boot_test.sh
@@ -0,0 +1,214 @@
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2026 The FreeBSD Foundation
+#
+
+# Tests for the 32 bit BIOS Parameter Block and FSInfo fields decoded by
+# readboot() in sbin/fsck_msdosfs/boot.c.  Each case sets one field to a
+# value whose most significant byte has its high bit set, which is the
+# range that a byte-by-byte "b[3] << 24" decode has to shift into the
+# sign bit of an int.
+#
+# Each case makes two assertions:
+#
+# 1. On stdout, that fsck_msdosfs(8) reports the full unsigned 32 bit
+#    value back.  This covers the decoding itself in an ordinary build.
+#
+# 2. On stderr, that nothing reports a runtime error.  A byte-by-byte
+#    decode of these values is undefined behavior, but every compiler we
+#    use wraps it into the same bit pattern, so it cannot be caught by
+#    the value alone.  In a WITH_UBSAN build it is caught here, because
+#    bsd.sanitizer.mk builds with -fsanitize=undefined and
+#    -fsanitize-recover=undefined, so the shift is reported on stderr and
+#    execution continues.  In a build without the sanitizer these
+#    assertions are trivially true.
+#
+# Note that assertion 2 also fails on unrelated undefined behavior that
+# these images reach anywhere in fsck_msdosfs(8), which is intentional.
+
+IMG=fat32.img
+
+# The high bit of the most significant byte is set in all of these.
+HIGH=2147483648		# 0x80000000
+HIGH1=2147483649	# 0x80000001
+
+# A UBSan report, as produced by a WITH_UBSAN build.  It never appears
+# in a build without the sanitizer.
+UB='runtime error'
+
+# Read an unsigned little-endian integer of $3 bytes at offset $2 of $1.
+bpb_read()
+{
+	od -An -v -tu1 -j "$2" -N "$3" "$1" | awk '
+	    { for (i = 1; i <= NF; i++) b[n++] = $i }
+	    END { v = 0; for (i = n - 1; i >= 0; i--) v = v * 256 + b[i]
+		  print v }'
+}
+
+# Write the unsigned 32 bit little-endian value $3 at offset $2 of $1.
+poke32()
+{
+	printf "$(printf '\\%03o\\%03o\\%03o\\%03o' $(($3 & 255)) \
+	    $((($3 >> 8) & 255)) $((($3 >> 16) & 255)) \
+	    $((($3 >> 24) & 255)))" |
+	    dd of="$1" bs=1 seek="$2" conv=notrunc status=none
+}
+
+# Byte offset of the FSInfo sector of $IMG.
+fsinfo_off()
+{
+	echo $(($(bpb_read ${IMG} 48 2) * $(bpb_read ${IMG} 11 2)))
+}
+
+# Create a 40 MiB FAT32 file system in $IMG.  One sector per cluster
+# keeps it comfortably above the 65525 cluster FAT32 minimum.
+make_image()
+{
+	atf_check -s exit:0 -o ignore -e ignore \
+	    newfs_msdos -C 40m -F 32 -c 1 -S 512 ./${IMG}
+	# A freshly created file system must be clean, and must not have
+	# tripped the sanitizer on its way through readboot().
+	atf_check -s exit:0 -o ignore -e not-match:"${UB}" \
+	    fsck_msdosfs -y ./${IMG}
+}
+
+atf_test_case hidden_secs_high_bit
+hidden_secs_high_bit_head()
+{
+	atf_set "descr" "Hidden sector count with the high bit set"
+	atf_set "require.progs" "newfs_msdos fsck_msdosfs"
+}
+hidden_secs_high_bit_body()
+{
+	make_image
+	poke32 ${IMG} 28 ${HIGH}
+
+	# readboot() decodes bpbHiddenSecs but nothing uses it, so the
+	# only thing to check is that the file system still comes out
+	# clean and that decoding it was well defined.
+	atf_check -s exit:0 -o not-match:'Invalid' -e not-match:"${UB}" \
+	    fsck_msdosfs -n ./${IMG}
+}
+
+atf_test_case fsinfo_free_high_bit
+fsinfo_free_high_bit_head()
+{
+	atf_set "descr" "FSInfo free cluster count with the high bit set"
+	atf_set "require.progs" "newfs_msdos fsck_msdosfs"
+}
+fsinfo_free_high_bit_body()
+{
+	make_image
+	poke32 ${IMG} $(($(fsinfo_off) + 0x1e8)) ${HIGH}
+
+	# The count is far larger than the number of clusters, so it has
+	# to be reported as wrong, with the decoded value spelled out.
+	atf_check -s exit:0 \
+	    -o match:"Free space in FSInfo block \(${HIGH}\) not correct" \
+	    -e not-match:"${UB}" fsck_msdosfs -n ./${IMG}
+
+	# Once repaired the bogus count must be gone for good.
+	atf_check -s exit:0 -o ignore -e not-match:"${UB}" \
+	    fsck_msdosfs -y ./${IMG}
+	atf_check -s exit:0 -o not-match:'Free space in FSInfo block' \
+	    -e not-match:"${UB}" fsck_msdosfs -n ./${IMG}
+}
+
+atf_test_case fsinfo_next_high_bit
+fsinfo_next_high_bit_head()
+{
+	atf_set "descr" "FSInfo next free cluster with the high bit set"
+	atf_set "require.progs" "newfs_msdos fsck_msdosfs"
+}
+fsinfo_next_high_bit_body()
+{
+	make_image
+	poke32 ${IMG} $(($(fsinfo_off) + 0x1ec)) ${HIGH1}
+
+	atf_check -s exit:0 \
+	    -o match:"Next free cluster in FSInfo block \(${HIGH1}\) invalid" \
+	    -e not-match:"${UB}" fsck_msdosfs -n ./${IMG}
+
+	atf_check -s exit:0 -o ignore -e not-match:"${UB}" \
+	    fsck_msdosfs -y ./${IMG}
+	atf_check -s exit:0 -o not-match:'Next free cluster in FSInfo block' \
+	    -e not-match:"${UB}" fsck_msdosfs -n ./${IMG}
+}
+
+atf_test_case root_cluster_high_bit
+root_cluster_high_bit_head()
+{
+	atf_set "descr" "FAT32 root directory cluster with the high bit set"
+	atf_set "require.progs" "newfs_msdos fsck_msdosfs"
+}
+root_cluster_high_bit_body()
+{
+	make_image
+	poke32 ${IMG} 44 ${HIGH}
+
+	# Out of range, so readboot() gives up before any phase runs.
+	atf_check -s exit:8 \
+	    -o match:"Root directory starts with cluster out of range\(${HIGH}\)" \
+	    -e not-match:"${UB}" fsck_msdosfs -n ./${IMG}
+}
+
+atf_test_case fatsecs_high_bit
+fatsecs_high_bit_head()
+{
+	atf_set "descr" "FAT32 sectors per FAT with the high bit set"
+	atf_set "require.progs" "newfs_msdos fsck_msdosfs"
+}
+fatsecs_high_bit_body()
+{
+	local nfats
+
+	make_image
+	nfats=$(bpb_read ${IMG} 16 1)
+	poke32 ${IMG} 36 ${HIGH}
+
+	# ${HIGH} FAT sectors times ${nfats} FATs overflows 32 bits.
+	atf_check -s exit:8 \
+	    -o match:"Invalid FATs\(${nfats}\) with FATsecs\(${HIGH}\)" \
+	    -e not-match:"${UB}" fsck_msdosfs -n ./${IMG}
+}
+
+atf_test_case huge_sectors_high_bit
+huge_sectors_high_bit_head()
+{
+	atf_set "descr" "32 bit total sector count with the high bit set"
+	atf_set "require.progs" "newfs_msdos fsck_msdosfs"
+}
+huge_sectors_high_bit_body()
+{
+	local bps spc rsvd nfats fatsz first clusters
+
+	make_image
+	bps=$(bpb_read ${IMG} 11 2)
+	spc=$(bpb_read ${IMG} 13 1)
+	rsvd=$(bpb_read ${IMG} 14 2)
+	nfats=$(bpb_read ${IMG} 16 1)
+	fatsz=$(bpb_read ${IMG} 36 4)
+	poke32 ${IMG} 32 ${HIGH}
+
+	# The root directory has no fixed entries on FAT32, so the data
+	# area starts right after the reserved sectors and the FATs.
+	first=$((rsvd + nfats * fatsz))
+	clusters=$(((HIGH - first) / spc))
+
+	# Too many clusters for FAT32; the count in the message is
+	# derived from the decoded sector count.
+	atf_check -s exit:8 \
+	    -o match:"Filesystem too big \(${clusters} clusters\) for FAT32" \
+	    -e not-match:"${UB}" fsck_msdosfs -n ./${IMG}
+}
+
+atf_init_test_cases()
+{
+	atf_add_test_case hidden_secs_high_bit
+	atf_add_test_case fsinfo_free_high_bit
+	atf_add_test_case fsinfo_next_high_bit
+	atf_add_test_case root_cluster_high_bit
+	atf_add_test_case fatsecs_high_bit
+	atf_add_test_case huge_sectors_high_bit
+}