diff --git a/src/libwolfboot.c b/src/libwolfboot.c
index 0a1988e3..b6cf25f8 100644
--- a/src/libwolfboot.c
+++ b/src/libwolfboot.c
@@ -1374,8 +1374,11 @@ uint16_t wolfBoot_find_header(uint8_t *haystack, uint16_t type, uint8_t **ptr)
         }
 
         len = p[2] | (p[3] << 8);
-        /* check len */
-        if ((4U + len) > (uint16_t)(IMAGE_HEADER_SIZE - IMAGE_HEADER_OFFSET)) {
+        /* check len (compare in a 32-bit domain: a uint16_t cast of the
+         * header budget wraps for headers >= 64 KiB and rejects every
+         * field) */
+        if ((uint32_t)(4U + len) >
+            (uint32_t)(IMAGE_HEADER_SIZE - IMAGE_HEADER_OFFSET)) {
             unit_dbg("This field is too large (bigger than the space available "
                      "in the current header)\n");
             unit_dbg("%u %u %u\n", (unsigned int)len,
diff --git a/tools/keytools/sign.c b/tools/keytools/sign.c
index 01cc2427..103fdd34 100644
--- a/tools/keytools/sign.c
+++ b/tools/keytools/sign.c
@@ -458,8 +458,11 @@ static uint16_t sign_tool_find_header(uint8_t *haystack, uint16_t type, uint8_t
         }
 
         len = p[2] | (p[3] << 8);
-        /* check len */
-        if ((4 + len) > (uint16_t)(CMD.header_sz - IMAGE_HEADER_OFFSET)) {
+        /* check len (compare in a 32-bit domain: a uint16_t cast of the
+         * header budget wraps for headers >= 64 KiB and rejects every
+         * field) */
+        if ((uint32_t)(4 + len) >
+            (uint32_t)(CMD.header_sz - IMAGE_HEADER_OFFSET)) {
             fprintf(stderr, "This field too large to fit into header "
                 "(%d > %d)\n",
                 (int)(4 + len), (int)(CMD.header_sz - IMAGE_HEADER_OFFSET));
diff --git a/tools/unit-tests/Makefile b/tools/unit-tests/Makefile
index 28f52840..a408501f 100644
--- a/tools/unit-tests/Makefile
+++ b/tools/unit-tests/Makefile
@@ -53,7 +53,8 @@ endif
 
 
 
-TESTS:=unit-parser unit-fdt unit-extflash unit-string unit-spi-flash unit-aes128 \
+TESTS:=unit-parser unit-parser-large-header unit-fdt unit-extflash unit-string \
+       unit-spi-flash unit-aes128 \
        unit-uart-flash \
        unit-aes256 unit-chacha20 unit-pci unit-mock-state unit-sectorflags \
        unit-max-space \
@@ -195,6 +196,7 @@ unit-aes128:CFLAGS+=-DEXT_ENCRYPTED -DENCRYPT_WITH_AES128
 unit-aes256:CFLAGS+=-DEXT_ENCRYPTED -DENCRYPT_WITH_AES256
 unit-chacha20:CFLAGS+=-DEXT_ENCRYPTED -DENCRYPT_WITH_CHACHA
 unit-parser:CFLAGS+=-DNVM_FLASH_WRITEONCE
+unit-parser-large-header:CFLAGS+=-DNVM_FLASH_WRITEONCE
 unit-fdt:CFLAGS+=-DWOLFBOOT_FDT
 unit-nvm:CFLAGS+=-DNVM_FLASH_WRITEONCE -DMOCK_PARTITIONS
 unit-nvm-flagshome:CFLAGS+=-DNVM_FLASH_WRITEONCE -DMOCK_PARTITIONS -DFLAGS_HOME
@@ -303,6 +305,9 @@ unit-extflash.o: FORCE
 unit-parser: ../../include/target.h unit-parser.c
 	gcc -o $@ $^ $(CFLAGS) $(LDFLAGS)
 
+unit-parser-large-header: ../../include/target.h unit-parser-large-header.c
+	gcc -o $@ $^ $(CFLAGS) $(LDFLAGS)
+
 unit-fdt: ../../include/target.h unit-fdt.c ../../src/fdt.c
 	gcc -o $@ $^ $(CFLAGS) -ffunction-sections -fdata-sections $(LDFLAGS) \
 		-Wl,--gc-sections
diff --git a/tools/unit-tests/unit-parser-large-header.c b/tools/unit-tests/unit-parser-large-header.c
new file mode 100644
index 00000000..7c98b8a4
--- /dev/null
+++ b/tools/unit-tests/unit-parser-large-header.c
@@ -0,0 +1,186 @@
+/* unit-parser-large-header.c
+ *
+ * Unit test for wolfBoot_find_header() with a manifest header at or above
+ * the 64 KiB boundary: the per-field budget check must compare in a
+ * 32-bit domain, since a uint16_t cast of (IMAGE_HEADER_SIZE -
+ * IMAGE_HEADER_OFFSET) wraps for headers >= 64 KiB and rejects every
+ * field, breaking the pack/parse roundtrip for large signed TLVs.
+ *
+ *
+ * Copyright (C) 2026 wolfSSL Inc.
+ *
+ * This file is part of wolfBoot.
+ *
+ * wolfBoot is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * wolfBoot is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+/* Must also define DEBUG_WOLFSSL in user_settings.h */
+#define WOLFBOOT_HASH_SHA256
+/* Exactly at the wrap boundary: the payload budget is 0x10000, which a
+ * uint16_t cast of the pre-fix check turned into 0 (rejecting everything). */
+#define IMAGE_HEADER_SIZE 0x10008
+#define WC_RSA_BLINDING
+#define ECC_TIMING_RESISTANT
+#include <stdio.h>
+/* Consume the unit target header up front so its include guard keeps
+ * libwolfboot.c's later include from running. image.h sanity-checks
+ * WOLFBOOT_SECTOR_SIZE > IMAGE_HEADER_SIZE, which the shared unit sector
+ * (0x400) cannot satisfy for a 0x10008 header; override it here. The
+ * parser under test does no flash I/O, so the sector size is otherwise
+ * unused. */
+#include "target.h"
+#undef WOLFBOOT_SECTOR_SIZE
+#define WOLFBOOT_SECTOR_SIZE 0x20000
+#include "libwolfboot.c"
+#include <check.h>
+static int locked = 0;
+
+/* Mocks */
+void hal_init(void)
+{
+}
+int hal_flash_write(haladdr_t address, const uint8_t *data, int len)
+{
+    (void)address;
+    (void)data;
+    (void)len;
+    return 0;
+}
+int hal_flash_erase(haladdr_t address, int len)
+{
+    (void)address;
+    (void)len;
+    return 0;
+}
+void hal_flash_unlock(void)
+{
+    ck_assert_msg(locked, "Double unlock detected\n");
+    locked--;
+}
+void hal_flash_lock(void)
+{
+    ck_assert_msg(!locked, "Double lock detected\n");
+    locked++;
+}
+
+void hal_prepare_boot(void)
+{
+}
+/* End Mocks */
+
+/* A 300-byte TLV payload: comfortably inside the 0x10000 budget, but far
+ * beyond the 244 bytes the wrapped uint16 budget would have allowed for a
+ * slightly smaller oversized header. */
+#define BIG_TLV_LEN 300
+
+static uint8_t big_hdr[IMAGE_HEADER_SIZE];
+
+static void build_big_header(void)
+{
+    uint32_t magic = WOLFBOOT_MAGIC;
+    uint32_t fw_size = 0x100;
+    int i;
+
+    memset(big_hdr, 0xFF, sizeof(big_hdr));
+    memcpy(big_hdr + 0, &magic, sizeof(magic));
+    memcpy(big_hdr + 4, &fw_size, sizeof(fw_size));
+
+    /* Single TLV at IMAGE_HEADER_OFFSET: HDR_VERSION with a 300-byte
+     * payload, followed by the end-of-options zero word. */
+    big_hdr[8]  = (uint8_t)(HDR_VERSION & 0xFF);
+    big_hdr[9]  = (uint8_t)(HDR_VERSION >> 8);
+    big_hdr[10] = (uint8_t)(BIG_TLV_LEN & 0xFF);
+    big_hdr[11] = (uint8_t)(BIG_TLV_LEN >> 8);
+    for (i = 0; i < BIG_TLV_LEN; i++)
+        big_hdr[12 + i] = (uint8_t)(0xA0 + i);
+    big_hdr[12 + BIG_TLV_LEN] = 0x00;
+    big_hdr[13 + BIG_TLV_LEN] = 0x00;
+}
+
+START_TEST (test_parser_large_header_finds_big_tlv)
+{
+    uint8_t *p;
+    int i;
+
+    build_big_header();
+
+    /* The version field must be located despite the header being at the
+     * uint16 budget wrap boundary. */
+    ck_assert_msg(wolfBoot_find_header(big_hdr + 8, HDR_VERSION, &p) ==
+                  BIG_TLV_LEN,
+        "Parser error: cannot locate version field in a >= 64 KiB header");
+
+    for (i = 0; i < BIG_TLV_LEN; i++)
+        ck_assert_msg(p[i] == (uint8_t)(0xA0 + i),
+            "Parser error: version payload does not match");
+
+    /* A non-existing field must still report not-found. */
+    ck_assert_msg(wolfBoot_find_header(big_hdr + 8, HDR_SHA3_384, &p) == 0,
+        "Parser error: found a non-existing field");
+}
+END_TEST
+
+START_TEST (test_parser_large_header_blobs)
+{
+    uint32_t ver;
+
+    build_big_header();
+
+    /* The blob accessor must read through the large header as well. */
+    ver = wolfBoot_get_blob_version(big_hdr);
+    ck_assert_uint_eq(ver, 0); /* 300-byte version is not a uint32: no field */
+
+    /* Rebuild with a 4-byte version payload: the accessor must return it. */
+    big_hdr[10] = 4;
+    big_hdr[11] = 0;
+    big_hdr[12] = 0x0d;
+    big_hdr[13] = 0x0c;
+    big_hdr[14] = 0x0b;
+    big_hdr[15] = 0x0a;
+    ver = wolfBoot_get_blob_version(big_hdr);
+    ck_assert_uint_eq(ver, 0x0a0b0c0d);
+}
+END_TEST
+
+Suite *wolfboot_suite(void)
+{
+
+    /* Suite initialization */
+    Suite *s = suite_create("wolfBoot");
+
+    /* Test cases */
+    TCase *parser_big = tcase_create("Parser large header");
+
+    /* Test function <-> Test case */
+    tcase_add_test(parser_big, test_parser_large_header_finds_big_tlv);
+    tcase_add_test(parser_big, test_parser_large_header_blobs);
+
+    /* Set parameters + add to suite */
+    tcase_set_timeout(parser_big, 20);
+    suite_add_tcase(s, parser_big);
+
+    return s;
+}
+
+
+int main(void)
+{
+    int fails;
+    Suite *s = wolfboot_suite();
+    SRunner *sr = srunner_create(s);
+    srunner_run_all(sr, CK_NORMAL);
+    fails = srunner_ntests_failed(sr);
+    srunner_free(sr);
+    return fails;
+}