diff --git a/core/embed/vendorheader/vendor_satoshilabs.json b/core/embed/vendorheader/vendor_satoshilabs.json
index 7c4c6f4567..0e0d2c7002 100644
--- a/core/embed/vendorheader/vendor_satoshilabs.json
+++ b/core/embed/vendorheader/vendor_satoshilabs.json
@@ -1,4 +1,5 @@
 {
+    "header_len": 4608,
     "text": "SatoshiLabs",
     "expiry": 0,
     "version": {
diff --git a/core/embed/vendorheader/vendor_unsafe.json b/core/embed/vendorheader/vendor_unsafe.json
index e7af4710e5..9e2d1620af 100644
--- a/core/embed/vendorheader/vendor_unsafe.json
+++ b/core/embed/vendorheader/vendor_unsafe.json
@@ -1,4 +1,5 @@
 {
+    "header_len": 2560,
     "text": "UNSAFE, DO NOT USE!",
     "expiry": 0,
     "version": {
diff --git a/core/tools/build_vendorheader b/core/tools/build_vendorheader
index be085c121e..e9264bf1dd 100755
--- a/core/tools/build_vendorheader
+++ b/core/tools/build_vendorheader
@@ -6,6 +6,15 @@ import click
 from trezorlib import firmware
 
 
+def minimum_header_len(spec):
+    spec = spec.copy()
+    spec["header_len"] = 512000
+    reparsed = firmware.VendorHeader.parse(firmware.VendorHeader.build(spec))
+    data_length = reparsed._end_offset - reparsed._start_offset
+    # data length + 65 for signatures, rounded up to nearest multiple of 512
+    return (data_length + 65 + 511) // 512 * 512
+
+
 @click.command()
 @click.argument("specfile", type=click.File("r"))
 @click.argument("image", type=click.File("rb"))
@@ -16,6 +25,19 @@ def build_vendorheader(specfile, image, outfile):
     spec["image"] = firmware.Toif.parse(image.read())
     spec["sigmask"] = 0
     spec["signature"] = b"\x00" * 64
+
+    min_length = minimum_header_len(spec)
+    if "header_len" not in spec:
+        spec["header_len"] = min_length
+    elif spec["header_len"] < min_length:
+        raise click.ClickException(
+            f"Specified header_len {spec['header_len']} too low. "
+            f"Minimum allowable value is {min_length}."
+        )
+
+    if spec["header_len"] % 512 != 0:
+        raise click.ClickException("Invalid header_len: must be a multiple of 512")
+
     outfile.write(firmware.VendorHeader.build(spec))
 
 
diff --git a/python/src/trezorlib/firmware.py b/python/src/trezorlib/firmware.py
index c6947fef75..21e67eab75 100644
--- a/python/src/trezorlib/firmware.py
+++ b/python/src/trezorlib/firmware.py
@@ -146,7 +146,7 @@ VendorTrust = c.Transformed(c.BitStruct(
 VendorHeader = c.Struct(
     "_start_offset" / c.Tell,
     "magic" / c.Const(b"TRZV"),
-    "_header_len" / c.Padding(4),
+    "header_len" / c.Int32ul,
     "expiry" / c.Int32ul,
     "version" / c.Struct(
         "major" / c.Int8ul,
@@ -159,17 +159,14 @@ VendorHeader = c.Struct(
     "pubkeys" / c.Bytes(32)[c.this.sig_n],
     "text" / c.Aligned(4, c.PascalString(c.Int8ul, "utf-8")),
     "image" / Toif,
-    "_data_end_offset" / c.Tell,
+    "_end_offset" / c.Tell,
 
-    c.Padding(-(c.this._data_end_offset + 65) % 512),
+    "_min_header_len" / c.Check(c.this.header_len > (c.this._end_offset - c.this._start_offset) + 65),
+    "_header_len_aligned" / c.Check(c.this.header_len % 512 == 0),
+
+    c.Padding(c.this.header_len - c.this._end_offset + c.this._start_offset - 65),
     "sigmask" / c.Byte,
     "signature" / c.Bytes(64),
-
-    "_end_offset" / c.Tell,
-    "header_len" / c.Pointer(
-        c.this._start_offset + 4,
-        c.Rebuild(c.Int32ul, c.this._end_offset - c.this._start_offset)
-    ),
 )