From 735d02584b4db8a1b923d546b7be94de28f0876a Mon Sep 17 00:00:00 2001
From: Andrew Murray <radarhere@users.noreply.github.com>
Date: Thu, 19 Mar 2026 10:38:28 +1100
Subject: [PATCH] Allow for different palette entry sizes when correcting
 offset

---
 Tests/test_file_bmp.py    | 16 ++++++++++++----
 src/PIL/BmpImagePlugin.py |  4 ++--
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/Tests/test_file_bmp.py b/Tests/test_file_bmp.py
index b61497812..c8ac46524 100644
--- a/Tests/test_file_bmp.py
+++ b/Tests/test_file_bmp.py
@@ -238,13 +238,21 @@ def test_unsupported_bmp_bitfields_layout() -> None:
         Image.open(fp)
 
 
-def test_offset() -> None:
+@pytest.mark.parametrize(
+    "offset, path",
+    (
+        (26, "pal8os2.bmp"),
+        (54, "pal8.bmp"),
+    ),
+)
+def test_offset(offset: int, path: str) -> None:
+    image_path = "Tests/images/bmp/g/" + path
     # Exclude the palette size from the pixel data offset
-    with open("Tests/images/bmp/g/pal8.bmp", "rb") as fp:
+    with open(image_path, "rb") as fp:
         data = fp.read()
-        data = data[:10] + o32(54) + data[14:]
+        data = data[:10] + o32(offset) + data[14:]
         with Image.open(io.BytesIO(data)) as im:
-            assert_image_equal_tofile(im, "Tests/images/bmp/g/pal8.bmp")
+            assert_image_equal_tofile(im, image_path)
 
 
 def test_use_raw_alpha(monkeypatch: pytest.MonkeyPatch) -> None:
diff --git a/src/PIL/BmpImagePlugin.py b/src/PIL/BmpImagePlugin.py
index 00975624a..a6724cab4 100644
--- a/src/PIL/BmpImagePlugin.py
+++ b/src/PIL/BmpImagePlugin.py
@@ -181,9 +181,10 @@ class BmpImageFile(ImageFile.ImageFile):
         assert isinstance(file_info["bits"], int)
         if not file_info.get("colors", 0):
             file_info["colors"] = 1 << file_info["bits"]
+        assert isinstance(file_info["palette_padding"], int)
         assert isinstance(file_info["colors"], int)
         if offset == 14 + file_info["header_size"] and file_info["bits"] <= 8:
-            offset += 4 * file_info["colors"]
+            offset += file_info["palette_padding"] * file_info["colors"]
 
         # ---------------------- Check bit depth for unusual unsupported values
         self._mode, raw_mode = BIT2MODE.get(file_info["bits"], ("", ""))
@@ -262,7 +263,6 @@ class BmpImageFile(ImageFile.ImageFile):
                 msg = f"Unsupported BMP Palette size ({file_info['colors']})"
                 raise OSError(msg)
             else:
-                assert isinstance(file_info["palette_padding"], int)
                 padding = file_info["palette_padding"]
                 palette = read(padding * file_info["colors"])
                 grayscale = True