diff --git a/Tests/LibGfx/TestImageDecoder.cpp b/Tests/LibGfx/TestImageDecoder.cpp
index 00a3a58b327..dec10f51416 100644
--- a/Tests/LibGfx/TestImageDecoder.cpp
+++ b/Tests/LibGfx/TestImageDecoder.cpp
@@ -265,6 +265,17 @@ TEST_CASE(test_small_24bit)
     EXPECT_EQ(frame.image->get_pixel(0, 4), Gfx::Color(1, 0, 1, 255));
 }
 
+TEST_CASE(test_stencil_mask)
+{
+    auto file = TRY_OR_FAIL(Core::MappedFile::map(TEST_INPUT("ilbm/test-stencil.iff"sv)));
+    EXPECT(Gfx::ILBMImageDecoderPlugin::sniff(file->bytes()));
+    auto plugin_decoder = TRY_OR_FAIL(Gfx::ILBMImageDecoderPlugin::create(file->bytes()));
+
+    auto frame = TRY_OR_FAIL(expect_single_frame_of_size(*plugin_decoder, { 320, 200 }));
+
+    EXPECT_EQ(frame.image->get_pixel(0, 4), Gfx::Color(0, 0, 0, 255));
+}
+
 TEST_CASE(test_ilbm_malformed_header)
 {
     Array test_inputs = {
diff --git a/Tests/LibGfx/test-inputs/ilbm/test-stencil.iff b/Tests/LibGfx/test-inputs/ilbm/test-stencil.iff
new file mode 100644
index 00000000000..233ceac178c
Binary files /dev/null and b/Tests/LibGfx/test-inputs/ilbm/test-stencil.iff differ
diff --git a/Userland/Libraries/LibGfx/ImageFormats/ILBMLoader.cpp b/Userland/Libraries/LibGfx/ImageFormats/ILBMLoader.cpp
index 3300ae649d2..7dfed023fec 100644
--- a/Userland/Libraries/LibGfx/ImageFormats/ILBMLoader.cpp
+++ b/Userland/Libraries/LibGfx/ImageFormats/ILBMLoader.cpp
@@ -182,7 +182,8 @@ static ErrorOr<ByteBuffer> planar_to_chunky(ReadonlyBytes bitplanes, ILBMLoading
     u16 pitch = context.pitch;
     u16 width = context.bm_header.width;
     u16 height = context.bm_header.height;
-    u8 planes = context.bm_header.planes;
+    // mask is added as an extra plane
+    u8 planes = context.bm_header.mask == MaskType::HasMask ? context.bm_header.planes + 1 : context.bm_header.planes;
     size_t buffer_size = static_cast<size_t>(width) * height;
     // If planes number is 24 we'll store R,G,B components so buffer needs to be 3 times width*height
     // otherwise we'll store a single 8bit index to the CMAP.
@@ -209,17 +210,17 @@ static ErrorOr<ByteBuffer> planar_to_chunky(ReadonlyBytes bitplanes, ILBMLoading
                 // when enough data for current bitplane row has been read
                 for (u8 b = 0; b < 8 && (i * 8) + b < width; b++) {
                     u8 mask = 1 << (7 - b);
-                    // get current plane
-                    if (bit & mask) {
+                    // get current plane: simply skip mask plane for now
+                    if (bit & mask && p < context.bm_header.planes) {
                         u16 x = (i * 8) + b;
                         size_t offset = (scanline * pixel_size) + (x * pixel_size) + rgb_shift;
                         // Only throw an error if we would actually attempt to write
                         // outside of the chunky buffer. Some apps like PPaint produce
                         // malformed bitplane data but files are still accepted by most readers
                         // since they do not cause writing past the chunky buffer.
-                        if (offset >= chunky.size()) {
+                        if (offset >= chunky.size())
                             return Error::from_string_literal("Malformed bitplane data");
-                        }
+
                         chunky[offset] |= plane_mask;
                     }
                 }
@@ -239,6 +240,10 @@ static ErrorOr<ByteBuffer> uncompress_byte_run(ReadonlyBytes data, ILBMLoadingCo
 
     size_t plane_data_size = context.pitch * context.bm_header.height * context.bm_header.planes;
 
+    // The mask is encoded as an extra bitplane but is not counted in the bm_header planes
+    if (context.bm_header.mask == MaskType::HasMask)
+        plane_data_size += context.pitch * context.bm_header.height;
+
     // The maximum run length of this compression method is 127 bytes, so the uncompressed size
     // cannot be more than 127 times the size of the chunk we are decompressing.
     if (plane_data_size > NumericLimits<u32>::max() || ceil_div(plane_data_size, 127ul) > length)