From c2b67a4fd91d56a12847dee1079b023cc7a76e82 Mon Sep 17 00:00:00 2001
From: tongwentao <twtzs@foxmail.com>
Date: Tue, 20 Oct 2026 15:29:57 +0800
Subject: [PATCH 1/2] Copy read-only image memory before frombytes and ImageCms
 writes

---
 Tests/test_image_frombytes.py | 28 ++++++++++++++++++++++++++++
 Tests/test_imagecms.py        | 22 ++++++++++++++++++++++
 docs/releasenotes/13.1.0.rst  |  8 ++++++++
 src/PIL/Image.py              |  4 ++++
 src/PIL/ImageCms.py           |  2 ++
 5 files changed, 64 insertions(+)

diff --git a/Tests/test_image_frombytes.py b/Tests/test_image_frombytes.py
index 06b4c5b3118..f5edec2dfd0 100644
--- a/Tests/test_image_frombytes.py
+++ b/Tests/test_image_frombytes.py
@@ -17,3 +17,31 @@ def test_sanity(data_type: str) -> None:
     im2 = Image.frombytes(im1.mode, im1.size, data)
 
     assert_image_equal(im1, im2)
+
+
+@pytest.mark.parametrize("mode", ("L", "RGBA"))
+def test_readonly_buffer(mode: str) -> None:
+    size = (2, 2)
+    length = 4 * Image.getmodebands(mode)
+    data = bytes(bytearray(length))
+    im = Image.frombuffer(mode, size, data, "raw", mode, 0, 1)
+    assert im.readonly
+
+    replacement = b"\x01" * length
+    im.frombytes(replacement)
+
+    assert im.tobytes() == replacement
+    assert data == bytes(length)
+    assert not im.readonly
+
+
+def test_mutable_core_with_readonly_image() -> None:
+    # Plugins may call frombytes() from load() before clearing the image's flag.
+    im = Image.new("L", (2, 2))
+    im.readonly = 1
+    core = im.im
+
+    im.frombytes(b"\x01" * 4)
+
+    assert im.im is core
+    assert im.tobytes() == b"\x01" * 4
diff --git a/Tests/test_imagecms.py b/Tests/test_imagecms.py
index b287004e010..4b498128aef 100644
--- a/Tests/test_imagecms.py
+++ b/Tests/test_imagecms.py
@@ -101,6 +101,28 @@ def test_sanity() -> None:
     hopper().point(t)
 
 
+@pytest.mark.parametrize("in_place", (False, True))
+def test_readonly_output(in_place: bool) -> None:
+    original = bytes([10, 20, 30, 255] * 4)
+    data = bytes(bytearray(original))
+    im = Image.frombuffer("RGBA", (2, 2), data, "raw", "RGBA", 0, 1)
+    assert im.readonly
+    profile = ImageCms.createProfile("sRGB")
+    transform = ImageCms.buildTransform(profile, profile, "RGBA", "RGBA")
+
+    if in_place:
+        expected = im.copy()
+        result = transform.apply_in_place(im)
+    else:
+        expected = Image.new("RGBA", im.size, (90, 80, 70, 255))
+        result = transform.apply(expected, im)
+
+    assert result is im
+    assert_image_equal(im, expected)
+    assert data == original
+    assert not im.readonly
+
+
 def test_flags() -> None:
     assert ImageCms.Flags.NONE.value == 0
     assert ImageCms.Flags.GRIDPOINTS(0) == ImageCms.Flags.NONE
diff --git a/docs/releasenotes/13.1.0.rst b/docs/releasenotes/13.1.0.rst
index 140bcbe3115..bf0d02ac006 100644
--- a/docs/releasenotes/13.1.0.rst
+++ b/docs/releasenotes/13.1.0.rst
@@ -49,6 +49,14 @@ TODO
 Other changes
 =============
 
+Copy read-only images before writing pixel data
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+:py:meth:`~PIL.Image.Image.frombytes` and :py:meth:`~PIL.ImageCms.ImageCmsTransform.apply`
+now copy read-only image memory before writing to it. This also applies to in-place
+ImageCms transforms, preventing writes into shared buffers or read-only memory-mapped
+files.
+
 Blurring I;16 images
 ^^^^^^^^^^^^^^^^^^^^
 
diff --git a/src/PIL/Image.py b/src/PIL/Image.py
index cefe6f1fc96..2ef9a17fec9 100644
--- a/src/PIL/Image.py
+++ b/src/PIL/Image.py
@@ -946,6 +946,10 @@ def frombytes(
         if self.width == 0 or self.height == 0:
             return
 
+        # Plugins may call frombytes() during load(), before clearing self.readonly.
+        if self.im.readonly:
+            self._copy()
+
         decoder_args: Any = args
         if len(decoder_args) == 1 and isinstance(decoder_args[0], tuple):
             # may pass tuple instead of argument list
diff --git a/src/PIL/ImageCms.py b/src/PIL/ImageCms.py
index 3dc30a7c96f..712ac9aea17 100644
--- a/src/PIL/ImageCms.py
+++ b/src/PIL/ImageCms.py
@@ -317,6 +317,8 @@ def point(self, im: Image.Image) -> Image.Image:
     def apply(self, im: Image.Image, imOut: Image.Image | None = None) -> Image.Image:
         if imOut is None:
             imOut = Image.new(self.output_mode, im.size)
+        else:
+            imOut._ensure_mutable()
         self.transform.apply(im.getim(), imOut.getim())
         imOut.info["icc_profile"] = self.output_profile.tobytes()
         return imOut

From 05336667d333279af4266ef59eae20d1dde1a162 Mon Sep 17 00:00:00 2001
From: tongwentao <twtzs@foxmail.com>
Date: Tue, 20 Oct 2026 16:12:06 +0800
Subject: [PATCH 2/2] Fix release note heading underline

---
 docs/releasenotes/13.1.0.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/releasenotes/13.1.0.rst b/docs/releasenotes/13.1.0.rst
index bf0d02ac006..cf2ca4bf4d7 100644
--- a/docs/releasenotes/13.1.0.rst
+++ b/docs/releasenotes/13.1.0.rst
@@ -50,7 +50,7 @@ Other changes
 =============
 
 Copy read-only images before writing pixel data
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 :py:meth:`~PIL.Image.Image.frombytes` and :py:meth:`~PIL.ImageCms.ImageCmsTransform.apply`
 now copy read-only image memory before writing to it. This also applies to in-place