From 2bd84c019c300b78e811743fbcddb67c9d9bf821 Mon Sep 17 00:00:00 2001
From: Cosmin Truta <ctruta@gmail.com>
Date: Fri, 7 Nov 2025 22:40:05 +0200
Subject: [PATCH] Fix a heap buffer overflow in `png_write_image_8bit`

The condition guarding the pre-transform path incorrectly allowed 8-bit
input data to enter `png_write_image_8bit` which expects 16-bit input.
This caused out-of-bounds reads when processing 8-bit grayscale+alpha
images (GitHub #688), or 8-bit RGB or RGB+alpha images (GitHub #746),
with the `convert_to_8bit` flag set (an invalid combination that should
bypass the pre-transform path).

The second part of the condition, i.e.

    colormap == 0 && convert_to_8bit != 0

failed to verify that input was 16-bit, i.e.

    linear != 0

contradicting the comment "This only applies when the input is 16-bit".

The fix consists in restructuring the condition to ensure both the
`alpha` path and the `convert_to_8bit` path require linear (16-bit)
input. The corrected condition, i.e.

    linear != 0 && (alpha != 0 || display->convert_to_8bit != 0)

matches the expectation of the `png_write_image_8bit` function and
prevents treating 8-bit buffers as 16-bit data.

Reported-by: Samsung-PENTEST <Samsung-PENTEST@users.noreply.github.com>
Reported-by: weijinjinnihao <weijinjinnihao@users.noreply.github.com>
Analyzed-by: degrigis <degrigis@users.noreply.github.com>
Reviewed-by: John Bowler <jbowler@acm.org>

CVE: CVE-2025-64506
Upstream-Status: Backport [https://github.com/pnggroup/libpng/commit/2bd84c019c300b78e811743fbcddb67c9d9bf821]
Signed-off-by: Peter Marko <peter.marko@siemens.com>
---
 pngwrite.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/pngwrite.c b/pngwrite.c
index 35a5d17b6..83148960e 100644
--- a/pngwrite.c
+++ b/pngwrite.c
@@ -2142,8 +2142,7 @@ png_image_write_main(png_voidp argument)
     * before it is written.  This only applies when the input is 16-bit and
     * either there is an alpha channel or it is converted to 8-bit.
     */
-   if ((linear != 0 && alpha != 0 ) ||
-       (colormap == 0 && display->convert_to_8bit != 0))
+   if (linear != 0 && (alpha != 0 || display->convert_to_8bit != 0))
    {
       png_bytep row = png_voidcast(png_bytep, png_malloc(png_ptr,
           png_get_rowbytes(png_ptr, info_ptr)));
