Pillow: Out-of-bounds access in _imagingmorph on a zero-width image

Summary

A PPM or PGM file can declare a width of 0, which Pillow accepts, producing an image of size (0, N). Passing that image to ImageMorph.MorphOp.apply() makes the native morphology code index a row buffer at outrow[width - 1], which is outrow[-1] when the width is 0. The process crashes.

Details

The PPM parser reads width and height with no lower-bound check:

• src/PIL/PpmImagePlugin.py:119

The native apply() writes the row edges with no guard that the width is positive:

• src/_imagingmorph.c:81 sets outrow[0] = outrow[width - 1] = 0;

match() and get_on_pixels() use the same edge-indexing pattern.

PoC

from PIL import Image, ImageMorph

# A width-0 image (also producible from a crafted PPM header). apply() indexes
# outrow[width - 1] == outrow[-1] in native code.
im = Image.new("L", (0, 5))
ImageMorph.MorphOp(op_name="dilation4").apply(im)

Run it:

$ python 03_poc.py
Segmentation fault (core dumped)

Observed on a source build of commit 3e2ab96.

Under AddressSanitizer:

==ERROR: AddressSanitizer: SEGV on unknown address (pc ... in apply)
    #0 apply   src/_imagingmorph.c:81
SUMMARY: AddressSanitizer: SEGV src/_imagingmorph.c:81 in apply

With width 0 the outrow[width - 1] write lands on an unmapped page, so it surfaces as a SEGV pinned to the edge-write line rather than a redzone overflow.

Impact

Out-of-bounds write in a C extension. Affects any application that runs a morphology operation on an image opened from untrusted input. Result is a crash and possible memory corruption.