GHSA-6r8x-57c9-28j4

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...