GHSA-9v9v-9qvf-h2hj

Description

### Summary On Windows builds, Pillow stores EMF and WMF image dimensions without a range check and passes them to the native `drawwmf` handler. That code sizes its allocation from dimensions truncated into 16-bit header fields but copies the result using the full 32-bit dimensions. A crafted metafile that is wide and short passes the decompression-bomb pixel check yet causes a heap over-read. ### Details The EMF branch computes the size straight from signed 32-bit header fields and stores it with no positivity or range check: - `src/PIL/WmfImagePlugin.py:121-129` reads the bounding box; `:148` stores it; the size reaches the native handler through `src/PIL/WmfImagePlugin.py:56`. In the native handler the DIB header fields `bcWidth` and `bcHeight` are 16-bit and truncate the allocation, while the copy length uses the untruncated ints: - `src/display.c:793-794` assign the 16-bit fields - `src/display.c:800` sizes the allocation from them - `src/display.c:827` copies `height * ((width * 3 + 3) & -4)` bytes using the full values The whole handler is compiled only on Windows (`#ifdef _WIN32`, `src/display.c:33-841`). ### PoC ```python import struct from PIL import Image def L(v): return struct.pack("<i", v) # Minimal EMF header. Bounding box is 70000 x 1, so the pixel count (70000) passes the # decompression-bomb check, but the width truncates to 16 bits for the allocation while # the copy uses 70000. emf = b"\x01\x00\x00\x00" + L(88) # record type + size emf += L(0) + L(0) + L(70000) + L(1) # rclBounds: x0, y0, x1, y1 emf += L(0) + L(0) + L(100) + L(100) # rclFrame (non-zero, used for the dpi calc) emf += b" EMF" # signature at offset 40 emf += bytes(44) open("poc.emf", "wb").write(emf) im = Image.open("poc.emf") print("size:", im.size, "pixels:", im.size[0] * im.size[1]) im.load() # over-read happens here on a Windows build ``` On Linux the first part is observable: the 88-byte file opens, `size` is `(70000, 1)`, and the bomb check passes. The native over-read only exists on Windows because `Image.core.drawwmf` is compiled out elsewhere (on Linux `im.load()` raises `OSError` instead). ### Impact CWE-125, out-of-bounds read in a C extension on Windows builds. A crafted EMF or WMF that passes the pixel check can read past a heap buffer, leaking memory or crashing the process.