Affected: python-pillow Pillow

Pillow is a Python imaging library. From 5.1.0 until 12.3.0, PdfParser.PdfStream.decode() in PIL/PdfParser.py calls zlib.decompress() with bufsize set to the PDF stream Length field without bounding the decompressed output size, allowing a crafted FlateDecode PDF stream to exhaust memory from a small file. This issue is fixed in version 12.3.0.

Affected: PyPI: Pillow

### 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()`:

```python # 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.…