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cbor2 — security advisory

Type: advisorySources: OSVAdvisory ID: UBUNTU-CVE-2026-26209

cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (<100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.

Products
cbor2
Affected versions
5.2.0-4, 5.2.0-4build1, 5.4.2-1, 5.4.2-1build1, 5.4.6-2, 5.4.6-2build1, 5.5.1-1, 5.6.0-1, 5.6.1-1, 5.6.2-1, 5.6.2-1build1, 5.6.5-1build1, 5.7.0-1, 5.7.1-1, 5.8.0-1, 5.8.0-2ubuntu1, 6.1.2-2
Fixed versions
Not specified
CVEs
CVE-2026-26209
Confirmed fixed CVEs
Not specified
Updated in this feed
31 Oct 2026, 00:51 UTC · Revision 1

Source evidence

UBUNTU-CVE-2026-26209

core-osv · Original source

Published 23 Mar 2026, 19:16 UTC

cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (<100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.

osv:UBUNTU-CVE-2026-26209

cbor2

Fixed versions: Not specified

cbor2

Fixed versions: Not specified

cbor2

Fixed versions: Not specified

cbor2

Fixed versions: Not specified

cbor2

Fixed versions: Not specified

Complete source record
{
  "id": "6f075f5b503f31716c9166d1ee4566ae10c31dcb389c18bfa387f09c88a6854e",
  "url": "https://osv.dev/vulnerability/UBUNTU-CVE-2026-26209",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "UBUNTU-CVE-2026-26209",
    "summary": "cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (<100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.",
    "evidence": "osv:UBUNTU-CVE-2026-26209",
    "products": [
      {
        "name": {
          "value": "cbor2",
          "evidence": "/affected/0/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [],
        "affected_versions": [
          {
            "value": "5.2.0-4",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "5.2.0-4build1",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "5.4.2-1",
            "evidence": "/affected/0/versions"
          },
          {
            "value": "5.4.2-1build1",
            "evidence": "/affected/0/versions"
          }
        ]
      },
      {
        "name": {
          "value": "cbor2",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [],
        "affected_versions": [
          {
            "value": "5.4.6-2",
            "evidence": "/affected/1/versions"
          },
          {
            "value": "5.4.6-2build1",
            "evidence": "/affected/1/versions"
          },
          {
            "value": "5.5.1-1",
            "evidence": "/affected/1/versions"
          },
          {
            "value": "5.6.0-1",
            "evidence": "/affected/1/versions"
          },
          {
            "value": "5.6.1-1",
            "evidence": "/affected/1/versions"
          },
          {
            "value": "5.6.2-1",
            "evidence": "/affected/1/versions"
          },
          {
            "value": "5.6.2-1build1",
            "evidence": "/affected/1/versions"
          }
        ]
      },
      {
        "name": {
          "value": "cbor2",
          "evidence": "/affected/2/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [],
        "affected_versions": [
          {
            "value": "5.6.5-1build1",
            "evidence": "/affected/2/versions"
          }
        ]
      },
      {
        "name": {
          "value": "cbor2",
          "evidence": "/affected/3/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [],
        "affected_versions": [
          {
            "value": "5.8.0-2ubuntu1",
            "evidence": "/affected/3/versions"
          },
          {
            "value": "6.1.2-2",
            "evidence": "/affected/3/versions"
          }
        ]
      },
      {
        "name": {
          "value": "cbor2",
          "evidence": "/affected/4/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [],
        "affected_versions": [
          {
            "value": "5.6.5-1build1",
            "evidence": "/affected/4/versions"
          },
          {
            "value": "5.7.0-1",
            "evidence": "/affected/4/versions"
          },
          {
            "value": "5.7.1-1",
            "evidence": "/affected/4/versions"
          },
          {
            "value": "5.8.0-1",
            "evidence": "/affected/4/versions"
          },
          {
            "value": "5.8.0-2ubuntu1",
            "evidence": "/affected/4/versions"
          }
        ]
      }
    ],
    "severity": "high",
    "structured": {
      "id": "UBUNTU-CVE-2026-26209",
      "aliases": [
        "UBUNTU-CVE-2026-26209"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        },
        {
          "type": "Ubuntu",
          "score": "medium"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "0"
                }
              ]
            }
          ],
          "package": {
            "name": "cbor2",
            "purl": "pkg:deb/ubuntu/cbor2?arch=source&distro=jammy",
            "ecosystem": "Ubuntu:22.04:LTS"
          },
          "versions": [
            "5.2.0-4",
            "5.2.0-4build1",
            "5.4.2-1",
            "5.4.2-1build1"
          ],
          "database_specific": {
            "source": "https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-26209.json"
          },
          "ecosystem_specific": {
            "binaries": [
              {
                "binary_name": "python3-cbor2",
                "binary_version": "5.4.2-1build1"
              }
            ]
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "0"
                }
              ]
            }
          ],
          "package": {
            "name": "cbor2",
            "purl": "pkg:deb/ubuntu/cbor2?arch=source&distro=noble",
            "ecosystem": "Ubuntu:24.04:LTS"
          },
          "versions": [
            "5.4.6-2",
            "5.4.6-2build1",
            "5.5.1-1",
            "5.6.0-1",
            "5.6.1-1",
            "5.6.2-1",
            "5.6.2-1build1"
          ],
          "database_specific": {
            "source": "https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-26209.json"
          },
          "ecosystem_specific": {
            "binaries": [
              {
                "binary_name": "python3-cbor2",
                "binary_version": "5.6.2-1build1"
              }
            ]
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "0"
                }
              ]
            }
          ],
          "package": {
            "name": "cbor2",
            "purl": "pkg:deb/ubuntu/cbor2?arch=source&distro=questing",
            "ecosystem": "Ubuntu:25.10"
          },
          "versions": [
            "5.6.5-1build1"
          ],
          "database_specific": {
            "source": "https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-26209.json"
          },
          "ecosystem_specific": {
            "binaries": [
              {
                "binary_name": "python3-cbor2",
                "binary_version": "5.6.5-1build1"
              }
            ]
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "0"
                }
              ]
            }
          ],
          "package": {
            "name": "cbor2",
            "purl": "pkg:deb/ubuntu/cbor2?arch=source&distro=stonking",
            "ecosystem": "Ubuntu:26.10"
          },
          "versions": [
            "5.8.0-2ubuntu1",
            "6.1.2-2"
          ],
          "database_specific": {
            "source": "https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-26209.json"
          },
          "ecosystem_specific": {
            "binaries": [
              {
                "binary_name": "python3-cbor2",
                "binary_version": "6.1.2-2"
              }
            ]
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "0"
                }
              ]
            }
          ],
          "package": {
            "name": "cbor2",
            "purl": "pkg:deb/ubuntu/cbor2?arch=source&distro=resolute",
            "ecosystem": "Ubuntu:26.04:LTS"
          },
          "versions": [
            "5.6.5-1build1",
            "5.7.0-1",
            "5.7.1-1",
            "5.8.0-1",
            "5.8.0-2ubuntu1"
          ],
          "database_specific": {
            "source": "https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-26209.json"
          },
          "ecosystem_specific": {
            "binaries": [
              {
                "binary_name": "python3-cbor2",
                "binary_version": "5.8.0-2ubuntu1"
              }
            ]
          }
        }
      ],
      "modified": "2026-10-17T12:56:13.077699258Z",
      "provider": "osv",
      "upstream": [
        "CVE-2026-26209"
      ],
      "references": [
        "https://ubuntu.com/security/CVE-2026-26209",
        "https://www.cve.org/CVERecord?id=CVE-2026-26209",
        "https://github.com/agronholm/cbor2/commit/e61a5f365ba610d5907a0ae1bc72769bba34294b",
        "https://github.com/agronholm/cbor2/pull/275",
        "https://github.com/agronholm/cbor2/releases/tag/5.9.0",
        "https://github.com/agronholm/cbor2/security/advisories/GHSA-3c37-wwvx-h642"
      ]
    },
    "related_cves": [
      {
        "value": "CVE-2026-26209",
        "evidence": "/upstream"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 354960,
  "published_at": "2026-03-23T19:16:00+00:00"
}