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GHSA-3C37-WWVX-H642

Vulnerability from github – Published: 2026-03-23 20:23 – Updated: 2026-03-25 20:38
VLAI
Summary
cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads
Details

Summary

  • The cbor2 library is 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 correctly uses Python's C-API for recursion protection (Py_EnterRecursiveCall), but this mechanism is designed to prevent a stack overflow by raising a RecursionError. In some environments, this exception is not caught, thus causing the service process to terminate.
  • While the library handles moderate nesting, it lacks a configurable, data-driven depth limit independent of Python's global recursion setting. An attacker can supply a crafted CBOR payload containing thousands of nested arrays (e.g., 0x81). When cbor2.loads() attempts to parse this, it hits the interpreter's recursion limit, causing the call to raise a RecursionError.
  • By sending a stream of small (<100KB) malicious packets, an attacker can repeatedly crash worker processes faster than they can be restarted, resulting in a complete and sustained Denial of Service.

Details

  • The vulnerability stems from the recursive design of the CBORDecoder class, specifically how it decodes nested container types like Arrays and Maps.
  • Inside decode_array (and similarly decode_map), the decoder iterates through the number of elements specified in the CBOR header. For each element, it calls self.decode() again to parse the nested item. This recursive call lacks a depth-tracking mechanism.
  • Vulnerable Code Locations:
  • cbor2/decoder.py (Pure Python implementation)
  • source/decoder.c (C extension implementation)
  • Execution Flow:
  • The cbor2.loads() function initializes a CBORDecoder and calls its decode() method.
  • The decode() method reads the initial byte and dispatches control to a specific handler based on the major type. For an Array (Major Type 4), it calls decode_array.
  • decode_array loops and calls self.decode() for each item, leading to deep recursion when parsing a payload like [...[...[1]...]...].

PoC

import cbor2

DEPTH = 1000

payload = b'\x81' * DEPTH + b'\x01'
print(f"[*] Payload size: {len(payload) / 1024:.2f} KB")
print("[*] Triggering decoder...")

try:
    cbor2.loads(payload)
    print("[+] Parsed successfully (Not Vulnerable)")
except RecursionError:
    print("\n[!] VULNERABLE: RecursionError triggered!")
except Exception as e:
    print(f"\n[-] Unexpected Error: {type(e).__name__}: {e}")

Impact

  • Scope: This vulnerability affects any application using cbor2 to parse untrusted data. Common use cases include IoT data processing, WebAuthn (FIDO2) authentication flows, and inter-service communication over COSE (CBOR Object Signing and Encryption).
  • Attack Vector: A remote, unauthenticated attacker can achieve a full Denial of Service with a highly efficient, low-bandwidth attack. A payload under 100KB is sufficient to reliably terminate a Python worker process.

Credit

This issue was discovered by Kevin Tu of TMIR at ByteDance. The patch was developed by @agronholm.

Severity
7.5 (High)
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 5.8.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "cbor2"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "5.9.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-26209"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-674"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-23T20:23:57Z",
    "nvd_published_at": "2026-03-23T19:16:39Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\n- The `cbor2` library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures.\n- This vulnerability affects both the pure Python implementation and the C extension (`_cbor2`). The C extension correctly uses Python\u0027s C-API for recursion protection (`Py_EnterRecursiveCall`), but this mechanism is designed to prevent a stack overflow by raising a `RecursionError`. In some environments, this exception is not caught, thus causing the service process to terminate.\n- While the library handles moderate nesting, it lacks a configurable, data-driven depth limit independent of Python\u0027s global recursion setting. An attacker can supply a crafted CBOR payload containing thousands of nested arrays (e.g., `0x81`). When `cbor2.loads()` attempts to parse this, it hits the interpreter\u0027s recursion limit, causing the call to raise a `RecursionError`.\n- By sending a stream of small (\u003c100KB) malicious packets, an attacker can repeatedly crash worker processes faster than they can be restarted, resulting in a complete and sustained Denial of Service.\n\n### Details\n\n- The vulnerability stems from the recursive design of the `CBORDecoder` class, specifically how it decodes nested container types like Arrays and Maps.\n- Inside `decode_array` (and similarly `decode_map`), the decoder iterates through the number of elements specified in the CBOR header. For each element, it calls `self.decode()` again to parse the nested item. This recursive call lacks a depth-tracking mechanism.\n- Vulnerable Code Locations:\n  - `cbor2/decoder.py` (Pure Python implementation)\n  - `source/decoder.c` (C extension implementation)\n- Execution Flow:\n  1. The `cbor2.loads()` function initializes a `CBORDecoder` and calls its `decode()` method.\n  2. The `decode()` method reads the initial byte and dispatches control to a specific handler based on the major type. For an Array (Major Type 4), it calls `decode_array`.\n  3. `decode_array` loops and calls `self.decode()` for each item, leading to deep recursion when parsing a payload like `[...[...[1]...]...]`.\n\n### PoC\n\n```\nimport cbor2\n\nDEPTH = 1000\n\npayload = b\u0027\\x81\u0027 * DEPTH + b\u0027\\x01\u0027\nprint(f\"[*] Payload size: {len(payload) / 1024:.2f} KB\")\nprint(\"[*] Triggering decoder...\")\n\ntry:\n    cbor2.loads(payload)\n    print(\"[+] Parsed successfully (Not Vulnerable)\")\nexcept RecursionError:\n    print(\"\\n[!] VULNERABLE: RecursionError triggered!\")\nexcept Exception as e:\n    print(f\"\\n[-] Unexpected Error: {type(e).__name__}: {e}\")\n```\n\n### Impact\n\n- Scope: This vulnerability affects any application using `cbor2` to parse untrusted data. Common use cases include IoT data processing, WebAuthn (FIDO2) authentication flows, and inter-service communication over COSE (CBOR Object Signing and Encryption).\n- Attack Vector: A remote, unauthenticated attacker can achieve a full Denial of Service with a highly efficient, low-bandwidth attack. A payload under 100KB is sufficient to reliably terminate a Python worker process.\n\n### Credit\n\nThis issue was discovered by Kevin Tu of TMIR at ByteDance. The patch was developed by @agronholm.",
  "id": "GHSA-3c37-wwvx-h642",
  "modified": "2026-03-25T20:38:41Z",
  "published": "2026-03-23T20:23:57Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/agronholm/cbor2/security/advisories/GHSA-3c37-wwvx-h642"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-26209"
    },
    {
      "type": "WEB",
      "url": "https://github.com/agronholm/cbor2/pull/275"
    },
    {
      "type": "WEB",
      "url": "https://github.com/agronholm/cbor2/commit/e61a5f365ba610d5907a0ae1bc72769bba34294b"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/agronholm/cbor2"
    },
    {
      "type": "WEB",
      "url": "https://github.com/agronholm/cbor2/releases/tag/5.9.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads"
}



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