CVE-2026-33936: python-ecdsa: Denial of Service via improper DER length validation in crafted private keys

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.