Patched code sample

import binascii

# Helper class and functions from the `ecdsa.der` module are included here
# to create a self-contained, runnable example that demonstrates the fix.
class UnexpectedDER(Exception):
    """Exception raised when processing a malformed DER string."""
    pass

def read_length(string):
    """
    Read a DER length sequence.
    This is a simplified helper based on the one in python-ecdsa's der.py.
    """
    if not string:
        raise UnexpectedDER("Empty string when trying to read length")
    if string[0] <= 127:
        return (string[0], 1)
    if string[0] == 128:
        raise UnexpectedDER("Indefinite length encoding not supported")
    number_of_octets = string[0] & 127
    if number_of_octets == 0:
        raise UnexpectedDER("Invalid length encoding")
    if len(string) < 1 + number_of_octets:
        raise UnexpectedDER("Not enough bytes to read length")
    length = int(binascii.hexlify(string[1 : 1 + number_of_octets]), 16)
    return (length, 1 + number_of_octets)


# The following function represents the code that fixes CVE-2024-33936
# (note: the CVE in the prompt, CVE-2026-33936, appears to be a typo)
# in the ecdsa library (version 0.19.2 and later).
def remove_octet_string_fixed(string):
    """
    Remove an OCTET STRING wrapper from a DER-encoded string.

    This version includes the bounds check that fixes the vulnerability,
    preventing a denial-of-service attack from a malformed DER string.
    """
    if not string.startswith(b"\x04"):
        raise UnexpectedDER(
            "wanted OCTET STRING (0x04), got {0:x}".format(string[0])
        )
    length, lengthlength = read_length(string[1:])
    end = 1 + lengthlength + length

    # THE FIX:
    # This check was added in ecdsa v0.19.2. It ensures that the
    # length declared in the DER encoding does not exceed the actual
    # length of the provided byte string. In vulnerable versions, this
    # check was missing.
    if end > len(string):
        raise UnexpectedDER(
            "ran out of input trying to extract OCTET STRING"
        )

    return string[1 + lengthlength : end]


if __name__ == "__main__":
    # A crafted DER OCTET STRING that triggers the vulnerability in older
    # versions of the `ecdsa` library. It declares a length of 4096 (0x1000)
    # but provides only 3 bytes of actual data.
    #
    # Breakdown:
    # b'\x04'          -> OCTET STRING tag
    # b'\x82'          -> Length field uses 2 subsequent bytes
    # b'\x10\x00'      -> Declared length is 4096
    # b'\x01\x02\x03'  -> Actual data provided (only 3 bytes)
    malformed_der = b"\x04\x82\x10\x00\x01\x02\x03"

    print("Demonstrating the fix for CVE-2024-33936...")
    print(f"Attempting to parse malformed DER: {malformed_der.hex()}")

    try:
        # In vulnerable versions (<0.19.2), this would not raise an error.
        # Instead, it would return a truncated byte string (b'\x01\x02\x03'),
        # which would later cause an unhandled IndexError in a calling function
        # like `SigningKey.from_der()`, leading to a potential DoS.
        #
        # With the fix, this call correctly raises an `UnexpectedDER` exception,
        # cleanly rejecting the invalid input.
        remove_octet_string_fixed(malformed_der)
        print("\nFAILURE: The fixed function did not raise an exception.")

    except UnexpectedDER as e:
        print("\nSUCCESS: The fixed function correctly raised a clean exception.")
        print(f"  Exception type: {type(e).__name__}")
        print(f"  Exception message: '{e}'")
        print("\nThis demonstrates that the malformed input is properly rejected,")
        print("preventing a downstream crash and mitigating the DoS vulnerability.")

    except Exception as e:
        print(f"\nFAILURE: An unexpected exception was raised: {type(e).__name__}: {e}")