Patched code sample

import sys
from cryptography.hazmat.primitives.asymmetric import ec
from cryptography.exceptions import InternalError

# This code demonstrates the fix for CVE-2026-26007.
#
# The vulnerability: In versions prior to 46.0.5, certain functions
# (e.g., EllipticCurvePublicNumbers.public_key()) did not validate that a
# public key point belonged to the correct prime-order subgroup for curves
# with a cofactor > 1 (e.g., SECT curves). This allowed an attacker to
# supply a point from a small-order subgroup.
#
# The impact: This could lead to security issues like leaking bits of the
# private key in ECDH or allowing signature forgery in ECDSA.
#
# The fix: The library was updated to perform a subgroup validation check.
#
# This script attempts to create a public key using a known invalid point
# on an affected curve (SECT239k1, which has a cofactor of 4). In a patched
# library (>= 46.0.5), this will fail with a ValueError, demonstrating that
# the validation (the fix) is working correctly.

try:
    # 1. Select a curve affected by the vulnerability.
    # SECT curves have cofactors > 1. SECT239k1 has a cofactor of 4.
    curve = ec.SECT239K1()

    # 2. Define coordinates for an invalid point from a small-order subgroup.
    # The point (x=0, y=1) on the SECT239k1 curve has an order of 2, which is
    # a divisor of the cofactor. This is NOT a point in the large prime-order
    # subgroup required for secure cryptographic operations.
    invalid_point_x = 0
    invalid_point_y = 1

    print(
        f"Attempting to load a public key with an invalid point (order 2) "
        f"on curve {curve.name}..."
    )

    # 3. Construct the public numbers object from the invalid coordinates.
    public_numbers = ec.EllipticCurvePublicNumbers(
        x=invalid_point_x,
        y=invalid_point_y,
        curve=curve
    )

    # 4. Attempt to create the public key. This is the step that demonstrates
    # the fix. In a fixed version, this method performs a subgroup check and
    # will raise an exception because the point is invalid. In a vulnerable
    # version, this would have succeeded.
    public_key = public_numbers.public_key()

    # If the script reaches this line, no exception was raised, meaning the
    # check was not performed.
    print(
        "\n[VULNERABILITY DETECTED]: Public key was created from an invalid point."
        "\nThe library version is likely vulnerable to CVE-2026-26007."
    )
    sys.exit(1)

except ValueError as e:
    # Catching a ValueError is the expected behavior in a fixed library.
    # This confirms that the point was validated and correctly rejected.
    print("\n[FIX VERIFIED]: The library correctly rejected the invalid public key.")
    print(f"Caught expected exception: {e}")
    print("This demonstrates the subgroup validation fix is in place.")
    sys.exit(0)

except InternalError as e:
    # Depending on the backend, an InternalError could also be raised.
    # Catching it provides a more robust demonstration.
    print("\n[FIX VERIFIED]: The library correctly rejected the invalid public key.")
    print(f"Caught expected exception: {e}")
    print("This demonstrates the subgroup validation fix is in place.")
    sys.exit(0)

except Exception as e:
    print(f"\nAn unexpected error occurred: {e}")
    sys.exit(1)