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The guard added for CVE-2024-33663 identified asymmetric keys by their text format only, matching PEM armor and SSH prefixes. A DER-encoded key is binary and has neither, so it was accepted as an HMAC secret: an attacker holding the service's public key could sign an HS256 token with its DER bytes and have it verify, when the verifying algorithms were not restricted. DER carries nothing to match on, so is_der_format() recognizes it by its ASN.1 structure instead. The check is pure Python rather than deferring to cryptography's key loaders, because the native backend is the one used when cryptography is not installed. Co-authored-by: Cursor <cursoragent@cursor.com>
generate_private_key() requires an EllipticCurve instance, so passing the SECP256R1 class raised a TypeError before the test could assert anything, leaving the CVE-2024-33663 regression uncovered. Co-authored-by: Cursor <cursoragent@cursor.com>
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Summary
is_der_format()and applies it in both HMAC backends (nativeandcryptography) alongside the existing PEM/SSH checks. Detection is structural ASN.1 rather than callingcryptographyloaders, so the native-only install path stays covered.test_incorrect_public_key_hmac_signing, which passed a curve class togenerate_private_key()and never reached its CVE-2024-33663 assertion on current cryptography.Fixes #414
Test plan
jwt.decode(..., algorithms=["RS256","HS256"])andjws.verify(..., algorithms=None). After the change both encodings raiseJWKError.cryptographyuninstalled): DER HMAC secret rejected; 176 tests passed under-m "not (cryptography or pycryptodome or backend_compatibility)".flake8,isort --check-only, andblack --checkclean.Made with Cursor