Unable to load certificate with negative serial number

[CBonnell]

Environment:

• Python 3.9.5 on Windows
• cryptography 35.0.0 installed in a venv via PyPi with pip version 21.1.1

Expected result:

• The PEM-encoded certificate https://censys.io/certificates/88497f01602f3154246ae28c4d5aef10f1d87ebb76626f4ae0b7f95ba7968799 (a root certificate trusted for TLS and distributed by Mozilla and other programs) is successfully loaded using x509.load_pem_x509_certificate.

Actual result:

• A ValueError occurs with "error parsing asn1 value: ParseError { kind: InvalidValue, location: ["RawCertificate::tbs_cert", "TbsCertificate::serial"] }"

Minimal repro:

pem = b'-----BEGIN CERTIFICATE-----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\nnJ2lYJU6Un/10asIbvPuW/mIPX64b24D5EI=\n-----END CERTIFICATE-----\n'

c = x509.load_pem_x509_certificate(pem)

I realize the negative serial number violates RFC 3280 and RFC 5280, but it was seemingly allowed by X.509 and RFC 2459, as they contain no explicit prohibition on negative serial numbers that 3280 and 5280 included. Given this root certificate is trusted in major root programs as a valid trust anchor for TLS, this restriction on negative serial numbers should (IMO) be relaxed until such certificates are no longer publicly trusted.

[tiran]

Indeed, the cert is in https://hg.mozilla.org/mozilla-central/file/tip/security/nss/lib/ckfw/builtins/certdata.txt and the system trust store of Linux distros.