Affected: authlib

Authlib is a Python library which builds OAuth and OpenID Connect servers. From 1.3.1 until 1.9.0, OctKey.import_key() checks asymmetric-key markers only at byte zero while Authlib's PEM loaders accept leading whitespace, comments, or a byte-order mark. In deployments that use the documented mixed algorithm registry and already configure verification with PEM or certificate bytes containing such a prefix, an unauthenticated attacker who knows those same public-key bytes can use them as an HMAC secret and forge HS256 tokens with arbitrary identity or authorization claims. This is a bypass of the remediation for CVE-2024-37568. The module-level jwt object reaches OctKey.import_key() in authlib/jose/rfc7518/oct_key.py, where POSSIBLE_UNSAFE_KEYS fails to identify prefixed asymmetric key material. This issue is fixed in version 1.9.0.

Affected: PyPI: authlib

## Summary

`OctKey.import_key()` carries the only guard that stops an asymmetric public key from being used as an HMAC secret in `authlib.jose`. That guard is `raw_key.startswith(POSSIBLE_UNSAFE_KEYS)` — anchored at byte 0 — while the loader that consumes the identical bytes on the asymmetric path (`cryptography.hazmat.primitives.serialization.load_pem_public_key` / `load_pem_x509_certificate`) skips anything before the `-----BEGIN` line. A PEM public key or X.509 certificate carrying **one leading byte** — a newline, a space, four spaces of indentation, a UTF-8 BOM, or a comment line — is therefore not recognised, falls through, and becomes the HMAC secret. An attacker holding only the issuer's public key then mints an `HS256` token with arbitrary claims that `jwt.decode()` accepts as authentic.

This is an incomplete fix of **CVE-2024-37568 / GHSA-5357-c2jx-v7qh**. That advisory's fix is exactly this guard (`git log -S 'POSSIBLE_UNSAFE_KEYS' -- authlib/jose/rfc7518/oct_key.py` returns a single commit, `3bea812` "fix: prevent OctKey to import ssh/rsa/pem keys", referencing issue #654), and the original attack works again unchanged whenever the stored PEM does not start at offset 0.

Reproduced end-to-end on **1.8.0**, the newest release (published 2026-08-30), and on `origin/main` (`c529a61`), with `cryptography` 50.0.2.

## Preconditions

Both are deployment properties; neither is attacker-controlled at request time, and the attacker needs no secret and no user interactio…