JWT verification downgrade via attacker-chosen alg

Improper JWT signature verification in Netflix Lemur before 1.9.2 allows attackers to influence token validation via a crafted alg header. The verifier in lemur/auth/service.py reads alg from the unverified JWT header and passes that attacker-controlled value into decode_with_multiple_secrets as the accepted algorithm list instead of enforcing a server-pinned allowlist. PyJWT 2.x blocks a simple alg=none bypass in the shipped configuration, so practical compromise requires a separate secret disclosure or a later asymmetric-signing migration that turns the sink into algorithm confusion and forged token acceptance.