Affected: Homebrew: fastmcp

### Summary

`joserfc.jwt.decode` accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or `None`. `HMACAlgorithm.sign` and `HMACAlgorithm.verify` in [`src/joserfc/_rfc7518/jws_algs.py:62-70`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/src/joserfc/_rfc7518/jws_algs.py#L62-L70) feed whatever `OctKey.get_op_key(...)` produced into `hmac.new(...)`, and `OctKey.import_key` only emits a `SecurityWarning` when the raw key is shorter than 14 bytes without rejecting zero-length input. Any application whose JWT secret is sourced from an unset environment variable, an unset Redis / DB row, a key finder fallback that returns `""`, or a `Hash.new("")`-style default verifies attacker tokens forged with `HMAC(key=b"", signing_input)` because the attacker trivially reproduces the same digest with no secret knowledge.

This is a cross-language sibling of jwt/ruby-jwt GHSA-c32j-vqhx-rx3x / CVE-2026-45363 (HS256/HS384/HS512 verify accepted an empty/nil HMAC key, filed 2026-05-13). ruby-jwt v3.2.0 added an `ensure_valid_key!` precondition that rejects empty keys at both sign and verify entry; joserfc has no equivalent. (The same primitive lives in the deprecated `authlib.jose` module by the same maintainer; filing this advisory against joserfc alongside a separate `authlib` advisory because the codebases are independent shipping artifacts on PyPI.)

### Affected versions

`joserfc` (PyPI) `<= 1.6.7` (lates…

Affected: authlib joserfc

joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. Prior to 1.6.8, joserfc.jwt.decode accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or None, because HMACAlgorithm.sign and HMACAlgorithm.verify in src/joserfc/_rfc7518/jws_algs.py pass the output of OctKey.get_op_key(...) to hmac.new(...) and OctKey.import_key in src/joserfc/_rfc7518/oct_key.py only emits a SecurityWarning for keys shorter than 14 bytes without rejecting zero-length input. This issue is fixed in version 1.6.8.

Affected: PyPI: joserfc

### Summary

`joserfc.jwt.decode` accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or `None`. `HMACAlgorithm.sign` and `HMACAlgorithm.verify` in [`src/joserfc/_rfc7518/jws_algs.py:62-70`](https://github.com/authlib/joserfc/blob/1ddca8f3c73ff47e3bc3ac06cb0c08a9535677ec/src/joserfc/_rfc7518/jws_algs.py#L62-L70) feed whatever `OctKey.get_op_key(...)` produced into `hmac.new(...)`, and `OctKey.import_key` only emits a `SecurityWarning` when the raw key is shorter than 14 bytes without rejecting zero-length input. Any application whose JWT secret is sourced from an unset environment variable, an unset Redis / DB row, a key finder fallback that returns `""`, or a `Hash.new("")`-style default verifies attacker tokens forged with `HMAC(key=b"", signing_input)` because the attacker trivially reproduces the same digest with no secret knowledge.

This is a cross-language sibling of jwt/ruby-jwt GHSA-c32j-vqhx-rx3x / CVE-2026-45363 (HS256/HS384/HS512 verify accepted an empty/nil HMAC key, filed 2026-05-13). ruby-jwt v3.2.0 added an `ensure_valid_key!` precondition that rejects empty keys at both sign and verify entry; joserfc has no equivalent. (The same primitive lives in the deprecated `authlib.jose` module by the same maintainer; filing this advisory against joserfc alongside a separate `authlib` advisory because the codebases are independent shipping artifacts on PyPI.)

### Affected versions

`joserfc` (PyPI) `<= 1.6.7` (lates…