PyJWT is a Python implementation of JSON Web Token standards. From 2.11.0 through 2.13.0, PyJWT's PyJWT._merge_options() method can modify a caller-supplied mutable options mapping when verify_signature is false. If an application reuses that same mapping for a later decode() or decode_complete() call and changes verify_signature to true, the mapping can retain false values for expiration, not-before, issued-at, audience, issuer, subject, and JWT ID checks. A signed token with invalid registered claims can then be accepted without disabling signature verification, but applications that create a fresh options mapping for each call are not affected.

Affected: PyPI: PyJWT

### Summary

`PyJWT.decode()`/`decode_complete()` mutates a caller-supplied `options` dict in place whenever `verify_signature` is falsy, adding `verify_exp`/`verify_nbf`/`verify_iat`/`verify_aud`/`verify_iss`/`verify_sub`/`verify_jti` keys directly onto that object. If application code reuses the same `options` dict across calls (a config object, a module-level constant, a wrapper's `self.options`), a later call that explicitly sets `verify_signature=True` for a full, normal verification silently inherits the leftover `verify_exp=False` etc. from an earlier `verify_signature=False` call on the same object -- and accepts an expired, wrong-audience, wrong-issuer, or wrong-subject token as valid with no exception. Signature verification itself is unaffected; this is specifically a claims-check bypass.

This is a regression of an already-fixed, closed issue: originally reported in #679 (2021), fixed in #743 (2022, via a shallow copy of `options`), and silently reintroduced by #1045 (merged 2025-03-05, a typing/logic refactor that dropped the 2022 shallow-copy line). It has been present since #1045 merged, so it likely shipped in several releases already in the wild.

### Details

File: `jwt/api_jwt.py`, `PyJWT._merge_options()` (called from `decode_complete()`, which backs both `jwt.decode()` and `jwt.decode_complete()`):

```python def _merge_options(self, options=None): if options is None: return self.options if not options.get("verify_signature", True):…