PYSEC-2026-1814

### Summary The wrong string if check is run for `iss` checking, resulting in `"acb"` being accepted for `"_abc_"`.

### Details This is a bug introduced in version [2.10.0](https://github.com/jpadilla/pyjwt/commit/1570e708672aa9036bc772476beae8bfa48f4131#diff-6893ad4a1c5a36b8af3028db8c8bc3b62418149843fc382faf901eaab008e380R366): checking the "iss" claim changed from `isinstance(issuer, list)` to `isinstance(issuer, Sequence)`.

```diff - if isinstance(issuer, list): + if isinstance(issuer, Sequence): if payload["iss"] not in issuer: raise InvalidIssuerError("Invalid issuer") else: ```

Since str is a Sequnce, but not a list, `in` is also used for string comparison. This results in `if "abc" not in "__abcd__":` being checked instead of `if "abc" != "__abc__":`. ### PoC Check out the unit tests added here: https://github.com/jpadilla/pyjwt-ghsa-75c5-xw7c-p5pm ```python issuer = "urn:expected"

payload = {"iss": "urn:"}

token = jwt.encode(payload, "secret")

# decode() succeeds, even though `"urn:" != "urn:expected". No exception is raised. with pytest.raises(InvalidIssuerError): jwt.decode(token, "secret", issuer=issuer, algorithms=["HS256"]) ```

### Impact

I would say the real world impact is not that high, seeing as the signature still has to match. We should still fix it.