# Which Problems Are Solved
We noticed logging where 500: Internal Server errors were returned from
the token endpoint, mostly for the `refresh_token` grant. The error was
thrown by the database as it received non-UTF8 strings for token IDs
Zitadel uses symmetric encryption for opaque tokens, including refresh
tokens. Encrypted values are base64 encoded. It appeared to be possible
to send garbage base64 to the token endpoint, which will pass decryption
and string-splitting. In those cases the resulting ID is not a valid
UTF-8 string.
Invalid non-UTF8 strings are now rejected during token decryption.
# How the Problems Are Solved
- `AESCrypto.DecryptString()` checks if the decrypted bytes only contain
valid UTF-8 characters before converting them into a string.
- `AESCrypto.Decrypt()` is unmodified and still allows decryption on
non-UTF8 byte strings.
- `FromRefreshToken` now uses `DecryptString` instead of `Decrypt`
# Additional Changes
- Unit tests added for `FromRefreshToken` and
`AESCrypto.DecryptString()`.
- Fuzz tests added for `FromRefreshToken` and
`AESCrypto.DecryptString()`. This was to pinpoint the problem
- Testdata with values that resulted in invalid strings are committed.
In the pipeline this results in the Fuzz tests to execute as regular
unit-test cases. As we don't use the `-fuzz` flag in the pipeline no
further fuzzing is performed.
# Additional Context
- Closes#7765
- https://go.dev/doc/tutorial/fuzz
* fix(oidc): return bad request for base64 errors
We've recently noticed an increased amount of 500: internal server error status returns on zitadel cloud.
The source of these errors appear to be erroneous input in fields that are supposed to be bas64 formatted.
```
time=2024-04-08T14:05:47.600Z level=ERROR msg="request error" oidc_error.parent="ID=OIDC-AhX2u Message=Errors.Internal Parent=(illegal base64 data at input byte 8)" oidc_error.description=Errors.Internal oidc_error.type=server_error status_code=500
```
Within the possible code paths of the token endpoint there are a couple of uses of base64.Encoding.DecodeString of which a returned error was not properly wrapped, but returned as-is.
This causes the oidc error handler to return a 500 with the `OIDC-AhX2u` ID.
We were not able to pinpoint the exact errors that are happening to any one call of `DecodeString`.
This fix wraps all errors from `DecodeString` so that proper 400: bad request is returned with information about the error. Each wrapper now has an unique error ID, so that logs will contain the source of the error as well.
This bug was reported internally by the ops team.
* catch op.ErrInvalidRefreshToken
