GHSA-jvvf-x445-j334

### Impact

`fast-uri`'s `mailto` scheme parser compares each query field name to the reserved names (`to`, `subject`, `body`) while the name is still percent-encoded, and only percent-decodes it when storing it as a generic header. On serialize, the decoded name is re-emitted, so a field name such as `%74o` (percent-encoded `to`) is not recognized as a recipient at parse time (`parse().to` shows only the legitimate recipient) but materializes as a literal `to=` field after `serialize()`, and reparsing then treats it as a recipient. The same technique smuggles `subject` and `body` through `%73ubject` and `%62ody`.

An application that parses an untrusted mailto URI, makes a display, allowlist, or logging decision on `parse().to`, then re-serializes the result and passes the serialized string to a mail client or an outbound send path can gain an attacker-chosen recipient, subject, or body that was not visible when the recipient list was checked. A scanner inspecting the raw input for an extra `to=` sees nothing, because the injected field appears only after `fast-uri` serializes.

### Patches

Upgrade to `fast-uri` 4.1.5.

### Workarounds

Percent-decode and compare mailto field names case-insensitively before trusting `parse().to`, or re-check the recipient list on the serialized output rather than only on the initial parse, until upgrading.