### Summary

PyMongo percent-decoded the entire host section of a connection string before splitting it into individual `host:port` entries. A percent-encoded `,` or `:` in a hostname therefore decoded into a real delimiter, injecting an additional attacker-chosen host and port into the client's seed list.

### Impact

An application that interpolates untrusted input into a MongoDB connection string -- for example, a tenant name or hostname fragment taken from a request -- could be made to add an attacker-controlled server to the seed list. Because `%2C` and `%3A` survive most URL-safety checks and only become delimiters inside PyMongo's parser, input that looks like a single hostname to the application becomes two hosts to the driver. The client may then perform topology discovery and authentication against the attacker's host, exposing credentials, or route operations to it.

Unix domain socket paths, the only host identifiers that legitimately require percent-encoding, are not affected.

### Patches

Fixed in PyMongo 4.18.2. Percent-decoding was moved into `split_hosts` and now applies only to Unix domain socket paths, identified by an unescaped `.sock` suffix before decoding, and only after splitting on `,`.

### Workarounds

Do not interpolate untrusted input into the host portion of a connection string. If unavoidable, reject or percent-decode-and-validate the value before building the URI.

### Details

The decoding was introduced in PyMongo 3.5.0 (PYTHON-1282), where `unquote_plus` was applied to the whole host section in `_validate_uri` and, later, `_parse_srv`, before the string was split on `,` and `:`.