AIKIDO-2026-814334

TL;DR

PyMongo decodes percent encoding across the entire host section of a connection string before splitting it into individual host:port entries. A percent encoded , or : inside a single hostname decodes into a real delimiter and injects an extra host and port into the client's seed list. When an application interpolates untrusted input into the host portion of a URI, a value that looks like one hostname to the application becomes two hosts to the driver, and the client can run topology discovery and authentication against the injected host. The fix moves decoding into split_hosts so the string is split on , first, and only Unix domain socket paths and IPv6 zone indexes are percent decoded afterward.