When installing a package from a Mercurial VCS URL (ie "pip install hg+...") with pip prior to v23.3, the specified Mercurial revision could be used to inject arbitrary configuration options to the "hg clone" call (ie "--config"). Controlling the Mercurial configuration can modify how and which repository is installed. This vulnerability does not affect users who aren't installing from Mercurial.
https://mail.python.org/archives/list/[email protected]/thread/F4PL35U6X4VVHZ5ILJU3PWUWN7H7LZXL/