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.
An issue was discovered in pip (all versions) because it installs the version with the highest version number, even if the user had intended to obtain a private package from a private index. This only affects use of the --extra-index-url option, and exploitation requires that the package does not already exist in the public index (and thus the attacker can put the package there with an arbitrary version number). NOTE: it has been reported that this is intended functionality and the user is responsible for using --extra-index-url securely