The Batch Cat WordPress plugin through 0.3 defines 3 custom AJAX actions, which both require authentication but are available for all roles. As a result, any authenticated user (including simple subscribers) can add/set/delete arbitrary categories to posts.
[
{
"product": "Batch Cat",
"vendor": "Unknown",
"versions": [
{
"lessThanOrEqual": "0.3",
"status": "affected",
"version": "0.3",
"versionType": "custom"
}
]
}
]