Mailvelope prior to 3.3.0 does not require user interaction to import public keys shown on web page. This functionality can be tricked to either hide a key import from the user or obscure which key was imported.
github.com/mailvelope/mailvelope/blob/master/Changelog.md#v330
www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/Publications/Studies/Mailvelope_Extensions/Mailvelope_Extensions_pdf.html
www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/Publications/Studies/Mailvelope_Extensions/Mailvelope_Extensions_pdf.pdf?__blob=publicationFile&v=3