An open proxy is an Internet proxy server which is accessible by unauthorised users, specificially those from elsewhere on the internet.

Generally, a proxy server allows users within a network group to store and forward web services such as DNS, email and web pages so that the bandwidth used by the group is reduced and controlled. With an 'open' proxy however users outside the authorised group are able to access the machine providing this service, usually for illegal purposes such as spamming other sites or creating zombie servers.

Because of the historically lax security on windows machines especially, it is possible for a user's machine to be running such servers without their knowledge.