An open proxy is an Internet proxy server which is accessible by unauthorised users, specifically 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 computers.

Because of the historically lax security on Microsoft Windows machines especially, it is possible for a user's machine to be running such servers without their knowledge by means of computer viruses designed for this purpose. Many third-party products which provide proxy servers for Windows machines do not ship with secure settings out of the box (most notable Wingate, which at one time was infamous for its insecure default settings), allowing anyone to use them; however, with recent rise in concerns about computer security, this situation has improved a lot.

Because proxies are implicated in abuse, system administrators have developed a number of ways to refuse service to open proxies. IRC networks such as the Blitzed network automatically test client systems for known types of open proxy. [1] Likewise, a mail server may be configured to automatically test mail senders for open proxies, using software such as Michael Tokarev's proxycheck. [2]

Internet Relay Chat (IRC) abusers also frequently use open proxies to cloak their identities.