An anonymous proxy server is a tool that attempts to make activity on the Internet untraceable. It accesses the Internet on the user's behalf, protecting personal information by hiding the source computer's identifying information.

There are many purposes for using anonymizers. Anonymizers help minimize risk. They can be used to expose human rights abuses without retribution, to speak about a taboo without loss of reputation, or to receive information within a repressive regime.

However, there are many ways in which this service can be abused. Criminals can use anonymizers along with encryption software to securely hide their identity and keep their message hidden from any police or intelligence agencies.

Anonymizers are not entirely secure. If an anonymizer keeps logs of incoming and outgoing connections and the anonymizer is physically located in a country where it’s subjected to warrant searches then there’s a potential risk that government officials can reverse engineer and identify all users who used the anonymizer and how they used it. Most anonymizers state they do not keep logs but there is presently no way to confirm that. However, if the user used another anonymizer to connect to the exposed anonymizer, that user is still anonymous. This is sometimes called daisy-chaining.

Further, an untrustworthy web based anonymizer is capable of man in the middle attacks. The anonymizer can read, inject, and modify content into the message that the user is sending as well as receiving. The anonymizer can intercept and record private unencrypted information such as username and password credentials, credit card numbers, e-mails, etc. that have been transported using the anonymizer. To avoid this, content should be encrypted and credentials should be exchanged outside of the anonymizer.

For even trustworthy anonymizers, anonymizers cannot filter out any malicious code that may reveal the identity of the user who wishes to remain anonymous. See malware. Care should be taken to prevent information leaks. For example, anonymizing an HTTP connection but not a DNS lookup can reveal the location of the viewer.

Anonymizers also present a high value target. Groups opposite the people who want to remain anonymous target public anonymizers, especially as they are often misused.