In cryptography, the man-in-the-middle attack or bucket-brigade attack (often abbreviated MITM), sometimes Janus attack, is a form of active eavesdropping in which the attacker makes independent connections with the victims and relays messages between them, making them believe that they are talking directly to each other over a private connection when in fact the entire conversation is controlled by the attacker. The attacker must be able to intercept all messages going between the two victims and inject new ones, which is straightforward in many circumstances (for example, the owner of a public wireless access point can in principle conduct MITM attacks on the users).
A man-in-the-middle attack can only be successful when the attacker can impersonate each endpoint to the satisfaction of the other. Most cryptographic protocols include some form of endpoint authentication specifically to prevent MITM attacks. For example, SSL authenticates the server using a mutually trusted certification authority.
This example shows the need for Alice and Bob to have some way to ensure that they are truly using each other's public keys, rather than the public key of an attacker. Otherwise, such attacks are generally possible, in principle, against any message sent using public-key technology. Fortunately, there are a variety of techniques that help defend against MITM attacks.
The integrity of public keys must generally be assured in some manner, but need not be secret. Passwords and shared secret keys have the additional secrecy requirement. Public keys can be verified by a Certificate Authority, whose public key is distributed through a secure channel (for example, with a web browser or OS installation). Public keys can also be verified by a web of trust that distributes public keys through a secure channel (for example by face-to-face meetings).
See key agreement for a classification of protocols that use various forms of keys and passwords to prevent man-in-the-middle attacks.
MITM should be seen as a general problem resulting from the presence of intermediate parties acting as proxy for clients on either side. If they are trustworthy and competent, all may be well; if they are not, nothing will be. How can one distinguish the cases? By acting as proxy and appearing as the trusted client to each side, the intermediate attacker can carry out much mischief, including various attacks against the confidentiality or integrity of the data passing through it.
A notable non-cryptographic man-in-the-middle attack was perpetrated by one version of a Belkin wireless network router in 2003. Periodically, it would take over an HTTP connection being routed through it: it would fail to pass the traffic on to destination, but instead itself respond as the intended server. The reply it sent, in place of the web page the user had requested, was an advertisement for another Belkin product. After an outcry from technically-literate users, this 'feature' was removed from later versions of the router's firmware.
Another example of a non-cryptographic man-in-the-middle attack is the "Turing porn farm." Brian Warner says this is a "conceivable attack" that spammers could use to defeat CAPTCHAs. The spammer sets up a pornographic web site where access requires that the user solves the CAPTCHAs in question. However, Jeff Atwood points out that this attack is merely theoretical — there is no evidence that any spammer has ever built a Turing porn farm". However, as reported in an October, 2007 news story while perhaps not being a farm as such, spammers have indeed built a Windows game in which users type in CAPTCHAs acquired from the Yahoo webmail service, and are rewarded with pornographic pictures. This allows the spammers to create temporary free email accounts with which to send out spam.