In cryptography and computer security, a man-in-the-middle (MITM) attack, or on-path attack, is a cyberattack where the attacker secretly relays and possibly alters the communications between two parties who believe that they are directly communicating with each other, where in actuality the attacker has inserted themselves between the two user parties. One example of a MITM attack is active eavesdropping, in which the attacker makes independent connections with the victims and relays messages between them to make them believe they are talking directly to each other over a private connection, when in fact the entire conversation is controlled by the attacker. In this scenario, the attacker must be able to intercept all relevant messages passing between the two victims and inject new ones. This is straightforward in many circumstances; for example, an attacker within range of a Wi-Fi access point hosting a network without encryption could insert themselves as a man in the middle. As it aims to circumvent mutual authentication, a MITM attack can succeed only when the attacker impersonates each endpoint sufficiently well to satisfy their expectations. Most cryptographic protocols include some form of endpoint authentication specifically to prevent MITM attacks. For example, TLS can authenticate one or both parties using a mutually trusted certificate authority.
Example
Suppose Alice wishes to communicate with Bob. Meanwhile, Mallory wishes to intercept the conversation to eavesdrop (breaking confidentiality) with the option to deliver a false message to Bob under the guise of Alice (breaking non-repudiation). Mallory would perform a man-in-the-middle attack as described in the following sequence of events.
Alice sends a message to Bob, which is intercepted by Mallory: Alice "Hi Bob, it's Alice. Give me your key." → Mallory Bob Mallory relays this message to Bob; Bob cannot tell it is not really from Alice: Alice Mallory "Hi Bob, it's Alice. Give me your key." → Bob Bob responds with his encryption key: Alice Mallory ← [Bob's key] Bob Mallory replaces Bob's key with her own, and relays this to Alice, claiming that it is Bob's key: Alice ← [Mallory's key] Mallory Bob Alice encrypts a message with what she believes to be Bob's key, thinking that only Bob can read it: Alice "Meet me at the bus stop!" [encrypted with Mallory's key] → Mallory Bob However, because it was actually encrypted with Mallory's key, Mallory can decrypt it, read it, modify it (if desired), re-encrypt with Bob's key, and forward it to Bob: Alice Mallory "Meet me at the park!" [encrypted with Bob's key] → Bob Bob thinks that this message is a secure communication from Alice. This example shows the need for Alice and Bob to have a means to ensure that they are truly each using each other's public keys, and not the public key of an attacker. Otherwise, such attacks are generally possible, in principle, against any message sent using public-key technology.
Types of MITM There are several attack types that can fall into the category of MITM. The most notable are:
HTTPS spoofing: The attacker tricks the victim into believing their connection is secure by substituting a fake SSL/TLS certificate. SSL/TLS stripping: Downgrades HTTPS traffic to HTTP, intercepting and reading unencrypted data. ARP spoofing: Sends fake ARP messages to associate the attacker’s MAC address with a target IP, intercepting local network traffic. DNS spoofing or poisoning: Redirects DNS queries to malicious servers, leading victims to fake websites. Session hijacking: Steals session cookies or tokens to impersonate a legitimate user in an active session. Man-in-the-browser (MITB): Malware alters browser activity, intercepting or manipulating transactions in real-time. Wi-Fi MITM (evil twin attack): Creates a fake Wi-Fi hotspot to intercept communications from connected devices. Email hijacking: Intercepts email exchanges to manipulate or steal sensitive information. Replay attacks: Captures and retransmits valid data to repeat actions or disrupt communication. Fake certificate authority (CA): Uses a fraudulent CA to sign fake certificates, tricking victims into trusting malicious connections.
Defense and detection MITM attacks can be prevented or detected by two means: authentication and tamper detection. Authentication provides some degree of certainty that a given message has come from a legitimate source. Tamper detection merely shows evidence that a message may have been altered and has broken integrity.
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