In computer security, a man-in-the-middle attack (often abbreviated mitm, or the same using all capital letters) is an attack where the attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other. Man-in-the-middle attacks can be thought about through a chess analogy. Mallory, who barely knows how to play chess, claims that she can play two grandmasters simultaneously and either win one game or draw both. She waits for the first grandmaster to make a move and then makes this same move against the second grandmaster. When the second grandmaster responds, Mallory makes the same play against the first. She plays the entire game this way and cannot lose using this strategy unless she runs into difficulty with time because of the slight delay between relaying moves. A man-in-the-middle attack can be used against many protocols. One example of man-in-the-middle attacks 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. 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 reception range of wireless access point can insert himself as a man-in-the-middle.
As an attack that aims at circumventing mutual authentication, or lack thereof, a man-in-the-middle attack can succeed only when the attacker can impersonate each endpoint to their satisfaction as expected from the legitimate end. All protocols include some form of endpoint authentication specifically to prevent attacks. For example authentication forwarded to either one or two parties using a mutually trusted certificate of authority.
First, Alice asks Bob for his public key. If Bob sends his public key to Alice, but Mallory is able to intercept it, a man-in-the-middle attack can begin. Mallory sends a forged message to Alice that purports to come from Bob, but instead includes Mallory's public key.
Alice, believing this public key to be Bob's, encrypts her message with Mallory's key and sends the enciphered message back to Bob. Mallory again intercepts, deciphers the message using her private key, possibly alters it if she wants, and re-enciphers it using the public key Bob originally sent to Alice. When Bob receives the newly enciphered message, he believes it came from Alice.
- 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 read it, or, modify it (as desired), re-encrypt with Bob's key, and forward it to Bob:
- Alice Mallory "Meet me at the van down by the river!" [encrypted with Bob's key] → Bob
- Bob thinks that this message is a secure communication from Alice.
- Bob goes to the van down by the river and gets robbed by Mallory.
This example shows the need for Alice and Bob to have some way to ensure that they are truly using each individual private key, rather than the public key of an attacker. Otherwise, such attacks are generally possible, in principle, against any message sent using public-key or private_key technology. A variety of techniques can help defend against attacks.
Detection and Defense
MITM attacks are largely detected, or prevented by two means: authentication, and tamper detection. Authentication provides some degree of a guarantee that a given message has come from a source. Means of tamper detection, by comparison, merely shows evidence that a message may have been altered, rather than providing any guarantees.
All systems that are secure against MITM attacks provide some method of authentication for messages. Most require an exchange of information (such as public keys) in addition to the message over a secure channel. Such protocols often use key-agreement protocols have been developed, with different security requirements for the secure channel, though some have attempted to remove the requirement for any secure channel at all.
A public key infrastructure, such as Transport Layer Security, may harden Transmission Control Protocol against Man-in-the-middle-attacks. In such structures, clients and servers exchange certificates which are issued and verified by a trusted third party called a certificate authority (CA). If the original key to authenticate this CA has not been itself the subject of a MITM attack, then the certificates issued by the CA may be used to authenticate the messages sent by the owner of that certificate. Use of mutual authentication, in which both the server and the client validate the other's communication, covers both ends of a MITM attack, though the default behavior of most connections is to only authenticate the server.
Attestments, such as verbal communications of a shared value (as in ZRTP), or recorded attestments such as audio/visual recordings of a public key hash are used to ward off MITM attacks, as visual media is much more difficult and time-consuming to imitate than simple data packet communication. However, these methods require a human in the loop in order to successfully initiate the transaction.
HTTP Public Key Pinning, sometimes called "certificate pinning", helps prevent a MITM attack in which the certificate authority itself is compromised, by having the server provide a list of "pinned" public key hashes during the first transaction. Subsequent transactions then require one or more of the keys in the list must be used by the server in order to authenticate that transaction.
Latency examination can potentially detect the attack in certain situations, such as with long calculations that lead into tens of seconds like hash functions. To detect potential attacks, parties check for discrepancies in response times. For example: Say that two parties normally take a certain amount of time to perform a particular transaction. If one transaction, however, were to take an abnormal length of time to reach the other party, this could be indicative of a third party's interference inserting additional latency in the transaction.
Quantum Cryptography, in theory, provides tamper-evidence for transactions through the no-cloning theorem. Protocols based on quantum cryptography typically authenticate part or all of their classical communication with an unconditionally secure authentication scheme e.g. Wegman-Carter authentication.
DNSSEC extends the DNS protocol to use signatures to authenticate DNS records, preventing simple MITM attacks from directing a client to a malicious IP address.
Captured network traffic from what is suspected to be an attack can be analyzed in order to determine whether or not there was an attack and determine the source of the attack, if any. Important evidence to analyze when performing network forensics on a suspected attack includes:
- IP address of the server
- DNS name of the server
- X.509 certificate of the server
A notable non-cryptographic man-in-the-middle attack was perpetrated by a Belkin wireless network router in 2003. Periodically, it would take over an HTTP connection being routed through it: this 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.
In 2013, the Nokia's Browser was revealed to be decrypting HTTPS traffic on Nokia's proxy servers, giving the company clear text access to its customers' encrypted browser traffic. Nokia responded by saying that the content was not stored permanently, and that the company had organizational and technical measures to prevent access to private information.
Notable man-in-the-middle attack implementations include the following:
- DSniff – the first public implementation of attacks against and SSH
- Fiddler2 HTTP(S) diagnostic tool
- NSA impersonation of Google
- Opendium Iceni Content-control software, used to perform inspection of HTTPS traffic at the gateway.
- Subterfuge – a framework to launch multiple MITM attacks
- Superfish malware
- Websense Content Gateway – used to perform inspection of SSL traffic at the proxy
- wsniff – , a tool for 802.11 HTTP/HTTPS based MITM attacks
- Aspidistra transmitter – a British radio transmitter used for World War II "intrusion" operations, an early man-in-the-middle attack.
- Babington Plot – the plot against Elizabeth I of England, where Francis Walsingham intercepted the correspondence.
- Boy-in-the-browser – a simpler type of web browser MITM
- Computer security – the design of secure computer systems.
- Cryptanalysis – the art of deciphering encrypted messages with incomplete knowledge of how they were encrypted.
- Digital signature – a cryptographic guarantee of the authenticity of a text, usually the result of a calculation only the author is expected to be able to perform.
- Evil Maid Attack – attack used against full disk encryption systems
- Interlock protocol – a specific protocol to circumvent a man-in-the-middle attack when the keys may have been compromised.
- Key management – how to manage cryptographic keys, including generation, exchange and storage.
- Key-agreement protocol – a cryptographic protocol for establishing a key in which both parties can have confidence.
- Man-in-the-browser – a type of web browser MITM
- Man-on-the-side attack – a similar attack, giving only regular access to a communication channel.
- Mutual authentication – how communicating parties establish confidence in one another's identities.
- Password-authenticated key agreement – a protocol for establishing a key using a password.
- Quantum cryptography – the use of quantum mechanics to provide security in cryptography (while older methods rely on one-way functions).
- Secure channel – a way of communicating resistant to interception and tampering.
- Spoofing attack
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