CAPEC-90 - Reflection Attack in Authentication Protocol

An attacker can abuse an authentication protocol susceptible to reflection attack in order to defeat it. Doing so allows the attacker illegitimate access to the target system, without possessing the requisite credentials.

Reflection attacks are of great concern to authentication protocols that rely on a challenge-handshake or similar mechanism. An attacker can impersonate a legitimate user and can gain illegitimate access to the system by successfully mounting a reflection attack during authentication.






  • Attack Methods 2
  • Protocol Manipulation
  • Spoofing
  • Purposes 2
  • Penetration
  • Exploitation
  • Sec Principles 3
  • Reluctance To Trust
  • Complete Mediation
  • Defense in Depth
  • Scopes 2
  • Bypass protection mechanism
  • Gain privileges / assume identity
  • Authorization
  • Access_Control
  • Confidentiality
  • Read application data
  • Confidentiality

Medium level: The attacker needs to have knowledge of observing the protocol exchange and managing the required connections in order to issue and respond to challenges

The attacker must have direct access to the target server in order to successfully mount a reflection attack. An intermediate entity, such as a router or proxy, that handles these exchanges on behalf of the attacker inhibits the attackers' ability to attack the authentication protocol.

All that the attacker requires is a means to observe and understand the protocol exchanges in order to reflect the challenges appropriately.

Use Authentication Mechanisms, Where Appropriate, Correctly

Step 1 -

The attacker opens a connection to the target server and sends it a challenge.

Step 2 -

The server responds by returning the challenge encrypted with a shared secret as well as its own challenge to the attacker.

Step 3 -

Since the attacker does not possess the shared secret, he initiates a second connection to the server and sends it, as challenge, the challenge received from the server on the first connection.

Step 4 -

The server treats this as just another handshake and responds by encrypting the challenge and issuing its own to the attacker.

Step 5 -

The attacker now receives the encrypted challenge on the second connection and sends it as response to the server on the first connection, thereby successfully completing the handshake and authenticating to the server..

The server must initiate the handshake by issuing the challenge. This ensures that the client has to respond before the exchange can move any further

The use of HMAC to hash the response from the server can also be used to thwart reflection. The server responds by returning its own challenge as well as hashing the client's challenge, its own challenge and the pre-shared secret. Requiring the client to respond with the HMAC of the two challenges ensures that only the possessor of a valid pre-shared secret can successfully hash in the two values.

Introducing a random nonce with each new connection ensures that the attacker cannot employ two connections to attack the authentication protocol