SS7 Attack: Bypassing SMS-Based Two-Factor Authentication (OTP)
The SS7 (Signaling System 7) protocol’s fundamental security flaw makes it one of the most effective and dangerous methods for bypassing Two-Factor Authentication (2FA) codes sent via SMS, also known as One-Time Passwords (OTPs).

This method is highly effective because it exploits a weakness in the global telecom network infrastructure itself, rather than targeting the user's phone or computer.

The Mechanism of the Attack
Initial Credential Compromise: The attacker first obtains the victim's primary login credentials (username and password) through a separate method, such as a phishing attack or data breach.

Login Attempt & OTP Request: The attacker attempts to log into the victim's sensitive account (e.g., bank, email, social media). Since 2FA is enabled, the service sends an OTP code via an SMS message to the victim's phone number.

SS7 Network Access: The attacker must have access to the SS7 network, often by purchasing or leasing access from a small, less-secure mobile network operator or a roaming hub.

SMS Interception/Redirection: Once on the network, the attacker sends a malicious signaling message (such as a fraudulent UpdateLocation request) targeting the victim's mobile phone number.

This message tricks the victim's home network (the Home Location Register or HLR) into believing the user has "roamed" onto the attacker's network node.

The HLR, following the inherent trust of the SS7 protocol, then reroutes all incoming communications—including the crucial OTP SMS—to the attacker's server instead of the victim's actual phone.

Account Takeover: The attacker intercepts and reads the OTP code immediately. They enter this code into the login prompt on the service's website, successfully completing the two-factor authentication process and gaining full access to the victim's account to steal funds or data.

Why This Works
The vulnerability is rooted in the fact that SS7 messages—including those that route SMS—are not encrypted and the system operates without strong authentication. The network blindly trusts the malicious UpdateLocation message that tells it to redirect the user's SMS traffic