2FA Testing Tips for Bug Bounty Hunters

Two-factor authentication (2FA) is a cornerstone of modern account security, adding an extra layer of protection by requiring users to verify their identity with something they have (like a one-time password or OTP) in addition to something they know (like a password). However, as with any security measure, improper implementation can lead to vulnerabilities, making 2FA a prime target for bug bounty hunters.

This guide delves into 21 key techniques for testing and identifying vulnerabilities in 2FA systems, helping bug bounty hunters strengthen applications while earning rewards.

1. Status Code Changes

When 2FA is enabled, the application typically responds with specific status codes (e.g., 200 for success, 403 for forbidden). By altering these codes during the verification process, you can identify potential bypass vulnerabilities.
Example:
Change 403 Forbidden to 200 OK in the response header and check if it grants access to protected areas.

2. Brute-Force OTP

A common misconfiguration is the lack of brute-force protection for OTP inputs. Test if the application allows multiple attempts to guess the OTP without locking the account or triggering an alert.
Steps: