How to fix Logic Flow Bypass
in .NET 8 Web API
Executive Summary
Logic flow bypasses in .NET 8 Web APIs occur when state transitions are handled client-side or when endpoints assume prerequisite conditions were met without server-side validation. Attackers exploit this by directly calling sensitive endpoints (e.g., /reset-password) while skipping verification gates (e.g., /verify-otp). To mitigate this, you must enforce cryptographically signed state transitions or use server-side session tracking to ensure the sequence is strictly followed.
The Vulnerable Pattern
[HttpPost("reset-password")] public async TaskResetPassword([FromBody] ResetRequest request) { // VULNERABILITY: The API assumes the user has already verified their OTP. // An attacker can skip the OTP step and call this endpoint directly for any email. var user = await _context.Users.FirstOrDefaultAsync(u => u.Email == request.Email); if (user == null) return NotFound(); user.PasswordHash = _passwordHasher.HashPassword(user, request.NewPassword); await _context.SaveChangesAsync(); return Ok("Password updated successfully.");
}
The Secure Implementation
The vulnerable code lacks state enforcement, allowing an attacker to manipulate the execution order. The secure implementation introduces a Scoped Flow Token. When the user successfully verifies their OTP, the server issues a short-lived JWT containing a claim (e.g., 'step': 'otp_verified'). The final password reset endpoint requires this token and validates that the email in the request matches the email in the signed token. This ensures the user cannot bypass the verification logic, as they cannot forge the server's signature on the flow token.
[HttpPost("reset-password")] public async TaskResetPassword([FromBody] ResetRequest request) { // SECURE: Validate a 'Flow Token' issued only after successful OTP verification. // The token contains a claim proving the 'otp_verified' state for a specific user. var flowToken = Request.Headers["X-Flow-Token"].ToString(); if (string.IsNullOrEmpty(flowToken)) return Unauthorized(); var principal = _tokenService.ValidateFlowToken(flowToken); var verifiedEmail = principal?.FindFirst(ClaimTypes.Email)?.Value; var stepClaim = principal?.FindFirst("step")?.Value; if (verifiedEmail != request.Email || stepClaim != "otp_verified") { return Forbid("Logic flow violation: OTP not verified for this account."); } var user = await _context.Users.FirstOrDefaultAsync(u => u.Email == request.Email); user.PasswordHash = _passwordHasher.HashPassword(user, request.NewPassword); // Invalidate the token or the specific flow state to prevent replay attacks await _tokenService.RevokeTokenAsync(flowToken); await _context.SaveChangesAsync(); return Ok("Password updated successfully.");
}
Your .NET 8 Web API API
might be exposed to Logic Flow Bypass
74% of .NET 8 Web API apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.
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