What exactly is a mortgage-backed security and how does the cash flow structure work?

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AcadiFi · Accepted Answer

A mortgage-backed security (MBS) is a bond whose cash flows come from a pool of residential mortgages. Unlike corporate bonds, MBS are amortizing, carry prepayment risk, and pass through payments net of servicing and guarantee fees.

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The mechanics differ from a corporate bond in three ways. First, cash flows are amortizing — every monthly payment contains both scheduled interest and scheduled principal, so the outstanding balance declines over time. Second, borrowers can prepay at any moment (refinance, home sale, default payoff), which makes the actual cash-flow timing uncertain. Third, the coupon passed to investors is the mortgage rate minus servicing and guarantee fees (typically 25-75 bps combined).

Agency MBS (Fannie, Freddie, Ginnie) carry credit guarantees, so the investor risk reduces to prepayment/interest-rate risk. Non-agency (private-label) MBS retain credit risk and usually require subordination structures.

For FRM, remember the payment priority: servicer collects, subtracts fees, remits to trustee, who distributes pro-rata (pass-through) or by tranche rules (CMO) to certificate holders. Want a worked amortization example? Our FRM fixed income module builds a full pool cash flow in Excel.

What exactly is a mortgage-backed security and how does the cash flow structure work?

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