What is securitization and how do mortgage-backed securities (MBS) actually work?

Securitization is the process of transforming illiquid assets (like individual mortgages or car loans) into tradable securities. It's one of the most important financial innovations — and one of the most exam-tested topics.

The Securitization Process:

Step by step:

1. Origination: A bank makes 5,000 individual mortgage loans totaling $1.5 billion
2. Pooling: The bank sells these loans to a Special Purpose Vehicle (SPV) — a separate legal entity
3. Tranching: The SPV issues securities in layers (tranches) with different risk/return profiles
4. Credit enhancement: The structure includes overcollateralization, subordination, and reserve accounts to protect senior tranches
5. Servicing: A servicer collects monthly payments from borrowers and distributes them to security holders

Key Terminology:

• Pass-through securities: All investors receive a proportional share of all cash flows
• CMO (Collateralized Mortgage Obligation): Redirects cash flows to create tranches with different maturities and risks
• ABS (Asset-Backed Securities): Broader term for securitized non-mortgage assets (auto loans, credit cards, student loans)

Major Risks:

1. Prepayment risk: Borrowers refinance when rates drop, returning principal early and forcing reinvestment at lower rates
2. Extension risk: When rates rise, borrowers don't prepay, extending the security's effective maturity
3. Credit risk: If underlying borrowers default, cash flows are reduced
4. Liquidity risk: Some tranches may have thin secondary markets

Why securitization matters:

It allows banks to free up capital, diversify risk, and provide funding for more lending. However, misaligned incentives (originate-to-distribute) contributed to the 2008 financial crisis.

Explore structured products in depth in our CFA Level I Fixed Income course.