A mortgage-backed security, or MBS, is an investment created by bundling hundreds or thousands of individual home loans into a single asset that trades on financial markets. Investors who buy into the pool earn returns from the monthly mortgage payments the underlying borrowers make. The market is enormous: as of December 2025, the outstanding balance of single-family agency MBS alone topped $9.2 trillion across Ginnie Mae, Fannie Mae, and Freddie Mac.1Ginnie Mae. Global Markets Analysis Report For an investor, an MBS offers exposure to the housing market without buying property or lending to a homeowner directly.
How an MBS Gets Built
The chain starts at a local closing table. A bank or mortgage lender funds a home purchase, and the borrower signs a promissory note plus a deed of trust pledging the property as collateral. Rather than sit on that loan for 15 or 30 years, the lender usually sells it into the secondary market, which frees up capital for new lending.
A secondary market buyer then groups purchased loans into a pool sorted by shared characteristics: similar interest rates, similar terms, similar borrower credit profiles. That homogeneity makes the pool’s performance easier to model.
To keep those loans legally separate from the buyer’s own balance sheet, the pool is placed inside a Special Purpose Vehicle (SPV). An SPV is a standalone legal entity, so if the sponsor later goes bankrupt, the mortgages inside are shielded from its creditors. The security’s value depends only on how the borrowers in the pool perform. The SPV then issues certificates representing fractional ownership of the pool, and those certificates are what investors buy.
Who Issues Mortgage-Backed Securities
Three kinds of institutions issue MBS, and the type of issuer largely determines how safe the security is and what loans sit inside it.
Ginnie Mae
The Government National Mortgage Association is a government corporation inside the Department of Housing and Urban Development.2Federal Register. Agencies – Government National Mortgage Association Ginnie Mae doesn’t buy loans. It guarantees the timely payment of principal and interest on securities backed by federally insured or guaranteed mortgages, mainly FHA and VA loans.3Ginnie Mae. Funding Government Lending Ginnie Mae MBS are the only mortgage-backed securities carrying the full faith and credit of the United States government, meaning the federal government stands behind payments even if the issuer or the borrowers default.4Office of the Law Revision Counsel. 12 USC 1721 – Management and Liquidation Functions of Government National Mortgage Association
Fannie Mae and Freddie Mac
Fannie Mae and Freddie Mac are government-sponsored enterprises (GSEs) that actually buy qualifying mortgages from lenders and package them into what the industry calls “agency MBS.” Fannie Mae operates under the Federal National Mortgage Association Charter Act; Freddie Mac operates under the Federal Home Loan Mortgage Corporation Act.5Office of the Law Revision Counsel. 12 USC 1716 – Declaration of Purposes of Subchapter6Office of the Law Revision Counsel. 12 USC 1451 – Definitions Both guarantee principal and interest on their securities, but the guarantee comes from the agencies themselves rather than the U.S. Treasury. The federal government placed both into conservatorship in 2008, but their MBS still don’t carry the same explicit full-faith-and-credit backing as Ginnie Mae’s.
To qualify for purchase by Fannie or Freddie, a mortgage has to fall within the conforming loan limit the Federal Housing Finance Agency sets each year. For 2026, the baseline limit on a single-family home in most of the country is $832,750, up $26,250 from 2025. In designated high-cost areas the ceiling rises to $1,249,125, or 150 percent of the baseline.7FHFA. FHFA Announces Conforming Loan Limit Values for 2026 Loans above those numbers are jumbo mortgages and cannot be securitized through Fannie or Freddie.
Private-Label Issuers
Large investment banks and other private financial firms issue their own MBS, usually called private-label securities. They carry no government or GSE guarantee, and they often include loans that fall outside Fannie and Freddie underwriting standards, such as jumbos, loans to borrowers with lower credit scores, or unconventional terms. Because the risk sits with investors, private-label MBS typically pay higher interest rates. These issuers also have to register their securities under the Securities Act of 1933 and the Securities Exchange Act of 1934,8Federal Register. Concept Release on Residential Mortgage-Backed Securities Disclosures and Enhancements to Asset-Backed Securities Registration and provide loan-level disclosures under Regulation AB.9eCFR. Subpart 229.1100 Asset-Backed Securities (Regulation AB) Agency MBS are exempt from those SEC requirements.
The Main Types You’ll See
Pass-Through Securities
The simplest structure. Borrower payments flow directly to investors on a proportional basis, minus servicing fees. Own ten percent of the pool, get ten percent of the monthly cash. Every investor shares the same exposure to prepayments, defaults, and interest rate moves.
Collateralized Mortgage Obligations
A collateralized mortgage obligation (CMO) takes the same pool of mortgages and slices the cash flows into tranches, each with its own rules for when it collects interest and principal. Some tranches absorb prepayments first and pay off quickly; others get no principal until earlier tranches are retired. That lets investors pick a slice that fits their time horizon.
The redistribution changes the risk profile too. Senior tranches get paid first and carry less default risk. Junior tranches absorb losses first and pay higher yields to compensate. Rating agencies score each tranche separately based on borrower credit, property values, and the equity cushion inside the underlying loans.
Commercial Mortgage-Backed Securities
CMBS are backed by loans on income-producing commercial properties: office buildings, shopping centers, hotels, apartment complexes. Pools are far smaller than residential ones, typically holding 40 to 250 loans instead of thousands.10Federal Reserve Board. Differences Across Originators in CMBS Loan Underwriting Because each loan is a bigger share of the whole, the credit quality of individual loans matters more in pricing a CMBS than in a residential pool where statistical averages across thousands of borrowers do most of the work.
What Investors Actually Receive
When a homeowner mails in the monthly payment, it goes first to a mortgage servicer, who collects payments, manages escrow for taxes and insurance, and chases delinquencies. The servicer takes a fee and passes the rest through. On Fannie Mae fixed-rate loans, the maximum servicing fee is 50 basis points (0.50 percent) of the outstanding balance per year.11Fannie Mae. C2-1.1-05, Servicing Fees Other programs are in a similar range.
Every payment an investor gets has two pieces: interest on the outstanding debt, and a chunk of principal being repaid. That’s different from a corporate bond that pays a fixed coupon until maturity. MBS payments swing month to month. When borrowers refinance, sell, or send in extra principal, cash comes back faster than the schedule. When borrowers stay put and pay only what they owe, principal returns slowly across the full life of the loan. The income stream is tied to real-world borrower behavior.
Default handling depends on the guarantee. Ginnie Mae issuers keep advancing scheduled payments to investors when borrowers fall behind.12Ginnie Mae. Chapter 18 – Mortgage Delinquency and Default Fannie Mae and Freddie Mac absorb losses through their own guarantees. In private-label deals, defaults reduce the cash available to investors, with junior tranches taking losses before senior ones.
The Risks That Make MBS Different
MBS carry two signature risks that don’t show up in ordinary bonds. Both trace back to the same fact: a homeowner can pay off a mortgage early.
Prepayment Risk
When interest rates fall, borrowers refinance at the new lower rate and pay off the original loan early. For investors, principal comes back sooner than expected, and it arrives exactly when reinvestment options offer lower yields.13Federal Reserve Bank of Kansas City. The Prepayment Risk of Mortgage-Backed Securities The rate rally that should have delivered a big capital gain delivers a smaller one.
Extension Risk
The mirror image. When rates rise, refinancing slows, prepayments dry up, and investors are stuck holding a below-market coupon for longer than they planned. MBS prices tend to fall faster than comparable Treasuries in a rising-rate environment because the expected duration keeps stretching.14Liberty Street Economics. Convexity Event Risks in a Rising Interest Rate Environment The industry term for this asymmetry is negative convexity: investors lose more than they’d expect when rates climb and gain less than they’d expect when rates fall.
Credit Risk
The chance that borrowers stop paying entirely. For agency MBS from Ginnie Mae, Fannie Mae, or Freddie Mac, the guarantees mean investor credit risk is minimal. Private-label MBS pass default losses straight through, with junior tranches hit first, so investors have to look hard at underwriting quality and borrower profiles in the pool.
The 2008 Chapter and What Changed
Mortgage-backed securities sat at the center of the worst financial crisis since the Great Depression. In the run-up to 2008, private-label MBS provided most of the funding for subprime and other high-risk mortgages. Lenders loosened underwriting because they could sell the risky loans into securitizations rather than hold them. Standard risk models had been built around traditional-mortgage default rates of roughly 1 to 2 percent and didn’t reflect the newer loan products.
When home prices peaked and turned, borrowers couldn’t refinance or sell their way out. Defaults surged, private-label MBS lost enormous value, and the market that had funded subprime lending seized up. Several subprime lenders went bankrupt, rating downgrades cascaded, and a broader credit freeze followed.15Federal Reserve History. Subprime Mortgage Crisis
The Dodd-Frank Act of 2010 rewrote the rules. Its central securitization provision requires the sponsor of a deal to hold at least 5 percent of the credit risk in the assets being securitized. Codified at 12 CFR Part 244, the “skin in the game” rule can be met with a 5 percent vertical slice of every tranche, a horizontal residual interest equal to at least 5 percent of the fair value of the securities issued, or a mix.16eCFR. 12 CFR Part 244 – Credit Risk Retention (Regulation RR) The goal is to give originators something to lose if the loans go bad. Agency MBS are exempt because the agencies themselves already guarantee investor payments.
How an Individual Investor Buys In
You can buy individual agency MBS through a brokerage account, though minimums vary. New Ginnie Mae securities start at $25,000, with additional purchases in $1,000 increments. Fannie Mae and Freddie Mac securities trade in $1,000 increments, which puts them within reach of smaller portfolios.
For most people, mutual funds and ETFs focused on mortgage-backed securities are the simpler route. They hold diversified MBS pools and handle the plumbing: tracking prepayments, analyzing tranches, reinvesting returned principal. Funds heavy in agency MBS generally sit in the “intermediate government” category.
Tax Treatment
Interest income from an MBS is taxed as ordinary income at the federal level. The return of principal isn’t taxed, since it’s your own money coming back. When an MBS is structured as a Real Estate Mortgage Investment Conduit (REMIC), as most CMOs are, each regular interest is treated as a debt instrument for federal tax purposes, and income is reported on the accrual method.17GovInfo. 26 USC 860B – Taxation of Holders of Regular Interests
Interest on Ginnie Mae securities may qualify for exemption from state and local income taxes because Ginnie Mae is a federal government entity. Interest on Fannie Mae and Freddie Mac MBS does not, because those are government-sponsored enterprises rather than federal agencies. If you buy at a discount or a premium, the tax picture gets more complicated, so it’s worth talking to a tax professional before you invest.