MBS
MBS (Mortgage-Backed Security)
Quick Definition
A mortgage-backed security (MBS) is a type of bond collateralized by a pool of mortgage loans. As homeowners make monthly mortgage payments (principal and interest), those cash flows pass through to MBS investors. Agency MBS are issued or guaranteed by Fannie Mae, Freddie Mac, or Ginnie Mae and carry the backing of the US government. Non-agency MBS carry credit risk from the underlying loans.
What It Means
Before MBS existed, banks originated mortgages, held them on their balance sheets, and had limited capital to make new loans. By packaging mortgages into tradable securities, banks could sell the loans and recycle capital into new mortgages, expanding credit availability dramatically. The 30-year fixed-rate mortgage that American homebuyers rely on exists in large part because of the MBS market.
The Federal Reserve became one of the largest participants in the MBS market through its quantitative easing programs. At its peak in mid-2022, the Fed held approximately $2.7 trillion in agency MBS. As of Q2 2026, Fed holdings have fallen below $2 trillion as the central bank allows MBS to roll off its balance sheet, though the pace of runoff remains slower than the stated $35 billion per month cap due to slower prepayments.
How MBS Are Created: The Securitization Process
- Origination: A bank or mortgage company originates home loans
- Pooling: Hundreds or thousands of similar mortgages are bundled into a pool
- Issuance: A trust issues MBS backed by the pool; investors buy the securities
- Servicing: A servicer collects monthly payments from homeowners
- Pass-through: Principal and interest payments pass through to MBS investors (minus servicing fee)
Agency vs. Non-Agency MBS
| Type | Issuer/Guarantor | Credit Risk | Yield vs. Treasuries |
|---|---|---|---|
| Agency MBS (pass-through) | Fannie Mae, Freddie Mac | Backed by US government | Spread of 50-200 bps over Treasuries |
| Ginnie Mae (GNMA) | Full faith and credit of US government | Zero credit risk | Tightest spreads |
| Non-agency (private label) | Private banks/trusts | Full credit risk from underlying loans | Wider spreads; varies by quality |
Fannie Mae and Freddie Mac are government-sponsored enterprises (GSEs) that buy conforming mortgages from banks, pool them, and guarantee the resulting MBS against default. If a homeowner defaults, Fannie or Freddie makes the MBS investor whole for principal and interest.
Conforming loan limits for 2026 are $806,500 for most US counties, with higher limits in high-cost areas. Loans above this limit are "jumbo" and not eligible for agency MBS.
MBS Cash Flow Complexities
Unlike a regular bond with fixed principal repayment at maturity, MBS cash flows are unpredictable:
| Complexity | Description |
|---|---|
| Prepayment risk | Homeowners refinance or sell homes, returning principal early; speeds up when rates fall |
| Extension risk | When rates rise, refinancing slows, extending the MBS duration beyond expected |
| Negative convexity | MBS price appreciation is capped when rates fall (prepayments accelerate); prices fall normally when rates rise |
| Contraction risk | Principal returned faster than expected during rate declines; must reinvest at lower rates |
This prepayment unpredictability makes MBS more complex to analyze than standard bonds. Investors cannot know exactly when they will receive their principal.
CMOs: Addressing Prepayment Risk
Collateralized Mortgage Obligations (CMOs) restructure MBS cash flows into tranches with different prepayment priorities:
| CMO Tranche | Prepayment Priority |
|---|---|
| PAC (Planned Amortization Class) | Stable, predictable cash flows within a band; most desirable |
| TAC (Targeted Amortization Class) | Protected on one side only (less stable) |
| Z-bond (Accrual) | Receives no cash flows until other tranches are retired; last to be paid |
| Support/Companion | Absorbs excess prepayments to protect PAC tranches; most volatile |
| IO (Interest Only) | Receives only interest; profits when prepayments are slow |
| PO (Principal Only) | Receives only principal; profits when prepayments accelerate |
The Federal Reserve and MBS in 2026
The Federal Reserve's relationship with agency MBS has shifted significantly. Between June 2022 and October 2025, the Fed's securities holdings declined by $2.2 trillion under its balance sheet reduction plans. In December 2025, the FOMC announced that reserve balances had declined to ample levels and initiated purchases of shorter-term Treasury securities to maintain reserves going forward. However, the roll-off of MBS holdings has persisted, with up to $35 billion of MBS rolling off each quarter, the proceeds reinvested in short-dated Treasuries.
As of March 2026, the Fed's agency MBS holdings stood at approximately $1,997 billion, down from $2,099 billion in September 2025, according to the Federal Reserve's balance sheet reports. The Fed's MBS portfolio is now dominated by 2% and 2.5% coupon MBS, a legacy of its QE purchases that focused on the coupons where origination was highest.
As the Fed has decreased its MBS portfolio, money managers and overseas investors have become the marginal buyers of MBS. Foreign demand, particularly from the UK and Japan, has been supportive in 2026.
MBS Spreads and Mortgage Rates in 2026
The MBS spread over Treasuries directly affects consumer mortgage rates:
30-year mortgage rate is approximately 10-year Treasury yield plus MBS spread plus origination margin
| Period | 10yr Treasury | MBS Spread | 30yr Mortgage Rate |
|---|---|---|---|
| 2021 (QE peak) | 1.5% | 0.8% | ~3.0% |
| 2022 (rate hike peak) | 4.2% | 1.5% | ~7.0% |
| Q4 2025 | 4.84% | ~1.1% | ~6.15% |
| Q2 2026 | 4.14% (2yr) to higher | +107 bps current coupon | ~6.5%+ |
According to Morgan Stanley's Q2 2026 Agency MBS and Housing Market Monitor, current coupon spreads tightened 2 bps to +107 bps over Treasuries. The Bloomberg US MBS Index yield rose to 4.89%, while the current coupon yield ended the quarter at 5.41%. Mortgage rates remain above long-term averages, driven by persistently elevated long-end Treasury yields, which continues to limit refinancing and housing market activity. This keeps MBS supply light and provides support to Agency MBS spreads.
The agency MBS market posted its third consecutive calendar year return in 2025, finishing up 8.33%. The index has now climbed out of the 2022 hole, which marked the worst calendar year on record for agency MBS total returns. The Agency MBS market is the second most liquid US fixed income market, behind only Treasuries.
Key Points to Remember
- MBS packages mortgage loans into tradable bond-like securities, enabling recycling of bank capital
- Agency MBS (Fannie, Freddie, Ginnie) are government-guaranteed; non-agency carry credit risk
- Prepayment risk is the defining complexity; homeowners' refinancing decisions make cash flows unpredictable
- Negative convexity means MBS behave worse than regular bonds in both rising and falling rate environments
- The Federal Reserve's MBS holdings fell below $2 trillion by Q2 2026, down from a peak of $2.7 trillion
- MBS spreads directly affect 30-year mortgage rates for American homebuyers
Common Mistakes to Avoid
- Assuming all MBS are risky: Agency MBS backed by Fannie Mae, Freddie Mac, or Ginnie Mae carry effectively zero credit risk. The 2008 crisis involved non-agency MBS backed by subprime loans, not agency MBS
- Ignoring prepayment risk when buying MBS: When rates fall, homeowners refinance and your principal comes back faster than expected, forcing reinvestment at lower rates. When rates rise, prepayments slow and your duration extends. Both scenarios can hurt returns
- Confusing MBS with CDOs: CDOs are collateralized debt obligations that may hold MBS tranches but also contain other debt. MBS are specifically backed by mortgage loans
- Overlooking the Fed's influence: The Fed's MBS holdings and its pace of balance sheet runoff directly affect MBS spreads and, by extension, the mortgage rates consumers pay
Frequently Asked Questions
Q: Is investing in MBS safe? A: Agency MBS (backed by Fannie Mae, Freddie Mac, Ginnie Mae) carry effectively zero credit risk. The government backstop means you will be paid principal and interest even if homeowners default. The risks are interest rate risk and prepayment risk. Your investment may be returned earlier (refinancing boom) or later (rising rate environment) than expected. Non-agency MBS carry real credit risk and require careful analysis.
Q: How did MBS contribute to the 2008 crisis? A: Non-agency MBS backed by subprime and Alt-A mortgages of very poor quality were issued in massive quantities from 2003 to 2007. Rating agencies gave AAA ratings to tranches of these securities based on flawed models. When housing prices fell and default rates spiked far above model assumptions, these MBS collapsed in value. The losses cascaded through CDOs (which held MBS tranches), then to AIG (which had sold credit default swaps on CDOs), triggering the systemic crisis.
Q: Can retail investors buy MBS? A: Yes, through ETFs and mutual funds. Vanguard Mortgage-Backed Securities ETF (VMBS), iShares MBS ETF (MBB), and many bond funds hold agency MBS. For individual agency MBS, minimum purchases are typically $25,000, and the prepayment complexity makes them better suited to institutional investors with specialized analytics.
Q: What happens to MBS when the Federal Reserve sells its holdings? A: The Fed is not selling MBS outright. It allows them to roll off its balance sheet as homeowners pay down or refinance their mortgages, with principal payments not reinvested. This gradual reduction reduces demand in the MBS market, which can widen spreads and put upward pressure on mortgage rates. However, money managers and foreign investors have absorbed much of the supply, limiting the impact.
Related Terms
ABS
An asset-backed security is a bond-like investment backed by a pool of consumer loans such as auto loans, credit card receivables, or student loans that generates cash flows passed through to investors.
Underwriting
Underwriting is the process by which an insurer evaluates risk to decide whether to offer coverage and at what premium. In 2026, AI and machine learning are transforming underwriting across insurance and mortgage lending, with Fannie Mae and Freddie Mac issuing formal governance frameworks.
Basis Point
A basis point is one one-hundredth of a percentage point (0.01%), the standard unit for interest rates, bond yields, and fee changes in finance, enabling precise communication about small rate movements.
Callable Bond
A callable bond gives the issuer the right to redeem the bond before maturity at a predetermined price, typically exercised when interest rates fall so the issuer can refinance at lower rates.
CDO
A CDO is a structured financial product that pools debt instruments and slices them into tranches with different risk and return profiles. CDOs fueled the 2008 financial crisis, but CLOs, a surviving variant, now represent a $1.5 trillion market.
Corporate Bond
A corporate bond is debt issued by a company to raise capital, paying investors regular interest and returning principal at maturity, with yields higher than government bonds to compensate for credit risk.
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