A conduit commercial mortgage-backed security, or conduit CMBS, is a bond backed by a diversified pool of commercial real estate loans that multiple lenders originated against standardized criteria and sold into a single trust. The trust issues bonds representing slices of the cash flow from those loans, and investors buy the slices that match their appetite for risk and yield. The “conduit” label refers to the assembly-line nature of the process: many loans, many borrowers, one securitization.
The underlying loans are made against office buildings, retail centers, warehouses, apartment complexes, and other income-producing properties spread across different markets. Bondholders never own the buildings. They own a right to a defined share of the mortgage payments those buildings generate.
What Kinds of Loans Go Into the Pool
A loan has to fit a standardized underwriting box before it can be securitized in a conduit deal, because rating agencies and investors need to be able to model the pool. The properties are typically stabilized, meaning they have an established record of occupancy and income. Loan sizes generally start at $2 million, with most falling between $2 million and $50 million.
Two ratios do most of the underwriting work. Loan-to-value is capped at 75% of the appraised property value, which forces the borrower to have real equity in the deal. Debt service coverage ratio has to be at least 1.25, meaning the property’s net operating income covers annual debt payments 1.25 times over. Lenders also look at debt yield, defined as net operating income divided by the loan amount, with 10% a common floor.1Trepp. CMBS 101: Trepp’s Essential Guide to the Life of A CMBS Loan Part 1
Almost every conduit loan is a ten-year, fixed-rate instrument amortizing on a 25- or 30-year schedule. That structure leaves a large balloon payment due at maturity, which the borrower expects to cover by refinancing or selling the property. If neither works, the loan defaults. Balloon risk is one of the defining features of the product.
Conduit loans are also non-recourse. If a borrower defaults, the lender’s remedy is limited to the collateral property, with narrow “bad boy” carve-outs for fraud or intentional misconduct. The lender cannot pursue the borrower’s other assets. That risk shift ultimately lands on the bondholders.
How the Loans Become Bonds
Lenders originate conduit-eligible loans and sell them to a sponsor, usually a large investment bank. The sponsor aggregates loans from several originators and transfers them into a special purpose vehicle, a legal entity created solely to hold the pool. Once the loans are inside the SPV, they belong to the trust, not to the sponsor or the original lenders. If any of those firms later fails, its creditors cannot reach the loans. This bankruptcy remoteness is the structural foundation of every CMBS deal.
For tax purposes the SPV is almost always structured as a Real Estate Mortgage Investment Conduit (REMIC). A REMIC must meet specific Internal Revenue Code requirements, including that substantially all of its assets be qualified mortgages, that it have exactly one class of residual interests, and that it use a calendar taxable year.2Office of the Law Revision Counsel. 26 USC 860D – REMIC Defined The REMIC election allows income to pass through to bondholders without an entity-level tax, which is what makes the economics work.
The SPV then issues bonds against the cash flow from the pooled loans and sells them to institutional investors. Proceeds flow back through the sponsor to the original lenders. Every mechanical detail of the deal thereafter is governed by the Pooling and Servicing Agreement (PSA), which sets out how payments get collected, how cash moves to different bond classes, who services the loans, and what happens when a loan goes bad.
The Tranche Waterfall
The single most important feature of a conduit CMBS is how the pool’s cash flow gets sliced. The bonds are divided into tranches, each carrying a different credit rating, yield, and position in the payment order. That is how one pool can serve both conservative pension funds and hedge funds hunting yield.
Senior tranches rated AAA sit at the top. They get paid first and are the last to absorb losses. Their protection comes from subordination: every tranche below them is a buffer. Losses eat through the bottom of the stack before they ever touch the AAA bonds. The tradeoff is a lower yield.
Mezzanine tranches carry ratings from AA down through BBB. They pay more than senior bonds and take losses before the AAA holders do.
At the bottom is the first-loss piece, known as the B-piece. If a loan defaults and generates a loss, the B-piece absorbs it first, and losses only cascade upward once the piece below is wiped out. The B-piece pays the highest yield in the deal, and the buyer taking that position ends up with far more influence over the transaction than the yield alone would suggest.
Why the B-Piece Buyer Matters
Because the B-piece buyer eats the first dollar of loss, they have every reason to scrub the pool before it closes. B-piece buyers run their own due diligence, including independent environmental reviews and property condition assessments. They hold “kick-out” rights that let them force the sponsor to remove specific loans from the pool before securitization. A property their consultant flags gets dropped.
Their influence continues after issuance. The B-piece buyer or an affiliated entity typically serves as the special servicer, or at least holds the right to appoint and replace it. The most subordinate outstanding bond class, called the controlling class, can designate a representative who weighs in on workout decisions for troubled loans. The special servicer retains ultimate authority under the PSA, but the controlling class receives detailed notifications about foreclosures, loan modifications, discounted payoffs, collateral releases, and property management changes. The party absorbing the first losses ends up with the loudest voice in how distressed assets get handled.
Who Runs the Loans After Issuance
Once the bonds are sold, day-to-day administration falls to specialized entities defined in the PSA.
Master Servicer
The master servicer handles routine administration for performing loans: collecting monthly payments, managing tax and insurance escrows, monitoring covenants, and passing collected funds to the trustee. The fee is a small percentage of outstanding loan balances. As long as a loan is current, the master servicer is the only entity the borrower deals with.
Special Servicer
When a loan goes sideways, it transfers to the special servicer, who is set up to maximize recovery on distressed assets. The triggers for a transfer are broader than plain payment default. A loan can move to special servicing for repeated late payments, unpaid taxes or insurance, occupancy falling below a defined threshold, the loss of an anchor tenant, an approaching maturity with no refinancing lined up, or physical deterioration of the property. The special servicer then works toward resolution, whether by restructuring the loan, negotiating a discounted payoff, foreclosing, or selling the property. Special servicers earn higher fees than master servicers, including workout fees tied to successful resolutions.
Trustee
The trustee acts as fiduciary for the bondholders, holding legal title to the mortgages on their behalf. The trustee makes sure the master and special servicers follow the PSA, receives their reports, and distributes principal and interest according to the waterfall. The trustee does not make loan-level decisions.
Prepayment Is Locked Down
Investors buying CMBS bonds are counting on a predictable stream of interest payments, so the deal aggressively protects that income. Conduit loans make paying off the loan early either impossible or expensive.
For at least the first two years after securitization, REMIC tax rules prohibit prepayment entirely. After that lockout, a borrower can exit, but only through one of two costly mechanisms.
Defeasance requires the borrower to buy a portfolio of government securities that replicate the remaining scheduled loan payments. Those bonds are transferred to a newly created entity that assumes the debt, and the original borrower walks away. The borrower still pays for every remaining interest payment, just indirectly through the bond portfolio. The process takes 30 to 45 days, involves several third-party professionals, and gets expensive when Treasury yields are low relative to the loan’s coupon.
Yield maintenance is simpler. The borrower pays off outstanding principal plus a penalty equal to the present value of the remaining loan payments, discounted at the yield on the Treasury security closest to the loan’s maturity. It’s faster than defeasance but can still be a big number when rates have fallen since origination.
Many conduit loans open up to penalty-free prepayment in the last few months before maturity. For most of the loan’s life, though, early exit is deliberately punitive. Borrowers who expect to sell or refinance within a few years often underestimate what leaving costs.
Risk Retention: Skin in the Game
After the 2008 crisis exposed how securitization could sever origination from the consequences of default, federal regulators required sponsors to keep exposure to the deals they package. Under Section 941 of the Dodd-Frank Act, the sponsor of a CMBS securitization must retain at least 5% of the credit risk of the securitized assets, and cannot hedge or sell off that retained piece.3SEC. Credit Risk Retention Final Rule
The sponsor can hold that 5% vertically (a strip of every tranche), horizontally (the most subordinate tranche equal to 5% of the fair value of all bonds issued), or in a combination.4eCFR. 17 CFR Part 246 – Credit Risk Retention In practice, most conduit deals satisfy risk retention horizontally, with a third-party B-piece buyer holding the required first-loss position for a minimum of five years. The rule, first enforced in late 2016, reduced the number of firms with the capital to sponsor conduit deals.
Main Risks for Investors
Conduit CMBS carries several risks beyond ordinary fixed-income credit risk, and which ones matter most depends on where an investor sits in the tranche stack.
- Balloon and extension risk. With ten-year loans on 25- to 30-year amortization schedules, every borrower faces a large balloon at maturity. If refinancing or a sale falls through, the loan defaults. The special servicer may extend or modify the terms, which delays payments to bondholders and stretches the bond’s effective life past what investors modeled.
- Concentration risk. Conduit deals are diversified by design, but individual pools can end up heavy in a particular property type or market. A pool weighted toward office properties carries different risk than one weighted toward industrial warehouses. The AAA label doesn’t tell you what’s in the pool.
- Interest rate risk. Because the underlying loans are fixed rate and prepayment is restricted, bondholders can’t benefit from reinvestment when rates fall. When rates rise, the bonds lose market value like any fixed-rate instrument, and the call protection means the portfolio doesn’t naturally turn over.
- Credit risk. The bonds ultimately depend on the properties generating income. Tenants leave, markets soften, values fall. Subordination protects senior tranches from moderate losses, but a severe downturn can erode even highly rated bonds once losses exceed the credit enhancement below them.
Conduit CMBS vs. Single-Borrower CMBS
Not every CMBS is a conduit deal. The private-label CMBS market splits into conduit deals and single-asset, single-borrower (SASB) deals, which securitize one large loan against one property or portfolio controlled by a single borrower. In 2024, SASB issuance accounted for roughly 68% of the $125.6 billion in private-label CMBS issued that year, making conduit deals the smaller share of the market.5Trepp. Private-Label CMBS Market Issuance Increased 21% in 2025
The distinction matters because the risk profile is different. A SASB deal concentrates all exposure in one property and one borrower, with no diversification cushion if things go wrong. A conduit deal spreads risk across dozens of properties, geographies, and borrowers, so a single default has a smaller effect on the pool. Diversification is the reason conduit CMBS exists as a product.