Money & Finance › Bubbles, crashes & crises › Subprime mortgages and the housing bubble
CDOs: repackaged mortgage debt
Peak 2006–2007· Asset or instrument
Fact, law or dataA rule, statute, definition or measured number.
Collateralized debt obligations: securities made by pooling debts, often mortgage bonds, and slicing the pool into layers ranked by who takes losses first.
See it on the Money & Finance map →How it works
A CDO buys a pool of bonds or loans and issues tranches. Cash flows down a 'waterfall': senior tranches are paid first and rated AAA, mezzanine next, and the equity slice takes the first losses for the highest yield. By 2005–07 many CDOs were built from the riskier BBB slices of subprime mortgage bonds, yet most of each deal was rated AAA, on the assumption that defaults would not all hit together. When home prices fell nationwide, they did.
Why it matters
CDOs spread subprime losses into banks, insurers and funds worldwide, helping turn a US housing downturn into the 2008 global financial crisis.
Think of it like
Like a capital stack of senior loan, mezzanine and sponsor equity, except every property in the pool sat in the same overheated market.
Facts, rules and data
The Financial Crisis Inquiry Commission's 2011 report; SIFMA data showing roughly $500 billion of global CDO issuance in 2006; the SEC's $550 million 2010 settlement with Goldman Sachs over its Abacus CDO.
Debates and risks
How blame splits among flawed rating models, conflicts of interest and investors who never looked inside is still debated. CLOs, a cousin built from business loans, remain a large market.
Connections (4)
▸Investment banks: underwriting, deals and trading packaged and sold CDOs: repackaged mortgage debtFact, law or data
The connection
Investment banks bought mortgage bonds, pooled them into CDOs, paid rating agencies, and sold the tranches to investors worldwide, earning fees at each step. Many also kept the 'super-senior' slices they could not sell, believing them safe. Those produced some of the crisis's biggest losses: Merrill Lynch, Citigroup and UBS wrote down tens of billions.
When and why
New York and London, 2004–2007; leading underwriters included Merrill Lynch, Citigroup, Goldman Sachs and Deutsche Bank.
How we know
The Financial Crisis Inquiry Commission report and the banks' own writedown disclosures; the SEC's 2010 Goldman Abacus case showed how one deal was assembled.
Debates and risks
How well bankers understood the risks they kept or sold, versus being fooled by their own models, is still argued.
▸Credit rating agencies: Moody's, S&P and Fitch gave AAA ratings to CDOs: repackaged mortgage debtFact, law or data
The connection
CDOs sold because most of each deal won AAA, letting pensions, insurers and banks buy them. Agencies used models assuming mortgage defaults would not rise together nationwide, and banks paid them per deal, a conflict critics say encouraged generous grades. When housing fell, the agencies cut tens of thousands of ratings, many from AAA to junk.
When and why
New York, 2004–2007, mainly Moody's and Standard & Poor's, with Fitch; mass downgrades came in waves from July 2007.
How we know
The Financial Crisis Inquiry Commission documented the ratings and downgrades; S&P paid $1.375 billion (2015) and Moody's $864 million (2017) in US settlements.
Debates and risks
Whether issuer-paid ratings were the main fault or investors' outsourcing of judgment, and whether the 2010 Dodd-Frank reforms fixed the problem, is debated.
▸Mortgage-backed securities (MBS) were repackaged into CDOs: repackaged mortgage debtFact, law or data
The connection
Private-label subprime mortgage bonds were sliced into tranches, and the lower-rated BBB slices were hard to sell. CDO managers bought them by the hundred, pooled them, and re-sliced the pool so most new tranches were rated AAA. Some CDOs then bought slices of other CDOs ('CDO-squared'), stacking the same mortgage risk again.
When and why
Wall Street structuring desks, 2004–2007; by 2006 slices of subprime mortgage bonds made up most of the collateral in new CDOs of asset-backed securities.
How we know
Deal documents analyzed by the Financial Crisis Inquiry Commission show the collateral; losses on those CDOs tracked subprime default rates closely.
Debates and risks
How much CDO demand itself pushed lenders to keep making weak loans, as many believe, is hard to measure precisely.
▸Syndicated and leveraged loans, and the CLOs that buy them are pooled into CLOs, cousins of CDOs: repackaged mortgage debtFact, law or data
The connection
A CLO is a type of CDO built from 150–300 leveraged loans instead of mortgage bonds. It sells layered notes, AAA at the top and equity at the bottom, and a manager actively trades the loans. Unlike 2000s subprime CDOs, CLOs hold diversified corporate loans, and very few AAA CLO notes have ever defaulted.
When and why
CLOs grew from the late 1990s; US new issuance set a record of about $209 billion in 2025.
How we know
S&P and Moody's default studies show CLO tranches held up far better than mortgage CDOs in 2008–2010.
Debates and risks
Whether CLOs' strong record survives a downturn with covenant-lite loans and lower recoveries.