The EU Carbon Border Adjustment Mechanism (CBAM) requires importers to declare the embedded carbon intensity of regulated goods including steel, aluminium, cement, fertilisers, and electricity. In practice, the majority of non-EU suppliers do not have the monitoring systems required to produce the specific carbon intensity data format that CBAM declarations require. This analysis explains the structural data gap, the default values CBAM authorities will apply when supplier data is unavailable, and the financial exposure created by defaulting to conservative estimates.