A Delivery Chain Breach: A UK bank opened the back door to China

In a supply chain attack against a system that produces some artefact (a container image, an executable, a VM image, whatever) an attacker can't modify what was injected after the fact (without building in some kind of remote update/attack persistence into the injection). This means that it could be audited and found later, for example the various projects designed to scan for the xz backdoor in executables.

But with this not only could the "attacker" (the CDN owner) change what is injected at any future time, there's no record of what was done as the author points out. A clever attacker could even selectively adjust what is sent based on the requester. They could distribute the unmodified code 99.99% of the time, but distribute malware 0.01% of the time — making it extremely hard to detect with simple 'spot checks'. They could exclude IPs known to be associated with the bank or CI systems from the malware distribution, so in-house malware checks would never see it.

Combined with other forms of intelligence, you could even build a system to target specific users with malware for very targeted attacks.

However, from the bank's perspective, of their supply chain, the component shown to a customer has not been handled by them: they never received it, to supply it.

At best they can order a copy of it for quality assurance purposes and hope it is identical to what the customer will receive.

Instead, their web page sends an order to the third party for delivery (script src tag refers to a foreign location) and crosses their finger that all will be good.

There are mechanisms to reduce delivery chain risks, such as SRI, but they were not used.