What is a downside of using a single CA model?

Choosing a single Certificate Authority (CA) model presents significant risks, one of which is that the CA server becomes highly exposed since it is responsible for issuing all certificates directly. This centralization means that if the single CA is compromised, all certificates issued by it are potentially at risk. The attack surface of the CA increases because it becomes a critical point of failure; a compromise could lead to fraudulent certificates being issued, which can undermine the integrity of the entire system relying on that CA. Furthermore, managing a single CA can lead to bottlenecks and reduced resilience, as any operational issues with the CA can disrupt the issuance and renewal of certificates for all entities using its services.

In contrast, the other options do not adequately reflect the inherent vulnerabilities or limitations of a single CA model. For instance, certificates being trusted by multiple providers hints at a distributed trust model rather than a single authority, while the absence of trust between different CAs relates more to decentralized structures rather than the limitations of a single CA. Similarly, the ability to allow multiple intermediate CAs to issue certificates is generally a feature associated with hierarchical models rather than an inherent drawback of utilizing a single CA.