Subordinate CA (Sub-CA, issuing CA)
Definition
Subordinate CAs are certificate authorities whose CA certificate is signed by another CA, typically a root; they usually issue end-entity certificates.
A subordinate CA (sub-CA) is a CA key pair together with the intermediate certificate a higher CA issued for it. The term names the authority, while "intermediate certificate" names its certificate, and the two are often used interchangeably. A sub-CA can be run by the same organization as the root or, as an externally operated sub-CA, by another company, and it may have sub-CAs of its own if its path length constraint allows it.
Why it matters
A sub-CA inherits the full trust of its root, so a compromised or misused sub-CA is as dangerous as a compromised root for every client that trusts that root. Root programs therefore track sub-CAs in CCADB and require audits, and a sub-CA can be limited technically with name constraints and EKU restrictions. Policies keep tightening: under Chrome Root Program Policy v1.8, new subordinate CAs disclosed from 2026-06-15 must assert only the serverAuth EKU. On 2026-10-20 Google and Mozilla confirmed compromised eMudhra intermediate CAs, and Chrome then removed the emSign ECC Root CA - G3 root.
Example
X509v3 Basic Constraints: critical
CA:TRUE, pathlen:0
X509v3 Extended Key Usage:
TLS Web Server Authentication
A sub-CA certificate like this may issue TLS server certificates but no further CA certificates.
Common misconceptions
- A sub-CA is less trusted than a root. Clients trust whatever chains to the root, so a sub-CA's certificates are exactly as trusted as the root's.
- A company can buy a public sub-CA and issue whatever it likes. Publicly trusted sub-CAs follow the same Baseline Requirements and audits; organizations that only need internal certificates use a private CA.