# What Is Subordinate CA (Sub-CA, issuing CA)?

> Subordinate CAs are certificate authorities whose CA certificate is signed by another CA, typically a root; they usually issue end-entity certificates.

Source: https://ssl.sh/glossary/subordinate-ca/
Last updated: 2026-11-10

A subordinate CA (sub-CA) is a CA key pair together with the [intermediate certificate](https://ssl.sh/glossary/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](https://ssl.sh/glossary/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
```text
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](https://ssl.sh/glossary/private-ca/).

## Related terms

- [中间证书](https://ssl.sh/zh/glossary/intermediate-certificate/): 中间证书是由根证书（或上级中间证书）签发、用来签发网站证书的 CA 证书，服务器必须把它和网站证书一起发送给客户端。
- [根证书](https://ssl.sh/zh/glossary/root-certificate/): 根证书是位于证书链顶端、由 CA 自签名的证书，预装在操作系统或浏览器的信任库中，作为验证其他证书的信任锚。
- [名称约束](https://ssl.sh/zh/glossary/name-constraints/): 名称约束是 CA 证书中的 X.509 扩展，用来限定该 CA 之下签发的证书只能包含哪些域名、IP 段、邮箱或 DN。
- [EKU](https://ssl.sh/zh/glossary/extended-key-usage/): EKU 即扩展密钥用途，是列出证书可用于哪些目的（如 serverAuth 服务器认证、clientAuth 客户端认证、代码签名）的 X.509 扩展。
- [证书颁发机构](https://ssl.sh/zh/glossary/certificate-authority/): 证书颁发机构（CA）是负责审核申请者、签发数字证书，并把公钥与域名或身份绑定在一起的机构。
- [CCADB](https://ssl.sh/zh/glossary/ccadb/): CCADB 是通用 CA 数据库，CA 在这里向各根证书计划披露其根证书、中间证书、审计报告、政策文件和 CRL 地址。
- [基本约束](https://ssl.sh/zh/glossary/basic-constraints/): 基本约束是 X.509 证书扩展，用来声明该证书是否属于 CA，以及其下方还允许出现多少层 CA。