# What Is Cross-Signing (Cross-certificate)? Definition and Examples

> Cross-signing is when an already-trusted CA issues a certificate for another CA's key, letting that CA's certificates chain to roots older clients trust.

Source: https://ssl.sh/glossary/cross-signing/
Last updated: 2026-11-10

Cross-signing creates a second certificate for the same CA name and public key, signed by a different CA that clients already trust. Certificates issued with that key then have two possible paths: one to the new CA's own [root](https://ssl.sh/glossary/root-certificate/), and one through the cross-certificate to the older root. A client follows whichever [path](https://ssl.sh/glossary/certificate-chain/) ends in its trust store.

## Why it matters
Getting a new root into every trust store takes years, and old devices may never update. Cross-signing lets a new CA serve those clients immediately. [Let's Encrypt](https://ssl.sh/glossary/let-s-encrypt/) is the best-known case: its ISRG Root X1 was cross-signed by IdenTrust's DST Root CA X3. DST Root CA X3 expired on 2021-09-30, Let's Encrypt's default chain stopped using the cross-sign on 2024-02-08, and the cross-signed certificate itself expired on 2024-09-30. Each step broke some outdated clients, which shows that a cross-sign is a bridge, not a permanent fix.

## Example
The two paths a Let's Encrypt certificate could take before 2024:

```text
Path A: example.com -> intermediate -> ISRG Root X1 (self-signed, in modern stores)
Path B: example.com -> intermediate -> ISRG Root X1 (cross-signed) -> DST Root CA X3
```

The leaf and intermediate are identical in both paths; only the last links differ.

## Common misconceptions
- **A cross-signed certificate uses a different key.** It carries the same subject and public key as the original CA certificate, which is exactly why both paths verify.
- **Cross-signing means two CAs validated your domain.** Only the issuing CA validated it; the cross-signer vouches for the CA, not for your site.
- **An old root expiring only affects old devices.** When DST Root CA X3 expired, some OpenSSL 1.0.2-based clients failed even on updated systems, because they stopped at the expired path instead of trying the valid one.

## Related terms

- [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。
- [证书链](https://ssl.sh/zh/glossary/certificate-chain/): 证书链是从网站终端证书经中间证书一直到受信任根证书的有序证书序列，每一张都由下一张签名。
- [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。
- [信任库](https://ssl.sh/zh/glossary/trust-store/): 信任库是操作系统、浏览器或编程语言运行时内置的一组根 CA 证书，验证证书链时以它们作为信任锚。
- [Let's Encrypt](https://ssl.sh/zh/glossary/let-s-encrypt/): Let's Encrypt 是由非营利组织 ISRG 运营的免费、自动化证书颁发机构，通过 ACME 协议签发域名验证型 TLS 证书。
- [根证书计划](https://ssl.sh/zh/glossary/root-program/): 根证书计划是浏览器或操作系统厂商用来决定其信任库收录、限制或移除哪些根 CA 的政策与流程。