A certificate authority (CA) is a trusted entity that issues and revokes public key certificates. A certificate binds a public key to a name, such as a website, and carries the CA's signature. Clients start from a limited number of CA keys they trust, so whoever holds a weak or stolen CA key can issue certificates that look genuine.
- Also Called1other nameCertification authority
- Related Terms2linked entriesAdversary-in-the-middle (AiTM) · Supply chain attack
How Does a Certificate Authority Work?
NIST defines a CA as a trusted entity that issues and revokes public key certificates. RFC 5280 says a certificate binds a public key value to a subject, and the CA’s digital signature asserts that binding.
Clients start from a limited number of CA public keys they trust. RFC 5280 describes a certification path: the website’s certificate is signed by one CA, plus zero or more CA certificates signed by other CAs, leading back to a trusted key. The CA at the top of that chain, whose key the client trusts, is usually called the root, and the CAs between it and the website’s certificate are called intermediates.
Why Does a Certificate Authority Matter?
Clients accept whatever a trusted CA signs. In October 2026 eight intermediate CA keys of eMudhra were reported as recoverable, because their private values were tiny integers below 256 (Mozilla bug 2083449). The eMudhra CA key compromise incident page has the dated record. Anyone with such a key could issue a certificate for any website. Combined with network interception, that enables an adversary-in-the-middle attack.
How Do You Defend Against a CA Failure?
- Keep the operating system and the CA bundles of your tools up to date, including those inside container images.
- Use revocation checking where your client supports it. RFC 5280 describes the certificate revocation list, a signed, time-stamped list of revoked certificates.
- Know which tools carry their own CA bundle, since removing a root from the operating system does not change them.
- Watch for certificates issued for your own names that you did not request.
Also Called.
- NameCertification authority
Questions.
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What is the difference between a root CA and an intermediate CA?
RFC 5280 describes a certification path. An end certificate is signed by one CA, and that CA's certificate is signed by other CAs up to a key the client trusts. The trusted top of the chain is the root; the CAs in between are intermediates.
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What happens when a CA is distrusted?
Clients stop accepting certificates that chain to it once their trust lists are updated. For eMudhra, Mozilla decided to remove the emSign ECC Root CA - G3, and software with its own CA bundle stays exposed until updated.
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How is a certificate revoked?
The CA lists it on a certificate revocation list, a time-stamped list of revoked certificates signed by the CA, which RFC 5280 describes. eMudhra revoked its eight affected intermediates on 18 October 2026.
Sources
In Our Research
- 1 eMudhra CA Key Compromise: What Python, uv and Container Teams Must Update Now October 2026
- 2 AI Gateway Exploitation: How Attackers Are Targeting LiteLLM, RAGFlow, and Kestra to Steal Your Model Provider Keys August 2026
- 3 Expired SSL Certificates: How a Lapsed Cert Becomes an Attack Vector July 2026
- 4 Lookalike Domains: How a Single Character Difference Becomes a Brand’s Worst Nightmare May 2026
- 5 PetitPotam Vulnerability Analysis Open Report April 2023
- 6 Preliminary Information Released for OpenSSL Vulnerability November 2022