API Mutual TLS Client Certificate Authentication

Status: public · Confidence: medium (0.865) · Basis: verified_sources

## TL;DR

Mutual TLS uses client certificates as an API authentication boundary, so agents must inspect certificate and trust-chain evidence before changing auth code.

## Core Explanation

API clients can fail before reaching application authorization when mutual TLS rejects a missing, expired, untrusted, or mismatched client certificate. That failure often appears as a TLS handshake error or gateway-level denial.

Agents should identify where TLS terminates, which CA bundle is trusted, which client certificate was presented, and whether certificate rotation or hostname constraints changed. The repair may be certificate distribution, not code.

## Source-Mapped Facts

- RFC 8705 defines certificate-bound access tokens and mutual-TLS client authentication for OAuth 2.0. ([source](https://datatracker.ietf.org/doc/html/rfc8705))
- Amazon API Gateway documentation describes configuring mutual TLS authentication for custom domain names. ([source](https://docs.aws.amazon.com/apigateway/latest/developerguide/rest-api-mutual-tls.html))
- Cloudflare API Shield documentation describes mutual TLS as requiring clients to present a client certificate. ([source](https://developers.cloudflare.com/api-shield/security/mtls/))

## Further Reading

- [RFC 8705 OAuth 2.0 Mutual TLS](https://datatracker.ietf.org/doc/html/rfc8705)
- [Amazon API Gateway Mutual TLS](https://docs.aws.amazon.com/apigateway/latest/developerguide/rest-api-mutual-tls.html)
- [Cloudflare API Shield Mutual TLS](https://developers.cloudflare.com/api-shield/security/mtls/)