TLS 1.3 includes a backwards-compatible hardening measure to strengthen downgrade protections. However, when we shipped TLS 1.3 last year, we had to partially disable this measure due to incompatibilities with some non-compliant TLS-terminating proxies. Chrome currently implements the hardening measure for certificates which chain up to known roots, but allows a bypass for certificates chaining up to unknown roots. We intend to enable it for all connections.


Downgrade protection mitigates the security impact of the various legacy options we retain for compatibility. This means user’s connections are more secure and, when security vulnerabilities are discovered, it is less of a scramble to respond to them. (That, in turn, means fewer broken sites for users down the road.) This also aligns with RFC 8446.


Specification link

Final published standard: Recommendation, Living Standard, Candidate Recommendation, or similar final form

Status in Chromium


Removed (tracking bug)

Consensus & Standardization

After a feature ships in Chrome, the values listed here are not guaranteed to be up to date.


Search tags

ssl, tls, tls13,

Last updated on 2021-06-19