F5XC TLS Security Levels Decoded: What High/Medium/Low Negotiate, and Why Your Scanner Flags a Load Balancer

Four ways to set TLS on an XC load balancer

An XC HTTP load balancer offers three predefined TLS security levels - High, Medium, and Low - plus Custom. Each predefined level is a shorthand for two things at once: a minimum and maximum TLS version, and a set of cipher suites. Custom lets you choose the versions and suites yourself.

Default is High

The point people miss is that Default is High. An HTTPS load balancer with an automatic certificate uses the Default level, and the Default level is the High level: minimum TLS 1.2, maximum TLS 1.3, and a short list of forward-secret AEAD ciphers - the TLS 1.3 suites plus ECDHE-ECDSA and ECDHE-RSA with GCM or ChaCha20. That is a strong posture out of the box.

The levels are cumulative

Going down from High does not swap the cipher list, it extends it. Medium keeps every High cipher and adds four ECDHE-CBC-SHA suites - still forward-secret, but CBC with SHA-1 rather than AEAD, so medium strength - and lowers the minimum version to TLS 1.0. Low keeps every Medium cipher and adds four static-RSA suites, and static RSA means no forward secrecy at all. So Low is a superset of Medium, which is a superset of High. Every level still caps at TLS 1.3.

Why a scanner flags TLS 1.0 or 1.1

This is the first of two questions F5 answers in a dedicated article. If a scanner reports TLS 1.0 or 1.1 enabled on your load balancer, the load balancer is on Medium or Low - those are the levels whose minimum version is TLS 1.0. The Default/High level is minimum TLS 1.2 and will not present the old protocols. So the fix for a TLS 1.0 finding is usually not a mysterious hidden setting; it is that the load balancer is on a lower predefined level than you thought (K000148226).

Why a scanner flags weak ciphers

The second question. If a scanner reports weak ciphers, it is almost always the static-RSA suites the Low level adds. Static RSA key exchange provides no forward secrecy, which every modern scanner grades down regardless of the symmetric cipher. If you are on Low for a legacy-client reason and a scanner is unhappy, those RSA suites are why (K000148079).

Choosing a level

For a modern posture, stay on High (the default) - it is TLS 1.2 and 1.3 with forward-secret AEAD ciphers, and with HSTS added it earns an A or A+ on public TLS graders. Drop to Medium or Low only for a specific legacy-client requirement, and know exactly what you are re-enabling: older protocols at Medium, and non-forward-secret ciphers at Low. The companion mapper shows the exact versions and cipher list for any level, and tells you which levels a given cipher belongs to when you are reading a scan.