OpenBMC bmcweb mTLS client-certificate UPN validation: Where mTLS is configured, bmcweb matches the certificate's UPN

Impact

Where mTLS is configured, bmcweb matches the certificate's UPN by walking dot-separated labels with no bound. A certificate issued for user@com authenticates against any host under that TLD, and a parent-domain certificate authenticates against every child deployment. For an operator running mTLS across a fleet - which is the hardened configuration, chosen by the most security-conscious teams - this means one certificate from anywhere in the hierarchy authenticates to every BMC in it, silently, with no failed-auth log to notice. It is the rare bug that punishes you specifically for having done the harder thing.

Who can reach it

Requires mTLS to be enabled on the BMC (not the default) and possession of any certificate the BMC's trust store chains to, including one issued for a parent domain or a different deployment. Network access to the BMC's HTTPS port.

What to do

Unpatched at disclosure. An April 2026 commit fixed only case-insensitivity in the comparison and left the suffix-walking logic intact, so do not assume a recent bmcweb clears it. If you run mTLS on BMCs, audit which CAs are in each BMC's trust store and narrow them to a per-fleet issuing CA that signs nothing else - that is a config-only change and is the effective mitigation today. Do not put a broadly-scoped corporate CA in a BMC trust store. A real fix will require a BMC firmware flash once upstream lands one.

References