All published s2s-proxy versions through 0.2.2 are affected. In versions 0.1.16 through 0.2.2, TLS server listeners use Go's RequireAnyClientCert mode when skipCAVerification is false. This mode checks that the client holds the certificate's private key but does not verify the certificate against the configured CA. An attacker can therefore use a self-signed certificate and key to establish a TLS and yamux connection, then invoke RPCs allowed by the proxy's configuration and Temporal credentials. No certificate or private key trusted by the deployment, and no Temporal credential, is required.
Exploitation requires network access to an affected TLS server listener configured with skipCAVerification set to false. Available RPCs and namespaces are limited by the proxy's allowlists and the permissions granted to its Temporal credentials.
Restrict the listener to known peer addresses or a private network, or use a TLS terminator that verifies client certificates against the intended CA. These controls reduce exposure but do not fix the proxy defect.
The fix is expected in version 0.2.3. Upgrade to version 0.2.3 or later when available.