CVE-2026-96770: s2s-proxy accepts untrusted client certificates
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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
s2s-proxyto a version that resolves this vulnerability.Fixed in 0.2.3
Event History
Frequently Asked Questions
Which deployments are exposed to unauthenticated clients?
All published s2s-proxy versions through 0.2.2 are affected. The described TLS client-certificate validation flaw applies to versions 0.1.16 through 0.2.2 when TLS server listeners run with skipCAVerification set to false.
What does an attacker need to exploit this issue?
An attacker needs only a self-signed certificate and its private key to establish a TLS and yamux connection. They do not need a certificate or private key trusted by the deployment, and they do not need Temporal credentials.
What access can an attacker obtain after connecting?
The attacker can invoke RPCs that are permitted by the proxy configuration and its Temporal credentials. The reachable actions therefore depend on the RPC permissions configured for the proxy and the authority of the Temporal credentials it uses.
How can I identify the affected TLS configuration?
Review TLS server listeners in s2s-proxy versions 0.1.16 through 0.2.2. A listener is affected when skipCAVerification is false and the server uses Go's RequireAnyClientCert mode, which requires proof of possession but does not validate the client certificate against the configured CA.