What’s Vulnerable › Vulnerabilities

CVE-2026-96770: s2s-proxy accepts untrusted client certificates

• Severity: critical (CVSS 9.3 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-23 · updated 2026-09-24
• Weakness: CWE-296

Description

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 products

• Temporal Technologies, Inc. s2s-proxy: from 0.0.1-alpha through 0.2.2

References

• https://docs.temporal.io/production-deployment/data-encryption#codec-server-setup
• https://github.com/temporalio/s2s-proxy/blob/v0.2.2/encryption/tls.go#L47-L62
• https://github.com/temporalio/s2s-proxy/commit/d100c2258dbc37d7df51298ea045315f36c7eb3b
• https://github.com/temporalio/s2s-proxy/commit/f1dc6b9a3919d37e82545ab461cbe6bb62618d3c
• https://github.com/temporalio/s2s-proxy/pull/296

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